LiFePO4 Batteries On Boats

02 LiFePO4 On Boats

LiFePO4 On Boats

I have had so many requests to open this article I have decided to remove the password protection. Please understand this article is a long way from being done but there is still a lot of information here. An article like this takes lots of time to write photograph and proof read and this web site is not my full time job. If you see typos feel free to point them out so I can edit and proofread faster…

Okay, okay I give up, the readers win…. (wink)

Where does the information in the article stem from?

Ever since I began the foray into LiFePO4 batteries readers of have been asking for more information. My experience and background with LFP date back to approximately 2008 when I began researching this technology and reading every white paper I could get my hands on. I quickly noted a complete lack of data for off-grid or fractional “C” type use.

This lack of available data resulted in setting up a test lab at Compass Marine Inc. (parent company of I then began experimenting with bare cells, pack level cells, charging schemes, balancing schemes, cell matching, float capacity loss testing, over-current protection testing, storage SOC & degradation, over-charging, charring at various C rates, Peukert testing, etc. etc. etc. etc. etc..  I have many thousands of hours of testing LFP cells in regards to house bank or off-grid type of use. I’ve destroyed in excess of $4k in LFP cells during this testing, both prismatic and cylindrical LFP cells, and have developed a pretty good understanding of how they behave and what practices can be damaging to these cells.

I am also an ABYC electrical systems specialist, a manufacturer of marine products and components such as alternators, and I specialize & focus on energy management systems for both design/engineering and installation. I am also an active member of the ABYC committee that is working on the safety standards for high capacity batteries.

Beyond all that we’ve had a LFP bank on our own boat since 2011. I am a huge fan of LFP banks, for many reasons, but I will not act as a “fan boy“, that would be unfair to you the reader… I will give you both sides of the story, not just the pluses.


#1 – does not sell LiFePO4 batteries or BMS systems. The advice here is not intended to get you to buy something from us, it is for you to learn something from so you get what you paid for. The information here is intended to give you as much information as I can so you don’t get screwed over. I don’t want to see you ruin a multi-thousand dollar investment, as many before you have already done.

#2 – If you intend to move forward with Li-Ion based bank please check with your insurer first. I am now receiving emails from readers who have been told they can’t have Lithium-Ion chemistry banks, including LiFePO4.

#3 – The ABYC High Capacity Battery TE or “Technical Information Report” is not yet complete though its been in the works since 2014. It’s still a good distance out for this to become a full blown standard. ABYC standards often start as TE’s then move to full standard status. Once this standard is out we will have a much better idea of how best to proceed with a LiFePO4 or other high capacity banks and be in compliance for what an insurer may want to see.

LFP Design & Consultation:

I no longer do any long distance consulting or design work for LFP batteries or systems. If the boat is not readily accessible here in Maine, Casco Bay to be specific, I can no longer do LFP consults. Why? Far too many owners insisting on cutting corners and multiple situations where end user implementation was not done to the design or design criteria have forced my hand. If you happen to glean something from this article, that’s great, but please don’t email me with further consulting questions on LFP.



Unless you want a big fat dent, where your wallet used to be, I do not believe LiFePO4 is ready for mass DIY prime time builds. Read with caution and especially focus on the things that you don’t want to hear rather than only what you want to hear. If you think you want LiFePO4 PLEASE READ VERY CAREFULLY. Once you are done reading this, and it makes sense to you, then please spend another 6-8 months reading everything you can including every single LiFePO4 white paper you can get your hands on.

Wallet Burns:

Since opening this article to the public I have now had what I consider a rather insane number of *LFP owners contact me who have ruined LiFePo4 batteries (not all marine based). In almost all of these cases of destroyed LiFePO4 batteries the resounding tone I hear come through is;

People on the internet made it sound easy.

The internet has a name for these kind of cheerleaders “fan boys“. Do yourself a favor and don’t jump to conclusions where large sums of money are involved over “fan boy” postings. PLEASE DO YOUR RESEARCH! YOUR RESEARCH DOES NOT END WITH THIS ARTICLE!

This article is BARELY scratching the surface and is not even touching on the actual science, which is complex, and it is not the intent of this article.

Sadly, only a few of the cases of destroyed LFP batteries I’ve been contacted about get reported in on-line in forums, on blogs etc.. The folks destroying these LFP batteries are not all boaters. Some are electric vehicle guys, some are RVer’s, one or two of them were off-grid and the rest are boaters.

The one thing they all seem to have in common was a grossly incomplete understanding of a what a BMS is, what a BMS should do, how to wire a BMS and how to properly & safely use, charge and discharge LFP banks within their safe operation range.

Here are the stats: (n=I’ve stopped counting):

#1 Cause of LFP Death = Over Charging – Almost always related incorrect absorption duration and too high an absorption voltage. Voltages were at or above 14.2V in every case and absorption duration well in excess of 30 minutes.

#2 Cause of LFP Death = Float Charging – In almost every case these owners insisted on floating LFP. Solar was #1 with shore power charging close behind. If you think a bank can’t be overcharged at a low pack voltage, think again, and please do more research. It is a fallacy that LFP can’t be overcharged at 1say 3.45V per cell, when this voltage is held there continuously.

#3 Cause of LFP Death = Over Charging Due to Cell Imbalance – In about half the cases the owners were monitoring only “pack level” voltage and in the other half it was incorrect pack balancing or poor cell matching when building the bank.

#4 Cause of LFP Death = Incorrectly set up BMS – I really hesitate to even call some of these “systems” a BMS but in each case the “protections” the owner had put in placed failed to protect the battery. About 4 of these cases were a simple “low voltage disconnect“. In one case the boat yard physically by-passed the BMS contactor.

#5 Cause of LFP Death = Over Discharging – These cases have been much rarer than over-charging but they still exist. Incorrect battery monitor SOC drift and lack or BMS protection lead to over discharged batteries. In a few cases parasitic loads were the cause & some were cell imbalance issues.

Please note that in all of these RUINED or drastically capacity diminished LFP banks there was not one incident of fire, explosion or dangerous off-gassing. Lots of bloated & ruined cells but no fire, just thousands and thousands of dollars in hard earned money GONE. LFP is pretty safe,  though it can still catch fire but it takes a lot of abuse, but it may not be entirely safe to your wallet.

What this article will discuss, and what it won’t discuss:

#1 This is meant as a general overview of LiFePO4 batteries (no other Li-Ion chemistry) for use as house banks on boats only. This is not about electric propulsion or electric vehicle use, just house bank use on cruising boats with both factory made systems and custom builds. I will sometimes refer to this type of use as “fractional C” use.

#2 It is the sharing of my learning, experimentation & installation/implementation of LiFePO4 batteries on boats. I do not consider myself an expert on the subject of LiFePO4. If even the Chinese manufacturers don’t fully understand this technology, inside and out, how can I? If there is no solid data on fractional C use then how am I to be an expert on it? Again, this article is the sharing of some of what I have learned about LFP, while keeping it to a “simple” level.

#3 This article will not show you every little “Betty Crocker” detail to build your own bank or give you every last ounce of detail & specific wiring diagrams. It is my belief that those final details need to be ironed out by whomever decides to DIY with the specific equipment chosen. If you do not have the ability or inclination to figure out the last few, connections of the dots, then perhaps you & LiFePO4 are not the right fit, at least right now.

#4 This article is not a suggestion or an endorsement for widespread DIY builds of LiFePO4 batteries. For those with the capabilities have at it, after checking with your insurer of course. this article is intended as educational in nature only. I am a firm believer that this is a level 11 on a scale of 1-10 for the average DIY to grasp. Unless you are an EE or have a very high, high, high electrical aptitude, and penchant for electrical systems, this is simply not prime time ready for the average DIY.

#5 LiFePO4 can put a HUGE dent in your wallet if not done correctly. In my experience there is lots of misguided advice on the net about doing this. One must read it all to decide which way will work best for them. I have a folder on my computer full of white papers and yet there are still massive holes in the data, when applied to fractional “C” use for house banks on boats. Many cells have been destroyed by folks trying to cut corners, take shortcuts or not doing enough research. I can only share what I have learned through research, physical testing & actual real world use.. This article is not to suggest there is only one way to use LiFePO4 on boats. I do however believe, for house bank use on boats, there is a right way and a wrong way. I will not tell you how you should do it, but I will share the why’s and how’s and why’s and how’s of how I came to decide on this or that methodology or practice.

What’s In The Box..?

The Shipment: These cells came from Balqon Corp in California and this is how they arrived, very well packed. I got a good deal on these cells but I can’t, with a good conscience, recommend dealing with Balqon. In my experience they are scatter brained, disorganized and shipping is extremely expensive. Some owners have waited many months to get their cells. If things change at Balqon, and they get better, I will be sure to let you know. Until then try to find USA stock of LFP cells, such as CALB, Sinopoly, GBS etc. elsewhere.

EDIT: Balqon has gone out of business.


Photo: The cells were very well packed and got here in great shape.

The Li-Ion chemistry chosen for this bank is called lithium ferrous phosphate, LiFePO4 or LFP for short. There are a few variations on this chemistry such as LiFeMnPO4 & LiFeYPO4 but the end result is still an LFP bank and has the same inherently safe characteristics. For this build I chose four 400Ah Winston LFP prismatic cells, and the bank is set up in a 4S configuration.

What the heck is 4S?

4S just means four 3.2V cells in series to make a 12V nominal pack. The pack/bank is really closer to a 13.3V pack as the resting & nominally loaded cruising voltage of these cells is around 13.2V – 13.35V.

If you want to do other than a 4S configuration the cells are ideally connected in parallel first then in series. Parallel first is done so that you only need to monitor 4 cell voltages. The parallel cells stay balanced and the BMS only has to monitor 4 cells for a 12V nominal system. if you choose series first you will require a lot more cell level monitoring. The nomenclature system you will most often see for 12V house banks is as follows.

4S = Four Series Cells

2P4S = Two Parallel Cells / Four Series Cells

3P4S = Three Parallel Cells/ Four Series Cells

4P4S = Four Parallel Cells/ Four Series Cells

There are many ways to configure LiFePO4 Cells in series or in parallel/series. I tend to prefer the simplicity of a 4S configuration when that size bank/cells works. 4S requires less overall connections and less work when doing cell balancing etc…. Some argue that if a cell is ruined with a 2P4S bank you could re-wire it and use the remaining cells.. At sea? Really? I much prefer the simplicity of a redundant lead acid reserve bank. Rewiring an LFP bank at sea is not a task you’d ideally  want to undertake at sea..

LiFePo4 Battery Bank Type Definitions:

Factory Integrated Lithium Battery – A lithium battery designed to work as a factory integrated system including the charge sources

System Integrated Lithium Battery – A lithium battery system with the capability of system interaction/communication with external charge sources, vessel loads, alarms or safety systems.

Drop-In Lithium Battery – A self-contained lithium battery with or without an integral BMS and contactor/s,that lacks external system communication with charge sources, vessel loads, alarms or safety systems.


Factory Integrated Systems – Victron & Mastervolt are about as close as it gets because they sell both the charge sources and the LFP batteries as a factory integrated system.

System Integrated Lithium System – Lithionics ex: Genasun & well executed DIY – Designed to work with third party products and can communicate with them.

Drop-In Lithium – If the internal BMS is sealed, and the battery can’t tell an external charge source when to stop charging or a load to stop discharging, then this is not a battery well suited for a cruising boat.

There are three basic options of getting LFP on your boat, with DIY being the least expensive, and most technical. The categories are:

DIY Builds:

This is a real cost saver but is not for the faint of heart or the limited skill DIY’er.. It is not recommended for the average boater. In the DIY build you source the cells, match the cells for Ah capacity, choose all the components, choose the BMS, choose the high voltage cut and low voltage cut relays, main contactor, wire and assemble everything, balance the pack and chose chargers, solar or alternator regulators to suit LFP charging. It is a very time consuming project.

Marine Specific Factory Made LFP Systems:

Genasun (discontinued), Mastervolt, Victron & Bruce Schwab OPE-Li3 / Lithionics all build LFP systems for marine specific applications. These systems are well engineered, well executed yet also pretty expensive. If you want LFP and don’t have the ability to DIY, these three companies I can certainly recommend. I personally have the most experience with Genasun, and the system was quite well done. The owner of the company, Alex MeVay, is an MIT grad and one of the brightest minds in the marine LFP field. Today I would strongly steer anyone towards the OPE-Li3 / Lithionics battery / BMS system.

Drop-In LFP:

Drop-In batteries are often sold in standard lead acid case sizes eg: group 27, 31, 4D, 8D etc. and they lack any external communication between the internal sealed BMS and the vessel.

I really even hate to mention this type of battery because it can have many issues, when installed on a cruising boat. Most of the drop-in batteries have been Chinese in origin and this is not necessarily a bad thing. Where they often fall flat is in marine specific engineering. These batteries were originally designed for telephone pole mounting where light weight and “drop-in” replacements for lead acid were critically necessary for the solar powered street lighting industry. The demand for this type of battery, especially in third world countries, is staggering. I know many boaters assume we are a large market but we are not and no these drop-in manufacturers are not specifically building “marine” batteries for us. They are simply slapping a “marine” sticker and perhaps even a well marketed brand name, on the plastic box.

Most of the “A” graded LFP cells used in these Chinese drop-ins are sold into the street lighting industry. Sadly for boaters this means the low grade “orphaned” cells wind up in batteries that may look the same but are sold on Ali-xxxx, eBay or through other less reputable sources. The problem is these batteries were never really designed nor intended for marine use. It has not however stopped some sleazy vendors from marketing them as such. Drop-ins will likely be the future of LFP, and there are currently a few good manufacturers working to improve the shortcomings but they are still not prime time ready. When they have improved the engineering it will certainly get mentioned here.

Drop-In Pit Falls:

#1 Current Handling – The current rating of the internal switch that protects the battery is often too small for the task. These batteries routinely use dinky little mosfet switches that often can’t handle the loads imparted by many boats. On board devices such as a bow thruster, windlass, large inverter, electric winches, electric cook tops, massive alternators or chargers and more are very very common on-board these days. These are exactly the devices many boat owners are hoping to see a gain in performance from when switching to LiFePO4. Look at the contactor ratings companies such as Lithionics, ex Genasun & Mastervolt etc. use/used for “marine specific” LFP batteries. What you’ll often see are 500A continuous rated Gigavac, Blue Sea ML Series, Tyco  or other similarly rated contactors being used. Companies building “marine specific” LFP batteries know what boats need. Companies re-stickering streetlight pole batteries do not use this type of contactor. What you’ll often find is some wimpy 50A continuous rated mosfet switch installed inside a 300Ah LFP battery! Ouch….

#2 Vibration – Many of the cheaply sourced drop-ins are using 18650, 26650 or 32650 cylindrical cells. In a worst case, a 100Ah LFP battery, built from 18650 cells, would need a grand total of 364 cells with two connections per cell! Hows that math work?

18650 Cell = 1.1Ah (An Ah rating for an 18650 cell above 1.1Ah should be considered suspect)

91 Cells Make Up Each 3.2V cell

Four 3.2V Cells Make Up a 12.8V 100Ah Battery

91 X 4 = 364  18650 Cells

Positive & Negative Connections Inside The Battery = 728

If the manufacturer uses 5Ah 32650 cells, and some do, we then only need 80 cells total, and 160 spot welds or bolted connections to potentially fail or work loose. (32650 cells are available in bolted or spot weld versions)

The connections, with 18650s’s, are almost always spot welded to end boards that make up the individual cells.  So in a single 100Ah battery, made of 18650’s, just to connect the cells, we have as many as 768 spot welds to rely on. Beyond that we have all the internal wiring and BMS connections. These spot welded assemblies are often just dropped into the polypropylene case with no other support or vibration dampening material.  Always be sure to ask your supplier to furnish third party vibration testing results.

#3 Internal Wiring – It is not uncommon to open a 100Ah drop-in battery and find 10GA or 12GA wire feeding the main positive and negative terminals. This does not even pass the Mickey Mouse School of Engineering sniff test.

#4 BMS – Some of the drop-in batteries coming out of China lack a BMS altogether and others only have a single low rated relay that disconnects the bank on low or high voltage only. They should also have temperature protection, for each 3.2V cell, but most don’t. The BMS protection relay (really just a mosfet switch), as discussed above, is often rated at a ridiculously low amperage capability of around 50A and maybe as high as 100A, if you’re lucky. The drop-in manufacturers are essentially relying on you paralleling multiple batteries and sharing the load across the questionably suitable under-engineered (for marine use) BMS switches.

#5 Non-Communicable BMS – This one is perhaps the most dangerous aspect on-board a cruising boat. For a trolling motor? Who cares.. It’s not powering anything critical.. For a house battery, on a boat that ventures off-shore, and is powering critical navigation and safety equipment, this can be a flat out dangerous situation. A non-communicable BMS is one that can not communicate externally with the vessels charge and load systems or even you the owner. It has no means of externally communicating or sending/sounding warning alarms or activating relays/triggers to properly and safely disconnect charge sources or give ample warning of an impending BMS disconnect.

Ask yourself what happens when your 150A alternator is in bulk charge, supplying all the current it can, and the internal BMS decides to “open circuit” or disconnect the battery from the boat? I’ll help out a bit here.

   A) The alternator diodes, unless avalanche style (pretty rare in high performance alternators), can be blown and the alternator will be rendered non-operable. Two years ago I  did exactly this, using the alternator test bench here at CMI. The alternator was running at full bore charging an LFP battery, the “system” I set up had a 3A dummy load on to simulate navigation electronics. With the alternator running at full bore I disconnected the battery just as an internal sealed BMS would do to protect the LFP cells. Poof went the alternator!! Worse yet the voltage transient I recorded on the “load bus” (navigation electronics) using a Fluke 289 was 87.2V!!!

   B) If the boat is wired, as is typical with drop in batteries, the voltage transient caused by the open circuited alternator will now directly feed the DC mains and potentially destroy your navigation equipment. A properly designed BMS, in this case, would open a relay that would de-power your charging sources on the input side, thus shutting the charge sources down correctly and safely with no risk of a damaging voltage transient. For a battery charger it would de-power the AC input side, for an alternator it would de-power the field wire or regulator B+, for solar it would open a relay in the PV feed etc. etc./ With a drop-in battery, that features a sealed BMS, you have no way to do any of this.


#6 Series Wired System – In a parallel wired bank one battery BMS dropping out only creates problems when it re-engages into a different SOC to the rest of the bank. With a series bank (or a single drop-in LFP battery) one batteries BMS taking itself off-line spells disaster at sea. I know one Alixxxxxx buyer who hit a bridge abutment in his electric boat using a 48V series bank of drop-in batteries. It did a few thousand in damage to the boat, and his pride, but it could have been much worse. The owner had ZERO warning the battery was about to disconnect itself before he 100% lost power while going under a bridge..

#7 Fan Boys/Girls – Please, please, please when watching videos on YouTube or reading blogs showing cruisers touting LFP drop-ins, keep in mind this is a technology they may no nothing about other than from the glossy marketing materials. Also, many of these installs may have been HEAVILY SUBSIDIZED BY THE BATTERY MAKER!!! I am one of the folks who has been contacted by these companies trying to get web presence from high viewer You-Tubers or high reader volume bloggers. We will continue to politely decline these offers until they can get their engineering above Mickey Mouse Club level and make their batteries safe and suitable for a cruising boat application. Unlike an RV you can’t just pull over and call AAA from the middle of the Pacific Ocean when the battery goes teats-up.

What would I need to see before I can support an LFP drop-in for a cruising boat?

#1 Externally Communicable BMS via a HARD WIRED external communication port, NOT via Bluetooth.

#2 Internal BMS contactors / switches capable of the amperage’s used on cruising boats. A 50A continuous rating on a 300Ah LFP battery is a JOKE.

 #3 Individual cells that have passed UL

#4 Third party vibration testing data or UL testing for the entire battery, not just the bare cells

#5 Verification of internal cell matching

#6 Transparency of what manufacturers cells are being used internally (there are a lot of horrible cell makers in China)

#7 Internal wiring gauge specifications – 10 or 12GA internal wire to feed the B- & B+ posts is 100% unacceptable.

#8 BMS warnings that can externally warn of a trend towards a disconnect.

#9 BMS low voltage, high voltage and over & under temp protection for each of the four 3.2V cells in the battery

The Future of Drop-In?

I do believe we will see at least a couple of manufacturers step up the game on all these points. I have discussed this with a couple of them who are actively working on making these batteries safer and better suited for marine house bank use including heavier duty BMS switches and external communication to avoid dangerous and damaging voltage transients. Some are already using UL tested cells but most lack UL for the complete battery. If it seems to good to be true, and it is sourced from eBay or Ali xxxxx it probably is! At this point in time I still urge a strong Caveat emptor on LFP drop-in’s for use on cruising boats. Safe enough now for a trolling motor on a bass boat? In some cases yes but not for a cruising boat.


When in doubt go with Mastervolt, Victron or the OPE-Li3 / Lithionics system. These companies have well thought out marine specific systems.

03 LiFePO4 On Boats

Reading the Specs on Drop-In’s

Sometimes one just has to laugh when reading the specification sheets on some of these drop-in batteries.

This drop-in LiFePO4 (LFP) battery is rated at 12V-300Ah with a maximum charge current of just 50A!

A 50A max charge current on a 300Ah battery is a charge rate of 0.16C


The specification sheet then claims 2000 100% discharge cycles so one might assume you can actually go to 0% on each and every cycle and have the battery last 2000 cycles. When you’re done buying your drop-in LFP’s, I have some waterfront land in Siberia I’d like to sell you. (grin)

The specification sheet also claims the battery can be charged in 60 minutes!! Holy $hite, that’s fast, but just to be safe, lets do the math..

300Ah battery at 0% SOC – 300Ah / 50A = 6 hours –Fail

300Ah battery at 20% SOC – 240Ah / 50A = 4.8 hours –Fail

300 Ah Battery at 83.4 SOC – 50Ah / 50A = 1 hour –Winner

You read that math correctly. The only way we can get this battery to charge in 60 minutes is if you only discharge to 83.4% SOC……. So much for those deep-cycling &  “fast charging” LiFePo4 batteries.

If you’re naiveinexperienced, dumber than a pound of mayonnaise, less than educated on a subject, drop-in battery makers will sell you anything you want to believe.

04 LiFePO4 On Boats

LiFePO4 vs. Lead – Pro’s & Con’s

No article would be complete without pro’s & cons and comparisons between lead and LFP.

LiFePO4 Pro’s:


2000+ “claimed” cycles to 80% DOD (depth of discharge). If we compare the best AGM batteries to LFP we find that reputable manufactures such as Enersys/Odyssey claim just 400 *lab rated cycles to 80% DOD. Please keep in mind that just like lead acid “cycle life testing” this testing is not done in a real world applicable environment. They are never floated, never over absorbed, never held at a high SOC for long periods, never over discharged and live is a cushy 77F to 80F test environment during testing. Vary any of those parameters and you will not get the same results.

I suspect, (wild ass guess), that LFP prismatic cells, from reputable manufacturers such as Winston, CALB, Sinopoly, Hi Power, GBS and others, can last approx 1600-2400 cycles even when pushed regularly to 80% DOD, in the real world of the marine environment, if treated correctly. Some LFP manufacturers claim more, but so do the lead acid makers, and these are always ideal “lab” conditions. Of course at this point in time this is a WAG (wild ass guess) on my part. I can assure you even 1600+ cycles is a very long time. I have been busting my hump since late 2010/2011 and have only managed to put a bit over 900 cycles on this bank. Please be advised that LFP cell makers have been slowly adding to marketed cycle life claims sometimes 1000 cycles at a whack. Very few of these cells have actually been tested for this and the data has simply been “extrapolated” One cell maker even went from a claim of 2000 cycles to 80% DOD to over 7000!

The average lead acid battery on boats is often dead well before 150 – 200 cycles and they rarely if ever even come close to the “lab rated” cycles. Do the math on your own bank, be honest about it, and see how many cycles you had, to 50% SOC, before your bank needed replacement. Most boat owners are shocked when they do this math.

* I have never seen a single lead acid battery bank hit its lab rating in the marine environment, especially not on cruising boats and still be above its 80% end of live Ah rating, NOT ONCE.. These lab numbers are fairy-tale ratings when translated into the real world. I also don’t trust the lab ratings of the LFP cells, when applied to real world conditions. House banks on boats are simply not being used in the same manner that factories test them. The factory for example does not let them sit in hot engine rooms, over-absorb them nor do they ever float them when testing for cycle-life.

Alex MeVay, the CEO of Genasun, firmly believed in 2000+ cycles to 70% DOD and he’s had banks out there since 2007/2008. This is utterly amazing cycle life. Based on what I am seeing at 900+/- cycles I am trending towards believing 2000 cycles is doable, if used correctly.


Approx 80% of an LFP banks capacity is fully usable, 20% DOD to 100% SOC. With lead acid you often have just 30-35% usable capacity (50% SOC to 80-85% SOC) due to charge acceptance current limiting. With LFP the current limiting or acceptance taper is very, very short in duration, even at relatively low 12V nominal pack voltages of 13.8V – 14.0V (3.45VPC to 3.5VPC) this duration can be as short as 10-30 minutes depending upon charge rate. If you increase the voltage of the pack towards 3.6VPC the CV duration almost entirely vanishes.


Very short current taper even with large current sources. With low current charging, such as solar, you can charge to nearly full before even attaining CV/absorption voltage. This of course is entirely dependent on your charging voltage and your current source. We charge at about 145A (160A externally rectified alternator  that runs at 140A – 150A continuously when hot) and our current taper lasts only 30-35 minutes. Compare that to HOURS and HOURS of current limiting using a 145A charge source on 400Ah of lead acid batteries. With a small charge current source, like a small PV system or wind, you can hit 99%+ SOC before the constant voltage (CV) stage is attained or before any current limiting can even occur. Our bank literally has to be chock full before our solar array can even get it to 13.8V. These batteries can take immense current, and charge extremely fast, but really tend to do extremely well with .3C to .5C in charge current.


Less than half the weight of lead, Ah to Ah, and almost always more compact. The 400Ah bank in this article weighs 134 pounds less than a 400Ah lead acid bank. However, to equal the usable capacity of a 400Ah LFP bank you would need approx 900Ah’s of lead. This makes the 400Ah LFP bank approx 400 pounds lighter than the equivalent usable capacity in lead acid!


The term “dead lead” is s term I coined in my electrical seminars. The typical lead acid bank consists of 65-70% of the weight being comprised of “dead lead” or the excess lead you carry around but that you can not use. If you have a usable capacity of just 30-35% of the bank, when out cruising, this means that you are carrying around 65-70% of that weight in unusable “dead lead” capacity. This 400Ah LFP bank weighs 130 pounds & 80% of it is easily usable. This means just 20% of it not easily usable or you simply don’t want to use it for best longevity. As a result, we carry around a measly 26 pounds of unusable battery on our 36 footer..

Lets go back to usable capacity for a moment. If we want to equal the usable capacity of this 400Ah LFP bank in lead, we would need 8 GC2 6V golf cart batteries or approx 900Ah’s. 35% of 900Ah is a usable capacity of 315Ah’s. 80% of the 400Ah LFP bank is a usable capacity or 320 Ah’s. The 900Ah lead bank weighs 520 pounds. If you use just 35% of that bank then you are hauling around 338 pounds of “dead lead” or 338 pounds of unusable capacity.. Twenty six pounds of unused LFP or 338 pounds of “dead lead“..?? Points to ponder…


LFP banks have a very strong & flat charge & discharge curve with a very steep & fast rise or drop at either end. These ends are called the “knee’s”. LFP cells will maintain voltages well above that of any fully charged lead acid bank, and remain very close to their 3.3VPC / 13.2V nominal voltage level, and hold quite steady voltages with little change almost all the way to 80% DOD. They will maintain a very tight voltage range even under “normal/typical” house loads. Espar heaters, refrigeration, water-makers etc. will all perform better. Equipment likes higher voltages. Even bilge pumps will pump more water. Voltage sag that can drop out electronics during bow thruster or windlass use is almost entirely eliminated.


Charge efficiency is also referred to as the Coulombic efficiency. These batteries are as near 100% efficient as I have ever seen on my test bench. Take 200Ah’s out and put 200 Ah’s back in and you hit the voltage and net accepted current at almost the exact same Ah’s out to Ah’s in. Until LFP I had never witnessed anything like this, even with the best AGM’s. Lead acid ranges from 70% to as high as 90% +/- efficient but you still need to put back in 10-30% more than you took out, and this is with “healthy” lead acid batteries. As they sulfate the charge efficiency or Coulombic efficiency gets even worse.


We know the Achilles heel of lead is sulfation and in order to fight off sulfation we need to charge them to 100% SOC as often as possible. This proves very difficult for many boaters and cruisers unless your boat resides at a dock after each sail or sits on a mooring with an adequate solar system. LFP batteries do not need to get back to 100% SOC so, FOGEDABOUTIT…….! This is a major win for LFP. When we come back from a weekend on the water, and our battery is at 50% SOC, I DON’T CARE!!!! I shut down the boat, and the solar and go home. LFP batteries actually prefer to sit at 50-60% SOC rather than at 90-100%.. As I said earlier this is a mental paradigm shift we need to overcome in our human behavior/thinking around our batteries…


Sulfation is by far and away the cancer and #1 killer of lead acid batteries. These batteries do not sulfate, no cancer, so there is no need or worry to constantly get back to 100% SOC before you leave your boat. LFP batteries actually prefer to be left at mid range SOC rather than full. Enjoy that sail home, without the motor!


Without question LFP is one of the safest of the Li-Ion battery formats. Many argue, and these arguments have certainly been well made, LiFePO4 is as safe or safer than lead acid. ALL BATTERIES ARE DANGEROUS, let us not forget that.

As Li-Ion technology goes LiFePO4 is currently one of the safest. Remember we are surrounded every day by far more volatile Li-Ion technologies in computers, iPads, iPods, tablets, video games, cell phones and even cordless tools. LFP is less energy dense than other more volatile Li-Ion formats, but when compared to lead acid everything looks energy dense. We have no need on boats for “Dreamliner” level energy density, thus we use the considerably safer LiFePO4/LFP technology not LiCoO2 like Boeing chose.. If you believe LiCoO2 & LiFePO4 batteries are the same PLEASE STOP READING HERE and go back & hug your lead acid batteries…..(grin)

I think this video done by Sinopoly can sum up the safety of LFP technology. Those crazy Chinese guys shot, burned, shorted and cooked these cells. Please DO NOT attempt this stuff at home. Take note that a single 60Ah 3.2V cell can throw in excess of 1800A of current into a dead short.. WOW! None of the testers got acid burns, were blinded or went home with holes in their clothes. (wink)..


That said and shown, it is a fallacy & myth to believe that LiFePO4 batteries can’t achieve thermal run away and catch fire. LiFePO4 batteries can catch fire and they can suffer from thermal run-away it is just much less common than other Li-Ion chemistries.  If an LFP battery does catch fire, you don’t want to be near it.

But, But Lead Acid…….

As I mentioned earlier no battery technology is 100% safe, let’s not kid ourselves.. Stupid charging practices, like using an automotive battery charger on a boat, can wind up with a situation like this.

Take a guess at what the battery acid did to the inside of this boat when this lead acid battery went KA-BOOM………!!

Yep BOOM, and its not LiFePO4!!! (wink)

05 LiFePO4 On Boats

06 LiFePO4 On Boats

LiFePO4 Con’s

PHOTO: In this photo I have constructed my cell compression case. Prismatic LFP banks need cell compression cases so that in an overcharge event the cell bulging may be controlled. I chose 1/4″ aluminum as it is easy to work with. The cover is 3/8″ Polycarbonate.

LifePO4 is tremendous technology but this technology comes at a price.

LiFePO4 Cons:


LFP batteries can be quickly compromised, have the capacity diminished or even destroyed if over charged. Unlike lead batteries over charging does not just gas off some electrolyte that can be replaced. In the case of prismatic cells it can cause the case to swell and the cells to be irreversibly ruined.  A lead acid battery will suffer some permanent capacity loss from chronic over charging, but can survive this, an LFP battery will not survive this without some damage. Over charging LFP batteries can result in a huge hole in your wallet! This means proper charging and most often a BMS system to ensure over charging can not happen. Please under stand however that over-charging is not just about voltage.


One area folks often misconstrue is thinking a lower charge voltage means it’s 100% safe. It’s not! I your charge sources are not suitable they can still over charge by holding the constant-voltage stage (absorption) for too long. Over absorbing, even at pack voltages as low as 13.65V, can result in “over charging” if the CV (constant voltage) stage of the charger is held long enough. An LFP cell can be fully charged as low as 3.42VPC if the voltage is held long enough. Far too many lead acid designed charge sources will hold the voltage more than long enough to damage or destroy LFP cells. The higher the target charge voltage the worse it will be for the cells when over-absorbing occurs. Some LFP manufacturers are finally starting to get this point, when selling into a lead-acid charger environment, and have reduced recommended max charging voltages accordingly where some others have not.

I currently have four prismatic cells in the shop, sent to me by a gentleman who “assumed” a “GEL” setting on his charger was “safe”. He did this based on Winston’s voltage specifications. He assumed, seeing as it was only 14.1V or 3.53VPC and well within the “spec“, that 14.1V was safe for a nominal 12V bank.  However, voltage is not the only factor to consider. You have voltage, duration at target voltage, and charge rate to also consider. LiFePO4 cells are best charged to 100% SOC then charging is 100% terminated. This does not happen with far too many lead-acid designed chargers.

The problem in his installation was the absorption DURATION, at 14.1V, was a monsterous 4 hours long with no way to change the length of the absorption cycle-timer. On top of a 4 hour absorption the chargers charge rate, which was very low in comparison to the banks Ah capacity, his cells were actually hitting 100% SOC before the voltage even got to 14.1V.

In other words he was technically over-charging his bank before he even got to 14.1V because charge rate also plays a role. Once his bank hit 14.1V the charger then continued to over-charge them for four more hours each time he went to 100% SOC. On top of all this his so called “smart charger” was actually really quite dumb and could reset the absorption timer when ever a large load kicked in and momentarily dropped the sensed voltage below the re-absorb trigger.

In just 150 +/- cycles his 180Ah cells could barely deliver 96Ah’s and they were puffed up like balloons.  2000 cycles????? This is far from it if you don’t charge them correctly.  His expensive Winston LFP cells were completely ruined in less than 150 cycles while using the GEL setting we so often read about as being “safe” for LFP. If his charger had stopped charging when the cells actually hit 100% SOC, it would have been much safer, but instead it kept charging for 4+ hours after the bank was full and could easily be re-triggered back into an absorption cycle pretty easily.

Contrast this real world scenario to a study conducted at a University using the same exact prismatic cells. They charged the cells to 4.0V then discharged to 0% then repeated. The cells survived 950 complete 100% discharge recharge cycles. The differnce here being the charging was 100% terminated/stopped when 4.00VPC was reached and the discharge turned on. This means the cells were only above 3.45VPC for a very, very brief period on each cycle.

The relationship between target voltage, duration at target voltage + charge current is where the damage occurs or does not. When setting up an LFP system these three factors can’t be ignored;

Target Charge Voltage

Duration Charger Holds Target Voltage

Charge Rate

These three items go hand in hand with LFP.

Charging LFP is simple – Charge to target voltage then STOP CHARGING.


Just like over charging over discharging can actually result in a polarity reversal and destruction of the cells. A lead acid battery may suffer some permanent capacity loss but can easily survive this, an LFP battery will not. Another HUGE WALLET DENT and another reason why an LFP system is designed and installed as a “system” not just “drop-in” look like lead batteries..


LFP cells should really not be charged at temps below 32F/0C. They can be discharged below the freezing point but not charged. Lead acid could really care less. This is yet another way to dent your wallet.

There is some confusion out there surrounding the LiFeYPo4 Winston or Voltronics cells. Winston claims that the addition of Yttrium allows the cells to be “charged” at temps as low as -40. I have asked Winston & their old US distributor Balqon, via email on multiple occasions, to furnish or name any third party testing that confirms it is safe to charge LiFeYPo4 cells at temps below 32F/0C. All I’ve received is dead silence. With Winston Chung’s past history all I can say is I DON’T TRUST SOME OF HIS CLAIMS. Do your own homework. Voltronics on the other hand, who uses Winston to manufacture their LiFeYPo4 cells, suggests not charging these cells below 32F/0C.

Until I see some verified & legitimate third party or University level testing, that can verify these cells can be charged SAFELY at temps below 32F/0C, I am holding steady that you should not do this. If anyone has a white paper I missed on cold weather charging of LiFeYPO4 please send it along.


Keeping the LPF cells balanced is of prime concern because it is high or low voltages that ruin LFP cells. In LFP cells we have what are referred to as the upper knee and the lower knee.

What the heck is a “knee”?

The upper and lower knees are where the cell hits full/empty and the voltage trend changes sharply upwards, in hockey stick fashion, or sinks like a rock. Over discharging and over charging is where most LFP cells are destroyed. One minute the bank is supplying or accepting massive amounts of current and the next minute a cell has gone overly full/empty and hockey sticks before the other cells do.

If one cell becomes out of balance it can hit full before the rest of the cells do and this one cell can be ruined. Cell balancing is most important & most critical when pushing or using high charging voltages, which IMHO and experience, is completely unnecessary for fractional C use. Charging these cells to more than 14.0V is really not  desirable and only leads you closer into the danger zone, especially if the cells were to drift.

This is why individual cell level monitoring, of cell voltage, is necessary. Pack voltage alone tells you nothing about an individual cell going off early, only what the overall pack voltage is.. A good BMS (battery management system) will cut off charging well before any damage can be done to an individual cell..


These are not lead acid batteries, not your fathers Oldsmobile, and they are not designed nor intended to be “float charged” in the typical sense. The only reason people even try to argue this point is because they are trying to save money, cut corners and adapt poorly or inadequately designed lead acid equipment to LFP technology. Not really a wise idea.

Look at any of your tablets, cell phones iToys etc. and they all terminate charge when the battery is FULL. They cut back in when battery terminal voltage has fallen to a preset level, but they do not keep feeding current to a full battery.

Floating LFP is a complex subject and I will touch on this later. Bottom line is to avoid floating LFP banks if you can. Some have argued that a float voltage of 3.35VPC or lower (13.4V for a 12V nominal bank) is not badly damaging. Remember this type of charging keeps you in the upper SOC range for long periods of time and these batteries prefer to sit at 40%-60% SOC when not being used, not 95%+ SOC. These cells are designed to be actively cycled not held continuously at higher SOC’s.

Can you float at 3.35VPC or lower? Sure you can do what ever you want to, but we don’t really know the long term affects other than to say it is likely going to shorten the life of the cells. Of the 80 or so white papers I have on LFP batteries not a single one of them has dealt with fractional “C” use and floating at 3.35V or lower, not one. What we do know is that the merely storing these batteries at anything close to a 100% SOC resting voltage (not even charging) can eat capacity at a rate exceeding 10% per year. Now stack a float voltage on top of that… Ouch!


Unfortunately most commercially avaible chargers, solar controllers and alternator regulators are of extremely limited design and are not well suited to charging LFP banks. Most of these manufacturers have zero, zilch, nada experience with LPF yet they have no qualms making up “Li-Ion” charge profiles, despite no physical or practical experience with them. In short many of these LFP charging claims are misleading at best and can be dangerous to the health of your cells.


Many of these profiles can destroy your very expensive LFP bank. ProMariner and Sterling Power are two chargers that immediately come to mind with a Li-Ion setting of 14.6V absorption and a 14.4V float. Keep in mind that these chargers also use a bulk duration multiplied by X type of algorithm to help determine absorption duration. This type of algorithm sets or extends the absorption duration based on the length of the bulk stage. Short Bulk = Short Absorption & Long Bulk (eg: LFP) = Long Absorption. Long absorption with LFP = BAD!

Seeing as bulk is very long with LFP how long do you think the LFP absorption duration will be? Holding these cells at 14.4V indefinitely (float) is plain and simple LiFePo4 homicide. As Homer Simpson says D’oh……..

Please do not use these settings to charge prismatic LFP cells. You will ruin your batteries. The sheer fact is the people who create these potentially dangerous charge profiles very often have zero experience with LFP banks, but, they do want to sell you a charger.

On the other side of the coin the Chinese are very eager to get into your pants. Like a 16 year old horny prom date, they also mislead and tell you it is safe to charge these banks at high voltages so end users/consumers THINK they can just drop them in and off they go.

I suspect much of this is simply a misunderstanding between what the consumer wants to believe and what the Chinese actually mean.

Follow me for a moment:

If LFP requires different charging, and lots of extras, the conversion from lead to LFP will be much slower, and the Chinese will sell fewer cells/batteries. How do they fix this?

Hmmm, lets sit in a smokey Chinese factory back room and figure this out.. Oh yeah, I know, just tell them they can be charged at normal lead acid voltages and we will sell more. If I recall the GEL battery makers already tried that and look where their market share is today. GEL is a tremendous lead acid technology for deep cycling but the greedy manufacturers destroyed their own market by advising far too high a charging voltage. I have both East Penn and Sonnenschein Prevailer GEL banks on customers boats that have gone 15-17 years, properly charged, and others dead in 13 months that have been incorrectly charged.


Let’s look at this another way. The Chinese say, in the manuals, it is okay to charge these batteries to 14.6V or 3.6VPC. Hmmm, what are WE missing? Here it is and it is very simple..

They EXPECT you to charge to 14.6V and then then STOP CHARGING. Once you hit 14.6V the Chinese expect charging to STOP entirely.

The reality is this is not at all how any commercially avaible marine chargers work.

“Charge to 14.6V”

is not the same as

“Charge to 14.6V and then let it remain charging at 14.6V for FOUR HOURS”

Are you beginning to see the disconnect between the Chinglish manuals and the reality of our poorly programmable lead acid charge sources?

Please do the research, then do your own testing, as I have, then you decide. After all it is your bank and your wallet. I suggest strongly looking into what manufacturers such as Mastervolt have learned over the last few years. Their charge voltage guidance has been reduced quite dramatically. Why? Well if the DESTROYED 10K Mastervolt battery in my shop is any indication then we have our answer. They apparently learned something about the Winston/Thundersky cells residing inside the pretty teal and gray box. What they learned was not from Winston Chung, it was from actual FAILURES when charging these banks to 14.6V (3.6VPC) instead of 14.2V (3.55VPC) or lower. These failures occurred with a fully gourmet 100% factory engineered & integrated LFP system. For a DIY without Mastervolts engineering skills, max out your charge voltage at 13.8V to 14.0V, with little to zero CV duration, and you will be in a much safer place.

You really need to source chargers, controllers and alternator regulators that allow 100% CUSTOM PROGRAMMING. Even this is not perfect but better than holding an absorption or a float cycle. Please do not source the cheapest piece of junk smart wanna-be Li-Ion charger.  Dip-switch (read; dip-shit) pre-selected, non-adjustable charge profile chargers, ones built by a charger maker who’s reading a misleading & unclear Chinese/Chinglish spec sheet, and does not understand what it even means, have no place being mixed with LFP.

The previously mentioned ProMariner ProNautic P and Sterling ProCharge Ultra chargers are excellent chargers, for lead acid, because they do have a custom program feature, for voltages only. You could use the custom profile but DO NOT use their “Lithium” charge profile. Where these chargers fall flat, for LFP, is the absorption duration or duration at constant voltage. These chargers lack the ability to shorten the absorption voltage duration to be suitable for LFP so you, as a user, would need to manually manage that. Where these chargers also fall flat, as most lead acid chargers do, is in the lack of a dedicated voltage sense lead. Also the largest charger they make is just 60A. Kind of small for an LFP bank. For an AC charger, a fully programmable unit is better, and by fully programmable, I mean absorption duration (set to 0) and voltage fully adjustable and FLOAT SET TO OFF. In most cases the best unit for LFP will be a fully programmable inverter/charger.


An LFP bank is not just a battery it needs to be treated as a COMPLETE SYSTEM. A car is simply not just wheels & a motor and an LFP battery is simply not a battery & terminals unless it has the rest of the system to go with it. This is why drop-in LFP batteries are a complete and utter travesty to me. I strongly suspect these lead acid look-a-like “drop-ins” will be the black eye on the industry. Sadly by the time it is realized many unscrupulous companies will have made their money by then and gotten out…

You read it here and my prediction is this;

Drop-in LFP systems will be the BLACK EYE that may destroy the entire LFP marine market just like what standard flooded charging voltages did to the GEL battery market.

I hope I am proven wrong on this point but it looks doubtful. I have reached out to these companies and tried like hell to get a human being on the phone who had even an ounce of clue about LFP batteries and I failed. Scary stuff.

GEL batteries are still the longest lasting of lead acid batteries but they SHOT THEMSELVES IN THE FOOT trying to sell into a market that did not require a “rest of the system” approach. They tried to tell end users to just “drop them in”, and they  got fried charging at FLA voltages. Dumb, really, really dumb. Apparently the Chinese LFP makers have not learned from battery industry history.

When I say system a good system will be designed from the ground up to include a BMS, LFP capable over-current protection (meeting the proper AIC / amperage interrupt current requirements), proper cell compression case (for prismatic cells), specifically chosen and 100% PROGRAMMABLE, regulators, controllers and a charge and loads bus as well as LVC and HVC safety cuts. This barely scratches the surface but you can see why none of this is “drop-in”.


Con? Not really but to some it may be.. No matter how you run the numbers LiFePO4 wins the cost per cycle comparison to AGM or GEL batteries, unless you improperly charge them and ruin them prematurely… This however does not mean a well designed and engineered system will not have a rather large sticker shock. I don’t personally find this a “con”, because I am capable of simple math, but some will see this as a con so I mention it.

Do the math on a $$ to cycles calculation and you will see that LFP wins. When doing this math remember that a 400Ah LFP banks has 312-320 usable Ah’s when out cruising and you can easily get back to 100% SOC. To get 320 usable Ah’s from a lead acid bank, when out cruising, (cycling between 50% & 85% SOC) you would need approximately 900Ah’s of lead to equal the 400Ah LFP bank.

You do not need as large an LFP bank as you do lead acid so you can’t honestly compare 400Ah of Li to 400Ah of lead on price / Ah’s as this is a grossly unrealistic comparison.. Interestingly 900Ah’s of lead, using 6V golf car batteries, weighs approx 528 pounds and 400Ah of LFP weighs 130 pounds.


By far the biggest con for LFP is relearning the human behavior developed on lead acid. Voltage for SOC is pretty much meaningless with LFP, so don’t bother. You don’t want to float LFP, unlearn float. Get out of the habit of charging to full, it’s not necessary. Stop worrying about not being able to start your motor with a low battery. Enjoy the piece and quiet and even at 80% DOD the LFP bank will laugh at the starter motor current. Starting a motor at 80% DOD for LFP is like Mike Tyson picking up a feather…….. Unlearn your lead acid learning’s and the transition to LFP will go much more smoothly. Human behavior learned from lead is IMHO one of the biggest cons for LFP.


In today’s day and age we live in a complex world, we always want to be within reach of internet, have multiple devices plugged in, marine electronics have become more complex and the entire boat is moving in a complex direction. LFP is going to add even more complexities to the boat. In a way it is more complex, with all the protection systems in place so as not to destroy the cells, but in another way, it is pretty simple.

How simple?

Charge to full and stop… Run the bank down to 80% DOD and recharge. If your system is set up correctly, repeat 2000 +/- times….


There is a lot of evidence out there that suggests these cells can easily cycle to 2000 or more cycles, under lab conditions, yet there is little information about prismatic cell shelf life. LFP does lose capacity, even when not used, but how much is still open for debate. The yellow 400Ah cells in this article are 2009 cells and as of December of 2014 still delivered 419.2 Ah’s under a 30A constant load at cycle 550 (edit 4/24/18 now nearing 1000 cycles). Of course this is an n=1, which in science, means squat.

These cells have never been stored at a high SOC nor in temps exceeding 80F so for LFP shelf life they have really had ideal conditions. Based on the scientific data we do have, shelf life does appear to be affected more dramatically at temps above 80F and in this regard LFP is not much different than lead acid. I guess no battery really likes heat. The bottom line is to keep them stored at cooler temps and do not store them at more than 50-60% SOC. High storage state of charge, and high temps during storage, can accelerate LFP shelf life degradation. It is possible, under the right circumstances, that a boat owner may never get the claimed cycles without shelf life eating away at the capacity too..


Lead Acid And High Current Loads

Lead acid batteries do not like to deal with high discharge loads such as inverters. When you apply a load larger than the 20 hour Ah rating the capacity of the bank gets smaller. Click the image to make it larger and see what I mean.

Lead acid batteries are rated at a 20 hour rating. This means a 100Ah battery can supply a 5A load for 20 hours before hitting a terminal voltage of 10.5V.

A 400Ah bank can supply a 20A load for 20 hours before hitting 10.5V. Any loads above the 20 hour rating diminish the capacity of the bank.

The 20 hour rating load is determined by; Ah rating divided by 20.

100Ah battery ÷ 20 = 5A

125Ah battery ÷ 20 = 6.25

225Ah battery ÷ 20 = 11.25A

LFP batteries are not rated at at 20 hour rate like a lead acid battery is. How they are rated for Ah capacity can cause lots of headaches trying to actually figure it out.

Some prismatic cells are rated at a .5C load or 50% of the Ah rating at 25 Celsius / 77F yet others are rated at 100% of capacity or a 1C Load at 77F. While these batteries do not have much capacity loss between high discharge, such as a 1C load, and mid discharge at a 0.5C load, there are small differences.

What this means is a 400Ah LFP battery rated at .5C can deliver all 400Ah’s at a 200A continuous load. At 1C it might deliver slightly less. CALB cells for example are rated at 1C / 77F so a 400Ah bank should deliver 400Ah’s with 400A load at 77F.

For off grid fractional “C” use I would not fret over the rated capacity as you will rarely if ever be drawing at anywhere even close to 0.3C (30% of the rated capacity in discharge) let alone a full 1C (100% of the rated capacity in discharge).

You would be wise however to capacity test your bank at somewhere slightly above your “average” DC load for your vessel, perhaps 15-20A for many cruising boats.. This will give you a real usable Ah capacity to work with.

Conversely a 400Ah lead acid battery with a Peukert of 1.27 will only deliver 215Ah’s at a 200A load and just 178 Ah’s into a 1C load. This is a big, big difference and the Peukert is theoretical only because a 400A load on a 400Ah lead acid bank will likely tank to less than 10.5V  in well under 10 minutes…..

07 LiFePO4 On Boats

08 LiFePO4 On Boats

BMS = Battery Management System

EDIT – January 2017 Clean Power Auto, the manufacturer of the BMS used in this article, recently announced a discontinuation of product sales to the DIY market and has closed it doors to DIY’s. Concerns over safety were a noted criteria for this decision. Dimitri has since joined forces with Lithionics where he can confidently sell factory made systems where he knows they will be safely engineered.

PHOTO: For my BMS box I used Velleman G-300 Series Project Box (LINK)

We’ve discussed the importance of not overcharging and over discharging so how do we go about preventing this? The BMS or Battery Management System does this.

The BMS:

The main purpose of a BMS is to simply protect the bank, at the cell level, from over charging or HVE’s (high voltage events) or over discharging  called low voltage events or LVE’s. That’s it, simple really….Think of a BMS as an insurance policy for your expensive cells.

I prefer to think of a BMS as a BIP, or battery insurance policy. It is there solely to ensure your cells can not be over charged or over discharged.

NOTE: There are two terms for these systems that use the acronym BMS.

Battery MANAGEMENT System –  This is what you want. It is automated and protects your bank even when you are being oblivious to what is going on.

Battery MONITORING System – This is a Band-Aid approach of audible alarms and visual gauges that YOU as a human need to monitor in order to protect your bank.

I would strongly urge you away from a human powered Battery MONITORING System. One of the guys in the Li arena for DIY’s who is extremely knowledgeable, one of the most knowledgeable, has even cooked some of his cells using a human powered battery MONITORING system. If this guy can ruin thousands of dollars in cells, and he is an expert in this field, what do you think your odds will be?

There are many facets of a good BMS design and I will highlight some important areas below.


HVC = High Voltage Warning or Cut: This warns of or stops an HVE or High Voltage Event.

HVC is a cutoff threshold for charge sources to prevent overcharging the cells. Depending upon the BMS this can either be done at pack level or cell level. Some do this at a warning level voltage before the shit hits the fan. Generally speaking if this is a warning level event they are often pack level voltages. Remember this is a WARNING LEVEL in either audible/visual alarm or an actual cutoff of charge sources only. A well designed BMS systems for LFP cells will usually cut the charge sources at 14.2V – 14.4V depending upon brand, model etc. Some are even custom programmable.. With an HVC set to warning level the HVC occurs before a main contactor (a big high current relay) for bank protection opens. It is important to properly wire the relays for such items as an alternator as you never want to open the alternator B+/output when it is supplying a load. The proper method for breaking HVC of an alternator is to cut the power to the voltage regulator. HVC should always be monitoring CELL LEVEL VOLTAGE, and not pack level voltage, thus it can break off charging if any cell should drift out of balance and protect it from over charging.


Please bear in mind that a DIY level BMS is not there to manage charging or charge sources by turning them on or off at certain points. All charging sources should be properly programmed so that HVC is not regularly triggered by the charge source. An HVC event should only ever occur if there is a fault or glitch in the charging system. These could include a voltage sensing issue, improper programming, a rare re-boot that clears programs a failed regulator etc. etc.. 99.9999% of the time your well engineered charging system should keep you out of the HVE range so that an HVC does not occur..

The BMS is your insurance against other system or human failures.

LVC = Low Voltage Warning or Cut: This warns or stops an LVE or Low Voltage Event.

The LVC is the opposite of the HVC and again, this should occur before the main bank protection contactor opens. This is a WARNING LEVEL alarm or cut. LVC occurs in the “safe-ish” range not in the “emergency” range. In a well designed system this will break the “loads” bus away from the bank but you will still have the charge bus to use, if appropriately wired.. A separate load and charge bus is just a smart design in a marine system. Both HVC and LVC should occur well before the ejection seat or main bank protection contactor is triggered. LVC, like HVC, should ideally always be monitoring cell level voltage, and not pack level voltage. This way it can break off loads if a cell should drift out of balance and protect it. If charged at safe voltages well out of knee range cell drift is rare but remember the BMS is your insurance.


A good battery management system will allow for audible alarms to sound at the time of HVC or LVC or even slightly before as a warning. If you don’t heed this warning the BMS will automatically protect the bank anyway.


MAIN BANK EMERGENCY DISCONNECT: This is the last ditch, oh $hit / ejector seat protection system to save your bank and WALLET.

In a well designed system this relay/contactor should NEVER even be attainable as HVC and LVC should be triggering/alarming and alerting you to an issue well before you break the main contactor. In many BMS systems HVC and LVC automatically reset when voltage rebounds or climbs, automatic is fine in LVC / HVC warning level ranges.. Some folks choose not to wire in HVC or LVC relays and I find this a tad risky and perhaps a tad penny wise, pound foolish. The main emergency contactor should be a MANUAL reset, not automatic system. It is there as an emergency back up insurance policy to HVC and LVC for your expensive bank. Once the main contactor is tripped you will need to MANUALLY re-boot the system… Think of this as your EPIRB. You never want to use it but it is there just in case. Ejector seat voltages are almost always based on cell level voltage signals and often run a range from 3.6VPC to 3.8VPC..


Let me first say;


Automated cell balancing is where the most debate around BMS systems stems from. Many BMS systems have the ability to balance the cells using diversion or small resistors to divert or waste current from the cells actively hitting high voltage levels to those cells that are not yet at high voltage levels or to just waste it as heat to slow charging to the high cells.. Sounds good right?

Picture four 5 gallon buckets filling with water for a moment. When one becomes full, ahead of the others, a small spigot dumps some of that excess “overflow” into the less full buckets but if the fill rate is fast enough it can’t handle all the excess “flow”.. This is sort of how “shunt balancing” or a diversion of current balancing system works. All good in theory, and even in practice when done right. The problem is that it’s often not executed well.

In reality diversion or shunt balancing needs to be done, and executed, with a good sound design and done, in my humble opinion, at LOW CURRENTS. BMS systems such as the Clean Power Auto House Power BMS do not allow shunt balancing until well after the HVC has cut away charging.

“But, but cell balancing won’t work automatically then?”


EDIT 2014: Dimitri has made some recent changes to the House Power BMS..

#1 The HPBMS now does HVC at 14.4V not 14.2V like earlier models. He can make a custom one at 14.2V if you want. I personally prefer a 14.2V HVC because this means that shunt balancing never happens automatically on my system. This is my own personal preference based on my own systems design…

#2 HVC now has some hysteresis programmed into it. Older models did not have any hysteresis.

#3 The 3.6V HPBMS cell boards, IIRC, begin shunting at 3.55V. The 3.8V cell boards begin shunting at 3.65V. I use 3.6V boards because I prefer to do any balancing myself and I have a 14.2V cell board. I originally had 3.8V cell boards but dropped back to 3.6V.. If you have the new 14.4V HPBMS board then shunting will occur automatically before HVC. If you don’t want that to happen you can bump to the 3.8V cell board. This moves ejector seat level to 3.8VPC and automated cell balancing will be above the HVC too.


HVC is pack level voltage (this is a warning level on the HPBMS)

LVC is pack level voltage (this is a warning level on the HPBMS)

Shunt balancing cell level voltage

Pack relay/main contactor/ejector seat is cell level (this is emergency level on HPBMS)

If you stay out of the HPBMS’s voltage ranges you will have a nice insurance policy…

HVC in the House Power BMS (HPBMS) was 14.2V and is now 14.4V. Cell balancing begins at about 3.55VPC, with the 3.6V cell boards.. In order for my own system to do shunt balancing I must disable HVC and then push the cells into the shunting voltage range manually. Again, this is my preference, for my own system. 700+ cycles at my design voltages has shown no balancing necessary, ON MY SYSTEM…..

If you MUST shunt balance:

It is my belief that a bench-top power supply with independent voltage and current control should be used to supply the LEAST amount of current that it takes to do the job of balancing whether you use the shunting of the BMS system or wiring the cells in parallel and doing it manually.. If you have a current source that can limit current similar to a bench top power supply then that will work too. The point being these resistors can only “shunt” so much. Bottom line… Keep your charge voltages below where pack balancing occurs and you will do just fine. My cells are now beyond 700 cycles most cycles to 80% DOD. They have only been balanced once, 700+ cycles ago… Proper initial balancing and safe charging voltages can result in your cells not drifting thus no need for automated cell balancing every cycle. Again, this is with safe & sane design voltages.

There are some other BMS systems that also take a similar approach to the HPBMS and break away the bank with HVC well before any shunt balancing occurs thus requiring manual & attended shunt balancing..

I will say this again:


I am a believer that shunt balancing should be a “monitored” event just like “equalizing” lead acid batteries. Again, my personal preference. The need for balancing LFP cells, in a well designed system, is proving to be far less often than equalizing lead acid batteries. Heck a majority of lead acid batteries are dead by 150 cycles on boats, and could only dream of 700 cycles to 80% DOD in the real world….. (wink)

If you do ever need to balance the cells I MUCH prefer doing this on the bench with a power supply. If this bank is any indication the need for this would be about once ever 5-6 years for a coastal cruiser and about every two years + for a full time cruiser. A hassle? Not at all….

This LFP bank is now beyond 700 cycles (took a very long time and lots of work to do that) and has exhibited virtually no capacity loss, no notable changes in internal resistance and no cell drift. In the first 50 cycles I actually saw a minor bump in capacity! Why? Likely due to the fact that I have chosen safe & sane charging levels and performed a good initial top balance. The cells really seem to like it…. This is the way “I” do it, you can always choose how you want to do it…

In a well designed fractional “C” system where safe & sane charging voltages are used the need for cell balancing will be extremely rare and you should rarely have a need to push the cells to cell balancing levels. This bank has now undergone 12 full 100% discharge capacity tests. The cells have only been top balanced once and are still in balance to 0.008V, during a .25C load or 100A on a 400Ah bank. I credit sufficient but not extreme charging voltages for this and purposely keeping this bank out of the knee ranges on charge and discharge cycles.

I am also a believer that high charging voltages, above 14.2V, per 12V nominal pack, simply result in more of a need for “balancing”. Pushing the charge voltages too high results in more need for balancing and it becomes a vicious cycle. A real catch 22. Interestingly this actually serves to create a need for a “balancing” BMS systems.. Go figure… These banks will deliver all the capacity in them when charged to just 13.8V – 14.0V. Why go any higher….??

Funny Story:

I was conversing with a gentlemen that emailed me who is an avid DIY EV enthusiast. He, like me, read and read for years before jumping in. Once he got his cells he began experimenting in his home shop to confirm, and put what he was reading, into practice. Long and short is he chose not to have balancing BMS and relied solely on one bottom balance when his cells were new. Yes he did have LVC and HVC protection in place just not a BMS that balances. He also chose safe & sane charging voltages.

At his first EV car show all he heard all day was how he was going to ruin his cells;

“You have to have a BMS that balances!”

“You must have cell balancing!”

“No cell balancing, are you insane?”

“Hey guys here’s a fire waiting to happen!”

One particularly obnoxious electrical engineer type berated him for nearly 40 minutes while himself admitting he was on his second set of prismatic cells in a few years.

“Wait until you get a few hundred miles on those and they are junk.”

To which the guy responded;

“A few hundred? These puppies only have 33 THOUSAND MILES and are just getting broken in.”

The electrical engineer walked off with his tail between his legs….. A lot of LFP is simple common sense…


Laying Out The BMS

For the BMS on this bank I chose to use the simple and cost effective House Power BMS. EDIT: Clean Power Auto Has Closed It’s Doors to DIY’s.

There are fancier and significantly more expensive BMS products out there but very few are geared towards off-grid or house bank use and usually offer more features and complexity than may be useful on a boat. Elithion and many other companies make BMS systems and there are enough to make your head spin. Genasun also used to make a beautiful BMS, they sold to DIY’s, but like Clean Power Auto they have since pulled out of the DIY market.

Are we starting to see a trend?

Clean Power Auto is no longer selling to DIY’s. A number of prismatic LFP cell makers have pulled out of the US market and those remaining can take months to get. Compass Marine will no longer consult long distance on LFP design because owners refused to follow design criteria. Genasun stopped Selling to DIY’s and then dropped marine LPF altogether. If you’ve done your research I suspect you know why this trend exists. I call it the lead acid mentality..

If you are still looking to build your own a good company, as an alternative, would be; Orion BMS

The HPBMS is the small PCB in the box. I cut shaped & routed a piece of high density phenolic to fit inside the Velleman box so the layout of the BMS could be done outside the box then simply dropped into it.

Additional items shown:

  1. Two 70A Cole Hersee RC-700112-DN SPDT 70A Relays. One for Alternator HVC and one for Solar HVC. (Ignore the relays in this image they were for layout and mock up only and are SPST not SPDT
  2. Blue Sea Terminal Strip
  3. Piezo alarm buzzer 12V.
  4. Momentary re-set switch.

09 LiFePO4 On Boats

10 LiFePO4 On Boats

Cell Matching

Any discussion about balancing the cells would be incomplete without discussing matching the cells for internal resistance and Ah capacity. When I got these cells there was less than a 0.32 Ah difference between the worst cell and the best cell. These cells were made in 2009 and were matched at the factory before being shipped to Balqon. Finding well matched cells today can be rather difficult. Over the last 8 years things seem to have gotten worse with LFP matching not better. I really don’t know why but it is what I am seeing.

If you don’t build your pack from well matched cells there’s not a lot that can be done to keep them in balance when working into the knees, so do yourself a favor and stay out of the knees. If you don’t have a way to confirm the Ah capacity of each cell, you’re essentially shooting darts blind. Any LFP pack build should start with well matched cells & please don’t be afraid to send them back.

Initial Cell Balancing

Cell balancing is an extremely important aspect of LFP banks. When you have lead acid batteries in series they can be purposely over charged/equalized to a 15.5V pack voltage and they will, in a sense, self balance. With LFP banks this will not happen due to the knee ranges. As a cell becomes full the voltage all of a sudden skyrockets and the cells need to be in balance in order to charge and discharge at matched voltages.


There is much controversy over top vs. bottom balance mostly due to confusion over differing uses.


A bottom balance simply means the cells are balanced at the lowest “safe” voltage and all cells will converge and match exactly at say 2.75 VPC. In the EV world bottom balancing is almost always the preferred method, and makes the most sense, with high loads and frequent opportunities to completely drain the bank. In EV the car is then brought back to the garage and charged with ONE charge source.

1-Discharge cell using a 20-30A load to 2.50V

2-Let the cell rest at room temp for 24 hours and allow voltage to rebound

3-The cell will now be resting somewhere between 2.75V and 2.85V

4-Apply the load and stop discharging at exactly 2.65V

5-Allow voltage to recover for about 6 hours

6-Repeat load discharge to 2.65V until the resting stable voltage of each and every cell is 2.75V

7-As you get closer and closer to resting voltage of 2.750V a small resistor can be used as opposed to the large load.

Once all cells rest at 2.750V and stay there the cells are bottom balanced.

NOTE: A guy recently dropped off 4 cells he was having trouble “balancing”. He was attempting a bottom balance and intending on using these for fractional “C” use stopping at 70% DOD.. He had spent countless hours trying to bottom balance these cells, and he did.

So what’s the problem? The problem is that at a 14.0V pack voltage he had one cell at 3.65V and one cell still at 3.380V!!!! His cells tested at varying capacities and thus the cell with the lowest capacity was firing into the upper knee sooner than the rest, even at a pack charge voltage of 14.0V. These were cells with an absolute MAX cell voltage of 3.600V. With a bottom balance and used cells (I don’t suggest buying used cells) he was sending one cell into the dangerous upper knee even at just a 14.0V charge rate. I conducted a top balance for him and the cells now all remain well balanced at the upper charging voltage range. On the low end one cell will still fall off the cliff early, but at 70% DOD that does not happen.


On boats we have multiple charge sources, shore charger, alternator, solar, wind, hydro or even hydrogen fuel cells. Our risk of cell imbalance is more pronounced at the top end rather than the bottom end. We run a much higher risk of over charging imbalanced cells than we do by over discharging, like the electric vehicle (EV) guys do, but it can still be a risk.. For off-grid / marine use top balancing is quite often the preferred method so the cells converge or are in excellent balance at the top, when fully charged, rather than when dead or fully discharged…

In theory the BMS would always protect the cells at either the bottom or the top end but keeping the cells well balanced ensures an extra level of protection, just as keeping charging voltages out of the upper knee range does. Don’t discharge below 80% DOD and have a max charge voltage of 3.5VPC / 14.0V for a 12V bank, and your cells will be very happy.

Whether you choose to top or bottom balance is a personal choice. I chose a top balance for this bank and even after 700 cycles the cells have tended to converge in cell voltage rather than diverge.

PHOTO: In the photo the four Winston cells have been individually & very carefully charged to 3.75VPC with the bench top power supply shown. The cells were then wired in parallel and allowed to sit for multiple days but weeks or months is even better, if you have the time…
TIP: When ordering cells ALWAYS order extra cell jumpers so that you can wire the cells in parallel and top balance if you choose to do so.

Cell Matching

Any discussion about balancing the cells would be incomplete without discussing matching the cells for internal resistance and Ah capacity. When I got these cells there was less than a 0.16 Ah difference between the worst cell and the best cell. These cells were made in 2009 and were factory matched.

Wire The Cells In Parallel

As I mentioned I first charged these cells, INDIVIDUALLY, to 3.75VPC and X current taper. The bench-top power supply allows you to set the voltage to 3.75 and let the cell become “full” at 3.75VPC. I held the voltage at 3.75V and allowed the current to tail off to 20A then stopped charging and moved onto the next cell.


It is fairly important that you top each cell up prior to wiring them in parallel. They ideally need to be very close in SOC/balance before placing the parallel jumpers in place. This helps to get most of the balancing done.

Within seconds of wiring these in parallel only 0.59A was moving between cells which means my balance to 3.75VPC was pretty close.. Leaving them in parallel will get them in closer balance BUT THIS CAN TAKE LOTS OF TIME..

As the voltages converge, after being wired in parallel, the movement of current between cells slows to a crawl, Ohms law, we are talking 0.001A level movements. This I why I simply let them sit for a week or more if possible or top balance them until my bench top power supply is flickering between 0.00A and 0.1A of current..

In theory cells way out of balance could take six months to a year to equal out when wired in parallel if not being “charged”… As resting voltage between cells become equalized the voltage differentials will make any current movement slow to a crawling pace.


Never trust the volt meter on the bench top power supply as there may be voltage drop or inaccuracies between the supply & actual battery terminals. ALWAYS measure the actual battery terminal voltage using a good quality DVM, when top balancing.

Winston cells should not be top balanced or have the voltage pushed beyond 3.800VPC, this despite what the crappy Chinese manual tells you. In reality, with four years of in-depth experimentation and many hundreds of hours of behavior observation, I personally would not advise pushing them to any more than 3.65V for a top balance as there is no need to do this for a fractional C system.
CALB cells also should not be pushed beyond 3.650VPC, when top balancing. 3.800V is the absolute max voltage you want to push any of these cells prismatic to in a closely monitored & attended top balance. During carefully monitored and attended top balancing is the ONLY time your LFP cells should ever be charged above 14.2V / 3.55VPC.

Remember, when you wire these cells in parallel you now have a 1600Ah 3.2V bank. Even the last 0.05V, from 3.75VPC to 3.80VPC requires many, many hours to get there.

Please note that these are Winston cells and Winston recommended the 3.8V top balance voltage. With CALB or other cell brands 3.6V- 3.65V is often the max you would want to push to. Please consult with the manufacturer of your cells for max top-balance voltages. Even with Winston a 3.65V max should work fine, but Winston insisted on 3.8V. I don’t actually agree with it, for fractional “C” use and lower charge voltages, it has however worked very well on this particular bank.

11 LiFePO4 On Boats

12 LiFePO4 On Boats

Current Moving Very Slow

The current moving between cells quickly slowed and kept slowing. I let the batteries sit wired in parallel for about 3 weeks with no charging, mostly because I got side tracked.

Balancing Parallel Cells To 3.800 VPC

This is why top balancing needs to be closely monitored. Like equalizing flooded batteries you simply DO NOT leave them unattended. Once the cells hit 3.800 VPC you will need to adjust the power supply very carefully so it does not overshoot 3.800VPC.

Watch the DVM NOT the power supply display. Allow the voltage to rise to 3.8000V (for Winston cells only!). Remember trust the DVM NOT the power supply. Other manufacturers, of prismatic cells, have considerably lower maximum balancing voltages such as 3.65VPC.

If you wire in parallel, hold voltage steady then allow the current to taper until flickering between 0.0A and 0.1A. Now the parallel pack is balanced and can be disconnected. Despite doing this initially with my own bank, I would not advise top balancing at 3.800V but rather somewhere closer to 3.65V for a fractional C bank charged at 14.0V or less..

After you do this disconnect the charge source and you could continue to let them sit, in parallel, for as long as you want, but charging to 0.0A – 0.1A is much faster…..

These days I top balance at 3.65V on Winston cells & 3.55V on CALB and I then allow the power supply to take the batteries to 100% full with the power supply bouncing between 0.0A and 0.1A. This actually puts the cells in balance at that voltage. At this point you can un-wire the cells and they are balanced.


I recently conducted an experiment on my test cells that pitted a “balancing BMS” against a 3.65V parallel top balance to 0.00A – 0.1A on the power supply. Holding the cells at 3.65V (CALB cells) the top balance in parallel actually balanced the cells in just under 3 hours. Using the “balancing BMS” after more than 7 hours at shunting / balancing voltages the cells were still not “balanced”….. Food for thought!

Please keep an infrared thermometer on hand and closely monitor cell temps. Hit all the cells at the same spot, when checking on the cells, to track any changes or anomaly’s. They should ideally remain below 80-85F, these did.

Further Thoughts:

After playing with multiple brands of prismatic cells for almost 4 years now it has become apparent that there is little to no difference between these cells in terms of upper knee voltages. With that in mind I would strongly suggest there is NO NEED to top balance Winston cells, for fractional “C” use, beyond 3.65V per cell.

13 LiFePO4 On Boats

14 LiFePO4 On Boats

Bench Top Power Supply

As I mentioned earlier I am a believer that if venturing into DIY LiFePO4 it should be done as a SYSTEM. Part of that system should include funds for a bench top power supply. In my opinion this tool should be a pre-requisite for DIY LFP.. Can you make do without? Sure, and I am certain Bode Miller could ski with only one leg, but why..? In the whole scheme of things they are inexpensive and they have multiple uses not just for charging or top balancing LFP.

The bench top power supplies I often use are made by Mastech, specifically the Mastech EX series. I own a 3030EX and a 3050EX. These are not the fanciest or the most expensive power supplies but they work and they work very well especially for the price. Years ago these devices would have run four figures each but today they are very reasonably priced.. A Mastech 3020EX (30V X 20A) will run you just $219.95. It will save you $400.00 in your time fiddling with top balancing alone. You will be looking for a 0-30V and 0-10A plus model. This is my 3050EX. The EX in the Mastech line signifies these units are specifically designed for charging batteries, usually Li batteries.. The dial second from the left is EX knob or the over voltage protection dial. Set this dial and the power supply will protect itself.

While the Mastech line represents a great value I tend to prefer my BK Precision Model 1900. The BK Precision 1900 is a 1-16V, 60A variable power supply with dedicated voltage sensing leads. The voltage sense leads to me are really the driving factor as you get far more accurate voltage at the terminals without worrying about voltage drop through the cables & terminals. It is a very nice piece of gear but runs close to $600.00. I bought mine from Test Equipment Depot.

Knobs and Displays:

Left Digital Display = Current Output

Right Digital Display = Voltage

Red Light = Constant Voltage Mode (power supply is limiting voltage to 13.8V)

Left Knob = Current Control Dial

Second From left Knob = Over Voltage Limit

Third From Left Knob = Constant Voltage Fine Tune Adjustment

Right Knob = Constant Voltage Coarse Tune Adjustment

As you can see in this picture with 15A of current flowing the Mastech and the Fluke are in close agreement but I still trust my Fluke a lot more than the voltage display on the power supply.
TIP: When charging LFP cells or banks with a bench top power supply PLEASE dial the current back by about 20%. This will allow the power supply to run almost indefinitely and not cause undue wear and tear on the unit. I run my 30A model at 24A and my 50A model at 40A… I often parallel them and charge at 64A when doing cycle testing….

Nothing makes top balancing easier than a bench top power supply:

#1 Charge individual cells to .05V below max top balance voltage and allow current to taper

#2 Wire cells in parallel and let sit for a couple of weeks (the longer the better)

#3 Charge cells to max top balance voltage Winston = 3.8V (now I’d do 3.65V) / CALB = 3.55V etc.

#4 Allow current to go to 0.0A to 0.1A

#5 Done

Human Error Over Charge!

This cell and three others were over charged by one of the brightest guys in DIY LFP banks. He is also an EE. $hit happens and I use the $ for an S for a reason.

Charging 4 x 90Ah cells in parallel with a 40 amp 12v charger, thought I’d turned the charger off, didn’t discover it till a few hrs later. The cell was at 4.55v from memory and so hot the terminal bolts burn into the finger tips. The strange smell of the electrolyte vapor, but no sign of any white cloud. The heat was similar to standing in front of an oil heater on full and was still quite noticeable the following morning. Only the 2 cells in the center bulged and they are the only two that failed. The cells at either end had better cooling, they bulged a bit, but they are still part of my battery bank 12 moths later.

Let’s break this down to see how easy these mistakes can happen..

♦These cells were in parallel which means a 3.2V nominal pack

♦A 12V charger was used instead of a power supply or charger capable of LIMITING the voltage to 3.XX

♦He thought he turned the charger off. This is a prime example of HEF (human error factor). No matter how smart we are, we are still capable of making human errors or being forgetful. This is just normal human nature.

♦Cells hit 4.55VPC!

♦Cells DID NOT EXPLODE, Catch fire or do anything other than get very hot!

♦The cells did not even smoke!

♦Two of the four cells actually survived this abuse!

♦Simply AMAZING!!!

Imagine how long your cells will live if you don’t allow HEF into the equation and you charge them PROPERLY for a fractional “C” system?

If an EE, and guy who knows more than just about anyone I know on LFP, & the subject of fractional “C” use can do this, you could too. I will mention it again, use a BMS on your bank for HVC / LVC and a bench top power supply for top balancing…

What actually happens if I do over charge?

Over charging forces lithium oxide to form on the cathode. The LFP cathode is, well, lithium iron phosphate. By causing an over charge you have now converted some of the lithium iron phosphate to Lithium oxide, not good! There is no reconversion or fixing this situation and the cell is now irreversibly damaged. Even slight over charging episodes can cause increases in internal resistance and cause a loss of capacity.

With the constant current>constant voltage (CC>CV) charging we use in the marine market, and multiple sources of it, your best and safest bet is to limit the constant voltage stage of charging to less than 14.2V or 3.55VPC (14.0V or 3.5VPC is even safer). Remember these cells are technically “full” at 3.4 VPC or 13.6V…

If your cells rest at room temperature for 3+ hours with no load and wind up at 3.4VPC they are full. Getting them full does require a voltage higher than 3.4VPC in order to get them to 3.4VPC rested, but not much higher. Charge to 13.8V or 14.0V and allow the current to tail off to about .025C (2.5A on a 100Ah cell) and your bank is now about as full as you’ll need it to be.

I charge this 400Ah Winston pack to 13.8V and 0.025C current and it still delivers 425Ah’s at a .25C load! This has now been tested and repeated twelve times in complete 100% discharge capacity test to confirm the Ah capacity…

Image Courtesy: Terry©

15 LiFePO4 On Boats

No Absorption Time at 3.65VPC / 14.6V / 29.2V

No matter how much I try to explain the confusion between the what the Chinese mean in their manuals, and the reality, folks still don’t seem to believe or trust what I have to say about safely charging LiFePO4, for optimal cycle life, with typical lead acid charging equipment.

Question to Winston Battery:

Q: If a 12V nominal 4S battery is charged to 3.65V per cell or 14.6V at a .3C charge rate how long can the constant voltage stage be held at 14.6V before cell damage begins to occur? 

Answer from Winston Battery:

A: To prevent the battery from over charge damage, stop charge after the standard 12V battery is up to 14.6V.

Despite what far too many LFP buyers want to believe about using lead acid chargers with LFP batteries, and charging technology that features a constant voltage absorption stage, there should be no absorption duration time if you charge to 14.6V or 3.65VPC.  In other words if you want to drop LFP cells into a lead acid charging environment you do not want absorb the battery, and especially not at 3.65VPC / 14.6V.

The graph below is from a University white paper where they did some rather abusive endurance testing. They charged Winston Prismatic cells to 3.8VPC – 4.0VPC and then immediately discharged them to 2.9VPC or 0% SOC. The cells survived for 950 cycles before reaching end of life as defined in the study.

“Wow RC it seems 4.0V or 16V for a 12V nominal pack is safe?”

Yes, for some cells it is, if done safely and correctly. This type of outcome can absolutely happen in a lab but it can’t be well translated to a boat using multiple charge sources, all at varying charge rates many of them using “absorption timers”. As I have mentioned before the peak safe voltages are just that “peaks”. As can be seen below the cells charged to just a hair over 3.8V then were immediately discharged.  A “peak” very short term duration at this voltage before a turnaround right into a high rate discharge is how this lab test was done.

What can be seen in this graph, quite clearly, is voltage knee, above the blue line or about 3.45VPC, becomes vertical and the discharge down to about 3.27VPC is also vertical. The green line is highlighting the area between 3.8VPC and about 3.27VPC where there is virtually no stored energy. The red circle is exactly what it is. Why would you push your cells into this area when there is no benefit, no quantifiable capacity to go after, and only the potential for harm if you are not operating in a laboratory?

Please Don’t Gloss Over the Data, If You Can Find it!

This image includes some very critical and important bullet points copied word for word from a “Drop-In Battery” charge guidance document. A document the reader who sent it to me only got AFTER purchasing the batteries.

Upon perusing their glossy web site, which, marketing wise, suggests these batteries can be dropped into any situation or application, I could not find the charge guidance anywhere. I have removed the brand from the wording in the image as my point here is not to attack the brand but to point out that you may NOT be getting all the facts on a manufacturers web site and to please DO YOUR HOMEWORK.

Here are the important bullet points from their charge guidance:

“Because of the different parameters required for charging lithium ion batteries as opposed to lead acid batteries, we do not recommend using a standard lead acid charger for your XXXXXX Battery. Using a lead acid battery charger with lithium ion technology presents risks of damage, decreased lifespan, and overall suboptimal performance.

• If your charger can be programmed to deliver constant current and charge up to 14.6V, there is no need for an absorb phase.

• If a float charge setting is necessary to program your charger, it should be set to 13.8V.

• In some cases, either due to customer request or expedited shipment, batteries are shipped at 40% charge. This is the best voltage for storage of lithium ion batteries. The 40% state of charge allows the batteries to be stored with minimal aging and self discharge.”

Wow, pretty interesting to say the least. In regards to floating, let’s examine the above statements.

#1 The manufacturer wants to see LFP batteries stored at a resting voltage that represents 40% SOC and they consider this the “best voltage for storage” for “minimal aging“. I agree 100% with storage between 40% SOC and about 60% SOC, in a cool environment (they fail to mention that), and so do the cell manufacturers.

#2 They then go on to suggest setting a float voltage at 13.8V…… Ouch!!! This is utterly contradictory to everything they just said above it.

#3 After saying that lead acid charger technology “presents risk of damage“,  “decreased lifespan” and “overall suboptimal performance” they go on to tell you how to program float, a LEAD ACID CHARGER FEATURE. Once more, I will repeat this, YOU DO NOT FLOAT LFP…..

EDIT: The manufacturer of this document has since added charge guidance to their web site. When the reader who sent this to me got his batteries this info did not exist on the web site. Neither he nor I could find it anywhere. What he thought was going to be an inexpensive LFP investment turned into multiple thousands in additional cost to “properly” charge his batteries to the manufacturers suggested guidance. I won’t even go into the other issues he’s had with his “drop-in batteries“.

More importantly, this manufacturer is now recommending a max charge voltage, for a 12V bank, of 14.0V for batteries charged in series or parallel unless each battery has it’s own charge source.Hmm… seems some LFP makers are indeed starting to get it….

16 LiFePO4 On Boats

Choose A Location For The New Bank

The nice thing about LFP banks is their weight and size are both smaller and lighter than a comparable lead acid bank. Due to these differences in weight and size I was able to relocate the entire bank to a nice dry and higher area of the vessel. I reconfigured a storage area to take the battery bank and it fit like a glove.

Take the time to reconsider where your bank will be and don’t just place it where the lead acid batteries were because there may be better alternatives.

Consider the following when choosing a location for a new LFP bank:

♦Weight Distribution

♦Moisture & Humidity

♦Corrosion Potential

♦Not In An Engine Space

♦Heat and Cold Potential


The Banks New Home

As can be seen the area chosen is high, dry and has good protection for the battery. Don’t be afraid to get creative in where you install the bank, but do be safe..

17 LiFePO4 On Boats

18 LiFePO4 On Boats

The Battery Compartment

Here’s a shot of the empty battery compartment, simple, clean and with hold down clamps that do not allow for any movement of the installed battery. These banks have the ability to throw massive amounts of current into a dead short so where and how the battery is mounted is an important aspect of the installation.

Make Sure The Battery Is Well Mounted

This clamp mechanism uses 3 X 3/8″ bolts and two pieces of 3/4″ thick HDPE board to clamp around the battery cases bottom draw bar. A slot was routed into each clamp at the perfect height to fit the battery case.

The battery case draw bars fit into the notches in the battery cells and compress the cells with 5/16″ SS threaded rod. The square aluminum stock is 1/2″ aluminum square tube which locks the cells in place and fits this hold down clamping mechanism. The battery cannot move at all when installed.

19 LiFePO4 On Boats

20 LiFePO4 On Boats

Over Current Protection

In this photo we can see the battery bank and the red 2/0 wire feeding the Class-T fuse holder. At a bare minimum you want to be using Class-T fuses as your main bank protection for an LFP bank.

This bank can easily throw 20,000A or more of current into a dead short and can damage and literally blow windows out of ANL fuses. I had this happen during the testing of some ANL fuses sent to me by a DIY LFP guy from Cruisers Forum, Thanks Bob E.. Class-T fuses are fully metal encased and are a very safe fuse.

All fuses have what is called an AIC rating or amperage interrupt capacity rating. This is the rating at which the fuse will fail safely. Class-T Fuses have the highest AIC rating of any fuse we use in the marine environment. There are fuses out there with higher AIC ratings but none of them have fuse holders avaible that are suitable for a marine application.

Ideally the main bank over current protection needs to be within 7″ of the battery bank but as we can see here that is often impossible. D’oh!!! There is technically more than 7″ of wire to get from the + battery post to the Class-T fuse here. Is this an unsafe installation, hell no, but sometimes the standards are a little to broad brushed to apply realistically to the real world so we do the best we can.

Unsafe ANL Fuse Failure

During the course of my testing & experimentation with LFP battery banks I blew approximately $400.00 worth of MRBF, ANL and Class-T fuses.

The only unsafe failures I had were off-brand elcheapo car stereo type ANL fuses. As can be seen here the windows literally exploded out of the fuse when it tripped. I did not have a single unsafe failure of a Cooper Bussmann/Blue Sea Ignition Protected ANL fuse but I only blew about 10 of them. I suppose if you blew 100 you may have an unsafe failure on an LFP bank..

Still, I would strongly urge Class-T as the bare minimum for LFP bank main overcurrent protection.

NOTE: Class T fuses do not have an ignition protection rating. As near as I can tell, from speaking with Blue Sea Systems, as well as Cooper Bussmann, they have not been specifically tested for this. This only means that they’ve not been tested, not that they would necessarily be unsafe.

Remember an ANL IP rated fuse has an AIC of 6000A and a Class-T non IP fuse has a 20,000A interrupt rating.. If you have a gasoline powered vessel, which requires ignition protected devices, consider this when engineering the over current protection for your LFP system..

21 LiFePO4 On Boats

22 LiFePO4 On Boats

Alternator Considerations For LiFePO4

If you decide on LFP then as I mentioned it is best designed as a system and a good alternator design and installation should always be part of that system.

Due to the extremely low internal resistance of these batteries, and the extremely flat voltage curve, LFP banks will tax an alternator to death if not properly installed. Because of the very low resistance the alternator will be in BULK mode for the vast majority of the charge cycle, (depends upon size) before even attaining absorption voltage.

With a high current alternator on lead acid, you can hit limiting/absorption voltage as low as 50% SOC where the alternator begins to catch a break…. You will not do this with an LFP, and your alternator will NOT get a break.

If you cycle the LFP bank to 80% DOD this means you are in BULK mode for approx 75% or more of the capacity of the entire bank bank before any sort of voltage limiting even begins..


This means the alternator has not brought the terminal voltage of the battery bank up to ABSORPTION or the limiting voltage. In BULK the alternator is working FLAT OUT in what is referred to as CC or constant current mode… Once the bank comes up to ABSORPTION VOLTAGE we switch to CV or constant voltage mode where voltage is held steady and current begins to taper off based on what the battery can accept at that SOC and voltage.

In BULK / CC the alternators capacity/ability is your limit.

In ABSORPTION / CV the battery determines how much current can flow at a specific SOC and terminal voltage.

Take a 400Ah LFP bank at 80% DOD and that is 320Ah’s that need to go back in. With a 130A rated alternator running hot, at about 100A, this means BULK charging will be about three hours long. There is no small-case alternator on the planet I know of or have tested that can run at full bore for three straight hours, into an LFP bank, inside the typical engine room on a boat, unless perhaps the diode rack has been mounted externally with its own cooling fan.

Let’s say you’re a marathon runner, and you can do the 26 miles at a pretty good jog. This is similar to a high capacity alternator feeding a large lead acid bank. You start out strong (BULK/CC) but as the race goes on you plateau & settle in at a sustainable pace (ABSORPTION/CV).

An alternator feeding an LFP bank is like trying to SPRINT the entire 26 mile marathon. Not going to happen….


Some factory alternators have a built in temp compensator and it resides in the voltage regulator circuitry to reduce current / voltage as the alternator heats up. This really defeats the purpose of “charging fast” or even having an LFP bank if you want to capitalize on the fast & efficient charging LFP batteries can offer..

While this alternator temp compensation feature is self protective of the alternator, in theory at least, it is really a very poor regulation choice for an LFP bank. I have seen Hitachi alternators so hot they have reduced the voltage output to 13.2V. Considering the resting voltage of an LFP bank is higher that, well….. Do it right and build this as a “system“..


Here we are looking at a Yanmar four cylinder engine with a Balmar / Alt Mount serpentine conversion and a Balmar AT series 165A alternator. The Balmar AT series is a hairpin wound small case alternator. This stator / rotor design is relatively new technology for the marine market and allows extremely high performance out of a small case alternator, but remember this is still a small case alternator.

Currently the Balmar AT series are one of the best small case alternators you can buy but they are pretty expensive. At the moment, in my opinion, they are the best suited small case alternator avaible for an LFP bank.  Unfortunately many sailboats don’t have the room for a massive large frame alternator with external custom brackets etc. If you do, that is great, and I would steer you in that direction.

Does this mean other small case high performance alternators can’t be used on LFP? Absolutely not, it just means the percentage of “rated output” you get out of the alt will be less than it is on an AT series and it will need to be dialed back in the regulator settings, for self protection, further than a Balmar AT series will.

Large Frame J180 Mount Alternator

When space allows a large frame extreme duty alternator is always better suited to charging LFP banks. These alternators are purposely designed from the ground up for driving large loads ofr long periods of time. Small case alternators require more current limiting or external rectification, to survive LFP banks than do large frame alternators.

To offer the LFP enthusiast an affordable large frame option, the CMI-200-J180 was recently developed. The CMI-200-J180 is the least expensive high performance externally regulated large frame alternator I know of. Fitting & mounting would need to be custom, on most marine engines, but for under $1000.00, including a  Balmar MC-614 External Regulator  there is no better deal out there.

PHOTO: In this photo we have a large case Balmar J180 mount alternator on a Balmar/AltMount custom bracket driven by a serpentine/rib belt. If you have the room, a large frame alternator is the best option to go with for charging LFP banks..
Image Courtesy: C. Kelley©

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24 LiFePO4 On Boats

Alternator Drive Belts

In order to get lots of current out of an alternator it requires a considerable amount of work on the part of an alternator belt. If designing the system for LFP there is no better option I know of than a multi-rib or serpentine belt kit for your alternator. I suppose you could design a geared PTO system too but $$$$$

A single serpentine belt is capable of driving 190A – 200A plus of current with less heat, less belt tension and less strain on water pump or alternator bearings.

NOTE: Universal & Westerbeke suggest the largest alternator they want to see on their engines is 190A. Yanmar has no such advisory that I have been able to find. With proper regulation you can run a 225A + alternator current limited to 190A, and do this all day long.

#1 Choice = Serpentine / Multi-Rib – Balmar/AltMount or Mark Grasser DC Solutions

#2 Choice = Dual belt configuration. This is a very distant second choice. Dual belt kits rarely if ever work as intended or share the load equally among belts. On an LFP bank they can become a belt dust nightmare. Matched pairs of belts are also getting extremely tough to find because even industry has moved away from v-belts. You will have belt dust issues with a dual-pulley/belt configuration driving LFP. Unless you already have dual belts, spend your money on a serpentine kit.

#3 Choice = 1/2″ Single V-Belt – A single 1/2″ v-belt driving LFP should be limited to approx 80A of current. I do not advise charging LFP with a single v-belt.

#4 Choice = 3/8″ Single V-Belt – A single 3/8″ v-belt driving LFP should be limited to approx 60A of current. I do not advise charging LFP with a single v-belt.

NOTE: I am ignoring common wisdom that a single 1/2″ belt can drive 100A and a 3/8″ belt can drive 80A. This is all well and good with lead acid but not for 3-4 hours plus at full bore with LFP. The same goes for large banks of AGM or GEL with a small alternator…

PHOTO: An Balmar serpentine pulley kit and a Mark Grasser DC Solutions Premier Series 140A alternator.

Alternator Voltage Regulation For LiFePO4

A good voltage regulator is critical for an LFP bank. It is my belief that no better regulator currently exists for LFP than the Balmar MC-614. This regulator allows every conceivable parameter to be adjusted from; voltage, to alt temp compensation, bulk, absorption and float duration, thresholds for transitions from bulk to absorption or absorption to float, the ability limit the field current and do “current limiting” of your alternator etc. etc. on and on. Of critical importance in LFP it also uses a dedicated voltage sense circuit (if properly wired and the Balmar manual is WRONG on this) that does nothing but sense voltage. This v-sense circuit carries virtually no current on it, other than a mA on the negative side. This allows the regulator to see an accurate reflection of the battery terminal voltage, provided you wire it correctly.


#1 User Defined Charging Parameters: Adjustable bulk, absorption and float voltages allow you to tailor the regulator to suit LFP banks. Float can be set low enough so that it essentially turns the regulator off when the bank is “full”.

TIP: In order to get the voltage settings low enough for LFP banks with a Balmar regulator one must work backwards in the custom programming menu by starting with float first, then absorption then bulk.

If you want BULK at 13.9V you need to lower/adjust ABSORPTION to 13.8V first. This is because BULK can’t be lower than ABSORB. There needs to be a minimum of 0.1V between “stages”. I use: BULK = 13.9V, ABSORB = 13.8V, FLOAT/OFF = 13.2V

#2 Dedicated Voltage Sense Circuit: This feature is not to be underestimated on an LFP bank. ACCURATE voltage sensing is of critical importance. The MC-614 + v-sense wire does nothing but sense voltage. The + v-sense wire is used with the regulator B- to create an accurate voltage sensing circuit. On the ARS-5, the next step down, this wire both power the regulator and senses voltage. This can create inaccurate voltage sensing due to the additional current carried on the v-sense/reg B+ wire to power the regulator.

TIP: In order for the v-sense “circuit” to work correctly, and accurately, you must wire the Balmar regulator negative lead directly to the negative post of the battery bank which you are measuring. Regulator B- is the other half of the voltage sense “circuit“.

#3 Current Limiting: Remember when I said there is no small case alternator on the planet that can run at full bore for 3-4 plus hours into an LFP load in boats engine bay..? Well this is where you fix this and help save and extend the life of your alternator.

Balmar calls this BELT LOAD MANAGER. It was formerly called AMP MANAGER, which I still feel is a better term, but it works for both belts and saving the life of your alternator. Current limiting allows you to essentially derate the output of your alternator by limiting the maximum field potential to the alternator. The field wire from the regulator is what drives the alternator. Simple stuff. By limiting the capability of the field wires maximum potential you in turn limit how much current the alternator can drive. This is a simple adjustment in the settings menu and should be made when the alternator has been run up to temp.

Every alternator will be slightly different but usually Belt Manager level 3 or 4 is a good place to start. I generally suggest buying an alternator that is larger than where you want the expected hot rated output to be and then dialing it back in Belt Manager..

On my own boat I now run a custom built 160A Mark Grasser DC Solutions externally rectified small frame alternator. This was the only way to get maximum output from the small case alternator. Before converting it to external rectification it was current limited to about 120A and it still occasionally bounced of the alt temp sensors limit.

With external rectification it will now drive at full bore for hours and rarely break 230F. This is exactly what I want to see. To not exceed 240F, before going to external rectification, I controlled this by limiting its max potential using Balmar’s Belt Load Manager feature. This kept the alternator from melting itself down and prolonged its useful life. In the end it was going to need to be a large frame or external rectification. Welding custom brackets for a large frame alt was more of a project than going externally rectified so I chose that route instead.

#4 Alternator Temperature Compensation: A regulator that offers an add an on-alternator temp protection sensor is additional insurance you won’t wind up with an Alternator fire.

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26 LiFePO4 On Boats

A Cooked Alternator Stator

This is exactly what happens when you don’t current limit an alternator that will be feeding an LFP bank. It will literally cook itself. Here the magnet wire coating has literally been cooked right off the stator windings. This happened on an AGM bank and this was a factory dumb regulated alternator with no built-in thermal compensation. It just ran and ran and ran until it burned itself out.

How can you avoid this? Use a Balmar MC-614 regulator, and current limit the alt, especially if you have an LFP bank!!!

RPM, Engine Room Temp & Current Limiting:

What does low engine RPM have to do with alternator heat? Many sailors and cruisers want to charge at low RPM while on the hook. Today’s high performance alternators can put out a substantial portion of their total output at just a fast engine idle. This is good, as it keeps your neighbors happier in the anchorage.

But wait, there is a catch, always is.. The problem with low RPM charging is that with a fast idle we have very slow alternator fan speeds. The speed of the rotor actually serves to keep the alternator cool.

Arguably the most abusive loads for a high performance alternator are not at cruise RPM, they are usually at fast idle. Keep in mind many of the new small case alternators can handle 17,000 – 19,000 shaft RPM. This creates excellent cooling but we never get there on most cruising boats. Because of this the alt is best set up and hot-load tested to fast idle RPM. This is where it will get the hottest.

Alternator Set Up & Load Testing:

It is not just good enough to program the regulator and walk away. Every alternator will respond differently to the field wire from the regulator. Proper set up will lead to a long alternator life and an alternator that can survive the abuse an LFP bank throws at it.

How do I do this?

It is not difficult. You will need the following:

#1 An on-board inverter capable of exceeding the alternators current capability, usually 2000W or more, or a portable inverter capable of at least 2000W or more.

#2 A good restive AC load such as a heat gun, hair dryer or portable heater. If the alternator and inverter are large enough you may need two of these devices.

#3 A remote temperature sensor attached to the alternator case that can be read with the engine room 100% closed up and sealed tight. Most DVM’s offer a remote temp probe.

Alternator hot load testing and set up:

Step #1 – Connect a DVM temp sensor (Fluke etc.) to the alternator and close engine room

Step #2 – Run the boat under cruising load, with the inverter loaded down via the heater, and at cruise RPM for at least 30 minutes.

Step #3 – Return to dock or mooring and leave the motor running at fast idle

Step #4 – Keep inverter/AC load running. This load should be in excess of alternators capability.

Step #5 – Monitor alternator temp and ensure temp does not exceed 225F?

Step #6 – If it goes over 225F, adjust Balmar Belt Manager to level #1

Step #7 – Continue load testing and monitor temp, did the alt still go over 225F?

Step #8 – If so move to Belt Manager Level #2

Continue this process until the alternator stays below 225F loaded to max output.

TIP: I start at Belt Manager Level #4 and work my way up. Most alts require level #3 or #4.

NOTE: Balmar’s Belt Manger used to be called Amp Manger in previous regulators. It is the feature you use to current limit your alternator and prevent it from cooking itself.

AC Battery Chargers & Inverter Chargers

Just like with external regulators we want to be able to control;

Max Current Output – A small generator may not be able to handle a large charger

All Charging Parameters – This is a must for an LFP bank

Dedicated Voltage Sensing – This is a critically important aspect for an LFP design.

High Current Output Capability – This compliments the bank and allows for faster charging. The largest charging capacity commonly found will be in an  inverter/chargers (I/C’s) often called combi’s. Stand alone chargers are usually smaller in current output.

NOTE: Most inverter/chargers drop current output as they heat up. Just because it says 130A does not mean it can do this indefinitely. When hot you may see considerably less than “rated” output.

Dedicated Voltage Sensing Rant:

Here we go again.. In a nut shell there are very, very few chargers or inverter/chargers out there that offer dedicated voltage sensing. This is really quite pathetic. Sadly the charging portion of most inverter/chargers is apparently an after thought for the engineers who design them. The engineers who fail to provide dedicated voltage sense leads, on battery chargers or inverter/chargers, have failed you, the customer, when you’re seeking fast charging performance..

Who are these failures?

Mastervolt – No dedicated voltage sensing

Magnum – No dedicated voltage sensing

Xantrex – No dedicated voltage sensing

Which manufacturers actually care about battery charging performance?

Victron – Victron I/C’s have dedicated voltage sense terminals right on the main unit. Kudos to Victron!!

Outback – Can be done but requires FLEXNET DC & MATE Remote Control.

This is not to say Victron makes the best inverter/chargers but God damn if their engineers actually understand charging. Battery charging that is. (wink)

Follow me on this. No matter how big you size the wire for with a 130A+ inverter/charger you will still have some voltage drop between the charger and the physical battery terminals. Most charger manuals only account for wiring voltage drop but we should remember that each termination, busbar, shunt, fuse, battery switch etc. results in even more voltage drop. It is not uncommon to see 0.4V -0.8V of drop, at full charging output, even on factory installed inverter/chargers. While the I/C makers often insist you keep the unit 5′ from the batteries, this is not always possible on a boat.

In the real world, voltage drop happens, and is simply a fact of life.

How do we fix that?

Simple, dedicated non current carrying voltage sensing leads connected directly to the battery terminals so the charger can compensate for slight voltage drops in the system wiring and not enter absorption or the voltage limited charging stage prematurely.

Consider that just a 3% voltage drop, something most boat owners feel is perfectly acceptable, winds up creating a .42V drop, at the battery terminals, when the charger is pumping maximum amperage and trying to attain 14.0V.

If we start with a target voltage of 14.0V, and drop or lose .42V, this means just 13.58V at the battery terminals. Even an LFP bank, which normally operates around 13.2V, can come up to 13.6V well before it should have its current limited by the charge source.

Due to voltage drop the charger simply begins limiting voltage because it thinks the batteries are at 14.0V, but they are not. The charger is thinking it’s in constant voltage mode but it should not be and instead should still be in BULK.. This type of situation literally murders fast charging performance, especially with the narrow voltage range of LFP.. This is not just bad on LFP banks but also bad form on large lead acid banks as well. because LFP has such a narrow voltage window, that it operates within for charging, any charge system voltage drop becomes a much larger issue charge performance wise.

How voltage drop murders charging speed:

At the charger end the charger sees 14.0V and enters CV mode or constant voltage mode. The charger now begins limiting voltage and current by controlling the output of the power supply, so as to not over shoot 14.0V. The problem is at the battery end, the voltage is below 13.6V and only so much current can flow into the battery at 13.6V, even an LFP bank. Voltage is the pressure that allows the charge current to flow into the batteries and LFP banks are not Ohm’s law exempt.

Attain a limiting voltage too early, due to voltage drop, and you have just extended your charging times and will have a longer current taper to get to 100% SOC, just like lead acid!

Dedicated voltage sensing at the battery terminals is critical to FAST CHARGING PERFORMANCE. If you use a generator to power an AC charger proper voltage sensing means less generator run time. If a charger or inverter/charger does not offer you this option please BUY ONE THAT DOES!

The Victron’s represent an excellent value in an LFP capable inverter/charger, especially one that has dedicated voltage sensing leads. By the time you are done with the Outback, by adding FLEXNET DC (allows for volt sensing) and the MATE (remote control), you are well in excess of the cost of a Victron Multi-Plus Combi. Course if you are in the US the Outback is a US company and supports US jobs.

Choose your AC charger carefully. The two biggies are full control over all charging parameters and dedicated voltage sensing..

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28 LiFePO4 On Boats

0.4C Charge Rate & 12v 100Ah Winston Pack

Okay you’ve heard me discuss how to safely charge these batteries when used as house banks at factional “C” usage, and here’s a prime example of what I am talking about.

As we can see in this image the battery has hit full at just 13.88V (pack voltage) with a .4C charge rate. A .4C charge means a 40A charge on a 100Ah battery pack. Anything above this voltage point is technically over charging the battery. If you stopped at this 13.9V level, and tested this pack for Ah capacity, you would see approx 98% – 107% of the rated Ah capacity. I know this because I have conducted these tests NUMEROUS TIMES…

This over charge can easily be denoted by the abrupt hockey stick rise in voltage once the cells hit “full”..

Baseline Test Results: Charging this 400Ah Winston pack upon delivery to 13.8V resulted in 425Ah’s at a .25C load or a 100 amp continuous load for over 4 hours straight.

Cycle #772 Test Results: Using this 400Ah Winston pack, as I have laid out in this article, still has the bank delivering 432Ah’s at a .075C (30A) discharge rate at cycle #772 and year 7.

Just for grins how many Ah’s would we get out of a “full” flooded lead acid bank when capacity tested at a .25C load? Let’s do the math:

400Ah FLA bank with a Peukert’s Constant of 1.3 driving a .25C load = 247Ah (Ouch)

WOW! Test a flooded lead acid battery at the same rate you can apply to a 400 LFP bank, .25C, which has only charged to 13.8V, and you get 178 Ah’s less capacity out of the FLA bank than you do from the LFP bank. The reality is that on boats we are not discharging at a .25C rate and we are discharging at fractional “C” rates.

PONDER THIS: The only benefit to charging to voltages above 13.8V – 14.0V is a slightly shorter current taper at the top end of charge.

I allow the current to fall to 10A at 13.8V before deeming the bank “full”. This equals 425Ah’s at a .25C load. All LFP banks will have slightly varying capacities, and the factory rating is at a .5C load. The 425Ah @ .25C correlates to just about 397 – 400Ah @ 13.8V and a .5C load. Why charge higher, & run more risk, if we can EXCEED rated capacity at just 13.8V – 14.0V at fractional “C” loads??

*Maximum Safe Charging Voltage: Max safe charge voltage for Winston cells, as house bank use = 14.2V

Optimal Charging Voltage: Charge Voltage for Winston Cells, as house bank = 13.8V to 14.0V

*When used with typical lead acid charging equipment.

If you choose CALB cells & a DIY build MAX should ideally be 14.0V.

Why? Take a look at what happens to the cell voltages as they get into the upper knee. We have the lowest cell at 3.69V and the top cell at 3.81V which is now into the danger zone! The pack voltage may still look okay but we are now cutting into the cycle life of a couple of the cells by over charging them..

While I have known lower charge voltages work fine for off-grid and fractional C use, and are arguably safer, research is finally coming out to back this up and to also show that higher voltages lead to shorter cycling life too.

What Has Industry Learned?

Tesla and other Li battery research institutions have now been able to show that regularly pushing Li chemistry cells to high charging voltages results in the build up of electrolyte oxidation by-products which adhere to the negative plate. This eventually leads to a shut down of the cells. It has been shown that higher charge voltages, with all Li chemistries, results in shorter cycle life. Essentially higher charge voltages result in more electrolyte oxidation clogging the negative plate. While the capacity may look good for a period of time the cells eventually fall of the proverbial cliff. In NMC cells (LiNiMnCoO2) cycle life degradation was accelerated as charge voltages were pushed higher. Testing showed that a max charge voltage of 4.20VPC (these are not LFP) showed very little capacity fade where as a max charge voltage of 4.35VPC resulted in less than 200 cycles and a charge voltage of 4.45VPC resulted in less than 60 cycles. It is not however just max voltages that affect the cells it is time at voltage. In the tests above the cells were simply charge “TO” the upper voltage but with the lead acid chargers we use voltages are “HELD” at a steady voltage for a period of time. In the marine environment, in order to compensate and not over-stress the cells with CV charging, we can simply lower the max charge voltage and this can help accommodate the cells. This serves to allow for the CV (constant voltage) stage to be safer for a slightly longer duration.

As you push into the upper knee the cells can rapidly run out of balance as one cell becomes more full faster than another. As cells age Coulombic efficiency can change, especially when over-stressed by using high charge voltages and CC/CV charging equipment. The actual cell to cell capacity can also change. By staying out of both knee ranges, voltage wise, the cells tend to cycle up and down with very little voltage drift. Regularly pushing into the upper knee simply creates the need for more cell balancing and many of the BMS companies pray on this. IMHO for fractional C use cell balancing is not necessary until AFTER the HVC point so that it can be done SAFELY, MANUALLY/ATTENDED and SMARTLY, just like equalizing a lead acid battery..

The cells in this image are cells that are well within the ideal balance range when kept between 3.0VPC and 3.45VPC but that drift and begin separating over that voltage. If your pack is not perfectly top balanced, as this one was, you will begin drifting even sooner.

If you can get 98-99.9% of the capacity out of the bank at a 13.9V – 14.0V maximum charge voltage, by stopping at a tail-current before you hit 100% SOC, why go any higher? The answer is DON’T and your batteries will stay in good balance for many hundreds of cycles. My own 400 Ah bank has now exceeded *825 cycles (1/15/2017) and is still in excellent balance and has lost virtually no capacity. This is nearly 3X the average cycle life of any lead acid battery I tend to see in the real world of the marine environment.

Please Source the Data:

Please, I ask anyone out there to present me any credible data that shows why going above 14.0V or 3.45VPC to 3.500V per cell is either good for the batteries or to give me just one solid reason why it is necessary to do so on each cycle? I have nearly 100 research/white papers in my data base and not a single one of them gives any good reason to push these cells into the upper knee with regularity. Not a single one.

What do we know about higher voltages and LiFePO4 battery longevity?

#1 Just letting these batteries sit idly at full charge degrades a Li batteries life. The manufacturers want them stored at approx 50% SOC for the longest life.

#2 Testing has shown that all Li batteries are degraded by merely charging “to” a high voltage and stopping. Imaging if they charged “to” that voltage and then “held it” as lead acid charge equipment does? I can’t imagine it is going to be better for them.


Please, please, please stop asking if it is okay to float your LiFePO4 batteries. The answer is still going to be NO. If you want to continually float your LFP bank you bought them for the wrong reason. These batteries LOVE TO CYCLE. They hate being at 100% SOC and even the mere act of storing them at 100% SOC, as we need to do with lead acid, is damaging to their life span. Adding a slight over voltage, above cell resting voltage, is even WORSE for them. DO NOT FLOAT LFP.

If you are in a situation where charge equipment can’t be turned off and would necessitate floating the LFP bank, you need to wire in a cross-over lead acid battery to handle shore-side or unattended duration’s of alternative energy charging. For dock side or unattended uses you need to be able to discharge the LFP to 50-60% SOC and TAKE IT OFF-LINE while allowing the small lead acid bank to run DC system loads along with the shore side charger or alternative energy systems.


I recently ended a very expensive experiment regarding storage at 100% SOC. The test duration wound up being 12 1/2 months using four 100Ah CALB cells where they were charged to 100% SOC and then left to sit idle with no connections to a BMS or other parasitic loads. The low temp recorded over the 12 1/2 months was 46°F and the high temp was 87°F and was meant to be a representation of the real wold.

A min/max capture thermometer was used to record the peaks. The cells, prior to letting them sit at 100% SOC for 12 1/2 months, were regularly testing at 101.2 to 101.3 Ah’s of capacity (previous 6 Ah capacity tests) as a 12V nominal bank. After 12 1/2 months the cells were discharged to a cut off voltage of 2.9V for the lowest cell. After 12 1/2 months of doing nothing but sitting there, at 100% SOC, the cells had lost 11.6% of their previous rigorously confirmed Ah capacity. Now imagine if you additionally stressed the cells by continually float charging them. Ouch!!!!

“The cells lost 11.6% of their confirmed capacity just sitting at 100% SOC”

How can these manufacturers suggest that the mere act of storage, at 100% SOC, is bad for the cells, which I have confirmed is, and then tell you “sure charge them to 14.6V”……? How can they say “store at 50-60% SOC yet then give you a “float” voltage? Really? Come on, let’s use some common sense. FLOATING LFP BATTERIES IS NOT GOOD FOR THEM. Sorry I don’t know how to be any clearer on this point.

I can sum up my feelings on the Chinese, and their often ridiculous charge voltage guidance, like this:

They figured out a great recipe, they can repeatably make the recipe, but they have NO IDEA WHY IT TASTES GOOD!

I would argue that pushing these batteries above 3.5VPC during regular charging is actually detrimental to longevity and not at all beneficial. Once again I challenge any and all Li battery researchers or scientists (I know many of you are reading this because I have your emails) to bring me any credible data to suggest a “need” for such insane charging guidance for the proposed use as a marine house bank.

At the very least pushing to these voltages causes a need for a balancing BMS. Pushing these cells beyond 3.5VPC / 14.0V/28V etc. can lead to nothing but problems in a fractional C system and holds your batteries in the upper knee, a range that can be detrimental to cycle life. IMHO if folks charged these batteries at sane voltages, and stayed out of the upper knee, there would likely be no need for a balancing BMS just HVC and LVC protections.

Like anything, the Chinese want to appease us and thus they tell us lead acid charging voltages are okay to use or they leave out critical points of how to charge these cells. Oh but wait “Don’t store these batteries at more than 50% – 60% SOC because these voltages are damaging“…. ? Huh? Really?

LFP batteries do not need to get back to 100% SOC, ever. Even if you only got to 95% and drained to 20% SOC you still get 75% usable capacity which is FAR greater than the 30-35% real world cycling you get from lead acid.

We also can’t forget what we are looking at here. A .4C charge rate on a boat with a 400Ah LFP bank would be a charge rate of 160A continuous. This means an alternator with a 200A rating current limited to 160A. Most boats will struggle just to get to a .3C to .4C charge rate and these batteries can take much more..

Image Courtesy: Terry©

LFP Cycle Life

This is one of the few tests I have seen on an LFP battery where supposedly they took it to 100% DOD every cycle. The cells were made by GBS and the cycle tests were to 100% depth of discharge. The tests were conducted at room temperature and the cell was discharged at a .5C load or 20A on a 40Ah cell.

At 2000 cycles, to 100% DOD, these cells were still putting up 35Ah’s or just 5Ah’s shy of the as new capacity rating. Even for lab type test this data is utterly amazing.

Please take this graph with a grain of salt because it comes from a manufacturer trying to sell you something. The inside buzz is that this testing was actually performed but still…

Image Courtesy: Elite Power Solutions©

29 LiFePO4 On Boats

30 LiFePO4 On Boats

Capacity Testing – Create A Baseline

With expensive batteries such as LiFePO4 I believe it is a wise idea to create a baseline capacity figure that you can repeat at least once per year. For the fist 11 capacity tests on this bank I used the set up pictured here. For loads I used an inverter and ceramic disc heater plus an incandescent bulb to get the current to approx 100A. This represented a load of approx .25C. To count Ah’s removed from the bank I used a Victron Ah counter.

This set up worked and so long as the cells were tested at the same temp, using the same equipment, the test was fairly repeatable with some level of accuracy. On the first capacity test after the initial top balance the bank yielded 425 Ah’s at a .25C load when charged to 13.8V and current allowed to taper to 10A..

With this initial capacity test I now had a solid baseline from which to monitor changes over time. I ran a capacity test approx every 50 cycles and the most recent was at cycle 701.

In order to run a capacity test you will need a load plus a way to track Ah’s. You will also need a way to track individual cell voltages and cut the test off when voltage drops on the lowest cell to 2.8V.

It is not important to test these cells at a .25C, .5C or 1C rate because the use on a vessel, as a house bank, will draw considerably less than that. Using higher C rates just makes the testing shorter in duration.

There will be some slight differences in capacity between say a .1C and a .25C test because these cells do have some Peukert effect, but this will be nowhere near as pronounced as it is with lead acid batteries.

Do yourself a favor and create a baseline so you know what you actually have! It’s kind of tough to program an Ah counter when you really have no idea of the actual capacity other than what the manufacturer tells you. There have been numerous reports of folks not getting the “rated” capacity in their cells, as well as more, and without a baseline you really have no way to know whether you started with less capacity or you caused premature capacity loss, through your treatment or mistreatment of the bank.

Moving Towards Better Accuracy In Capacity Testing

One of the services I offer my customers is 20 hour capacity testing for their expensive AGM or GEL batteries. Some even take advantage of it for expensive flooded deep cycle batteries. The only true way to test a lead acid battery, for use as a deep cycle bank, is to physically capacity test it using industry standard testing procedures. The 20 hour test is most representative of the loads used on boats and thus a true 20 hour capacity test is what I use.

Many years ago I built an Ah capacity tester using an Ah counter, relays and DC loads but the accuracy was not as good as I would like because the DC load needs to stay steady the entire duration and this means manual manipulation. As the voltage decays the current you initially set at 12.7V changes, thus changing the batteries discharge rate. With an non steady discharge rate, on lead acid batteries, this results in less than stellar accuracy.

With LiFePO4 holding the current steady, while capacity testing the bank, is less critical because of the very, very low Peukert effect.

A few years ago I had Mark Grasser, of Mark Grasser DC Solutions, build me a custom DC constant load tester for Ah capacity testing batteries at the 20 hour rate. This device worked well but still, I wanted better control, easier set up and better overall accuracy.

I finally invested in a lab grade DC electronic loads with battery testing capability. What a world of difference! The accuracy of these devices is amazing but they are not inexpensive.

At capacity test #12 (cycle #550) I switched this battery bank over to the new DC electronic load tester. All testing from this point forward has been be done using this device. My new baseline was established at cycle #550 and the bank delivered an astounding 419.2 Ah’s at a 30A constant load after 550 cycles!

I hesitate to compare this capacity figure to the previous 11 capacity tests but it was very close to them. From any 400Ah rated bank with 550 cycles on it this is something I would have never believed, had I not done the testing myself.

This bank is proving my initial skepticism as perhaps being misguided? While the discharge rate was lower using the DC electronic load the Peukert effect on this bank is also low. What really matters, or should, to myself or anyone using these as a house bank, is how many Ah’s you can harvest at your average house loads?

The answer for this bank is in excess of 400Ah’s even at 550 cycles in…

I do believe I have lost some capacity in those 550 cycles, but not as much as I initially thought I would. The last capacity test, using the old testing rig, delivered 423 Ah’s.. Tests using the old method & equipment yielded anywhere from 421Ah’s to a high of 426Ah’s. This test was at 30A, not 100A, but with a higher accuracy piece of equipment and no inverter, just a pure DC load.

Still this bank delivers more Ah capacity at a 30A load, which is multiples more than our average on-board load, and that is good! We have 419 Ah’s of capacity, at a 30A constant load, after 550 cycles. I’m a happy camper!!

PHOTO: The 400Ah bank, a 60A lab grade power supply for charging, and the 40A lab grade DC electronic load tester. Most of the smaller wires, in this spaghetti mess, are voltage sensing leads.

31 LiFePO4 On Boats

32 LiFePO4 On Boats

Capacity Test #12 @ Cycle 550

Here is a close up shot of the data the DC electronic load captures when set to battery Ah testing mode. (click the image to make it larger)

I set the capacity tester to 30A so as not to over heat it by running at full bore for 10+/- hours. The unit is rated at 40A but like anything electronic it likes to be run at less and will last longer doing so.

It should be noted that my charge source and DC Electronic Load tester both utilize dedicated voltage sensing leads that do not carry any current. This means accurate charging voltages physically measured at the battery terminals, and also accurate cut-off voltages, measured at the battery terminals.

The cut off voltage for this capacity test was set to 11.2V or 2.8VPC. A Cell Log 8 was used to trigger alarms if any cell dropped below 2.78V.

The Capacity Testing Process:

#1 Charging = 13.8V and current allowed to taper to <10A

#2 Cell Temps = 76F – 77F

#3 DC Load = 30A constant

#4 Voltage Cut Off = 11.2V / 2.8VPC

#5 Capacity Measurements = Ampere Hours & Time At Load


Cycle Number = 550
DC Constant Load = 30A
Ah’s Delivered = 419.2 Ah’s
Time @ 30A = 13:58:55

Do yourself a favor and create a baseline capacity figure that is both repeatable and useful to you!



Cycle Test #772 Discharge Graph

This discharge curve may be helpful to some. The data is the same data in the video above only in a graphical format. Of interesting note is how little capacity is stored at loaded voltages at a low .075C discharge rate or 30A for a 400Ah rated pack.

In the graph the 400Ah rated battery has only delivered 2.83Ah’s by the time the voltage curve levels out at 3.32V under a 30A load. When these cells are at 100% SOC they can have a resting voltage of about 3.38VPC to 3.40VPC or 13.52V to 13.60V, for a 12V nominal pack, but there is really very little stored energy between 3.4VPC and 3.32VPC about 0.65% of Ah capacity to be exact.

On this pack the stored energy between 3.4VPC and 3.32VPC was 2.83Ah’s. This is the rather abrupt near vertical portion of the blue voltage line at the very beginning of the curve. Also, at low rate discharges, such as this .075C discharge rate, the curve is more gradual and it is not until about 2.9VPC that the voltage starts to hit the knee and drop rapidly, in a near vertical fashion.

On a 400Ah rated pack, at a 30A / .075C discharge rate, the working voltage range between about 99.3% SOC and 0% SOC is only a 1.66V difference for this 12V nominal pack.


One Comment

  1. Paul Walter June 3, 2017 at 1:59 pm - Reply

    Do you know any quality person/business willing&able to install a Complete lifepo4 system connected to solar panels in a cargo van? Size of battery bank 200-300 AH. Will be traveling so anywhere in country is posible.
    Thank you.

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