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. An article like this takes lots of time to write photograph and proof read and this is my busiest time of year. Please be patient. 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)
Ever since I began my foray into LiFePO4 batteries readers of this site have been asking for more information. I thought now was as good a time as any to share my experience.
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!!! 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.
Since opening this article to the public I have now had what I consider and 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 not the intent of this article.
Sadly, only a few of the cases of destroyed LFP batteries I have been contacted about are 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 had 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=68):
#1 Cause of LFP Death = Over Charging – Almost always incorrect absorption duration and too high an absorption voltage. Voltages were above 14.2V in every case and absorption duration 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 13.8V pack voltage think again and please do more research. It is a complete fallacy that LFP can’t be overcharged at 13.8V, when this voltage is held there.
#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.
#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“.
#5 Cause of LFP Death = Over Discharging – In most of these cases incorrect battery monitor SOC programming and lack or BMS protection lead to over discharged batteries. Some were cell imbalances and one or two not accounting for parasitic loads.
Please note that in all 68 of these RUINED LFP banks, that I have to date, 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.
I apologize but I have stopped all long distance consulting on LFP. If the boat is not readily accessible here in Maine, Casco Bay to be specific, I simply can no longer do LFP consults. Why? Far too many owners insisting on cutting corners and multiple situations where things were not done to design or design criteria. Bottom line is this; if I can’t lay my hands on it, I am no longer designing or consulting on it. If you happen to glean something from this article, that is fine, but please don’t email me with further questions on LFP. This article is for INFORMATIONAL & EDUCATIONAL PURPOSES ONLY.
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 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…
#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 a poor fit, at least right now….
#4 This article is not a suggestion or an endorsement for widespread DIY builds of LiFePO4 batteries. It 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 Li 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, elsewhere.
Photo: The cells were very well packed and got here in great shape.
The Li 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 Li 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…
There are three basic options of getting LFP on your boat, with DIY being the least expensive, and most technical. The categories are:
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, all the components, choose the BMS, choose the high voltage cut and low voltage cut relays main contactors, 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 LFP Systems:
Genasun, Mastervolt & Victron build LFP systems for marine specific applications. These systems are well engineered, executed yet also expensive. Lithionics is also moving along nicely with a marine specific design. If you want LFP and don’t have the ability to DIY, these three companies I can certainly recommend and I suspect Lithionics will have a marine system soon. I personally have the most experience with Genasun, and the system is quite beautiful. The owner of the company, Alex MeVay, is an MIT grad and one of the brightest minds in the marine LFP field.
I really even hate to mention this type of system because it has so many issues, when installed on a boat, that I find it tough to swallow making mention of them. Some of these systems lack a BMS altogether and others only have a single relay that disconnects the bank on low or high voltage. This relay is often rated at a ridiculously low amperage capability of around 100A. If it seems to good to be true, it probably is! Caveat emptor on Li Drop-In’s.
I URGE STRONG CAUTION ON ANY “DROP-IN” LFP BATTERY!!!!
When in doubt go Genasun, Mastervolt or Victron. Lithionics is also coming along nicely with a marine system. These companies have well thought out marine specific systems..
LiFePO4 vs. Lead – Pro’s & Con’s
No article would be complete without pro’s & cons and comparisons between lead and LFP.
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.
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. The 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 for approx 28 months and have only managed to put 550 cycles on this bank and that is with accelerated cycle testing..
Of course the average lead acid battery on boats is often dead well before 150 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 entirely trust the lab ratings of the LFP cells, though some have done the testing and have seen this.
Alex MeVay the CEO of Genasun firmly believes in 2000+ cycles to 70% DOD. This is utterly amazing cycle life if it can really happen. Based on what I am seeing at 550+/- cycles I am trending towards believing it..
USABLE CAPACITY RANGE:
Approx 80% of an LFP banks capacity is fully usable. 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 current limiting or acceptance taper is very, very short in duration, even at relatively low charging voltages of 13.8V – 14.0V.
Very, very short current taper even with large current sources. Charge to nearly full before even attaining absorption voltage. This of course is entirely dependent on your charging voltage and your current source. We charge at 120A steady (160A alt set up to run at 120A continuously) and our current taper lasts only 30-35 minutes. Compare that to HOURS and HOURS of current limiting using a 120A charge source on 400Ah lead acid batteries. With a small charge source, like solar or wind, you will hit 99%+ SOC before any current limiting occurs. Our bank literally has to be chock full before our solar array can even get 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!
GET OUT THE DEAD LEAD:
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’s 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” house loads. Espar heaters, refrigeration, watermakers 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.
NO NEED TO RECHARGE TO 100% SOC:
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, WHAT’S THAT?:
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!!!
SAFE Li TECHNOLOGY:
Without question LFP is the safest of the Li 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 goes LiFePO4 is currently the safest. Remember we are surrounded every day by far more volatile Li technologies in computers, iPads, iPods, tablets, video games, cell phones and even cordless tools. LFP is less energy dense than other more volatile Li 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)..
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)
PHOTO: In this photo I have constructed my cell compression case. LFP banks used 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.
LFP batteries will be quickly compromised or DESTROYED if over charged. Unlike lead batteries over charging does not just boil off electrolyte 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 will survive this, an LFP battery WILL NOT. Over charging LFP batteries WILL 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.
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”..
CHARGING BELOW FREEZING:
LFP cells should really not be charged at temps below 32F. They can be used below these temps but not charged. Lead acid could really care less. This is yet another way to DENT YOUR WALLET!
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 battery or cell hits full/empty and the voltage shoots for the moon 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 150% UNNECESSARY for fractional C use. Charging these cells to more than 14.0V is really 100% unnecessary 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 charging well before any damage can be done.
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 50-60% SOC when not being used not 90%+ SOC..
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.
INDUSTRY LIES & MISLEADING MARKETING:
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” charge profiles, despite no physical or practical experience with them. In short many of these LFP charging claims are misleading and can be dangerous to the health of your cells.
PLEASE DO NOT BUY THE HYPE/BOVINE DUNG WHEN CHARGER MAKERS TELL YOU THEIR CHARGER HAS A “LITHIUM” CHARGE PROFILE!!!
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 setting of 14.6V absorption and a 14.4V float. As Homer Simpson says D’oh……..
DO NOT use these settings to charge prismatic LFP cells. You will eventually DESTROY your batteries. Here’s a reminder hint you DON’T FLOAT LFP 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 here. 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. D’oh….. If I recall the GEL battery makers already tried that and look where their market share is now.
DO NOT TRUST THE HORRIBLY WRITTEN CHINESE MANUALS FOR CHARGE VOLTAGE GUIDANCE! THEY JUST WANT TO GET INTO YOUR PANTS POCKET AND EMPTY YOUR WALLET.
Let’s look at this another way. The Chinese say, in the manuals, it is okay to charge these batteries to 14.4V – 14.6V. 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 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 inside the pretty tea; and gray box, and it was not from the Chinese, it was from actual FAILURES when charging these banks to 14.6V instead of 14.2V 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 a13.8V to 14.0V and you will be in a safer range.
You really need to source chargers, controllers and alternator regulators that allow 100% CUSTOM PROGRAMMING. Do not source the cheapest piece of junk smart wanna-be Li charger. Dip-switch (read; dip-shit) pre-selected, non-adjustable charge profile chargers built by a charger maker, who’s reading a misleading Chinese/Chinglish spec sheet, and does not understand what it even means, have no place mixing with LFP.. Wow, that was a mouth full…
The previously mentioned ProMariner ProNautic P and Sterling ProCharge Ultra chargers are decent chargers because they DO have a very limited custom program feature. USE the custom feature but DO NOT use their Lithium charge profile. Where these, & most chargers, fail is in the lack of a dedicated voltage sense lead and the largest charger they make is just 60A. Kind of small for an LFP bank.
IT IS A SYSTEM, NOT JUST A BATTERY:
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. These “drop-ins” will be the black eye on the industry but 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 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 the “rest of the system”. 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 have not learned from battery industry history… I won’t even bother to go into the why’s or how’s of why I am not sold on LFP “drop in’s” because these companies are nothing more than money grubbing opportunists, IMHO.
When I say system a good system will be designed from the ground up to include a BMS, LFP capable over-current protection, 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”.
INITIAL STICKER SHOCK:
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. 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 LA 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.
HUMAN BEHAVIOR & LEARNING CURVES:
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”, 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.
Charge to full and stop… Run the bank down to 80% DOD and recharge. Repeat 2000 +/- times…. If you want a simple system then LFP is not going to be the direction for you. Perhaps the Pardey’s do have it all figured out… (wink)
There is a lot of evidence out there that suggests these cells can easily cycle to 2000 or more cycles 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 7/7/15 over 700 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…..
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 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..
AUTOMATIC CELL BALANCING:
Let me first say;
I DO NOT BELIEVE AUTOMATED CELL BALANCING IS NECESSARY FOR A WELL EXECUTED FRACTIONAL “C” SYSTEM !!!!
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.
NOTE: With The HPBMS:
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 DO NOT ALLOW MY BMS SYSTEM TO BALANCE AUTOMATICALLY !
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 5700 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….??
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:
- 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
- Blue Sea Terminal Strip
- Piezo alarm buzzer 12V.
- Momentary re-set switch.
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.
TOP BALANCE vs. BOTTOM BALANCE:
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.
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.
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.
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.
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
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!
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©
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:
♦Moisture & Humidity
♦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..
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.
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..
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 alt running hot at about 100A this means BULK charging will be about three hours. There is no alternator on the planet 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. There is not a single small case alt that will survive this for long without a proper installation.
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 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.
Currently the Balmar AT series are the meanest, bad a$$ small case alternators money can buy. I used the $$ signs in the word ass for a reason, they are not inexpensive…
Nothing can touch the Balmar AT in this case size range. At the moment, in my opinion, they are the best suited small case alternator avaible for an LFP bank.. Unfortunately many sailboats don’t have the room for a massive large case alternator with external bracket etc. If you do, that is great, and I would steer you in that direction.
Does this mean other small case high performance alternators can’t be used on LFP? Absolutely not, it just means the percentage of “rated output” you get out of the alt will be less than it is on an AT series and it will need to be dialed back in the regulator settings, for self protection, further than a Balmar AT series will.
Large Frame Alternator
When space allows a large frame/case alternator is generally better suited to LFP banks. They still prefer current limiting/Belt Manager but they are purposely designed from the ground up, and intended for, driving large loads. Other than the Balmar AT series, small case alternators are adapted and modified to driving large loads. Small case alternators require more current limiting to survive LFP banks than do most large case alternators.
PHOTO: In this photo we have a large case alternator on a Balmar/AltMount custom bracket driven by a serpentine/rib belt. If you have the room, this is perhaps the best option to go with for charging LFP banks..
Image Courtesy: C. Kelley©
Alternator Drive Belts
In order to get lots of current out of an alternator it requires a considerable amount of work on the part of an alternator belt. If designing the system for LFP there is no better option I know of than a multi-rib or serpentine belt kit for your alternator. I suppose you could design a geared PTO system too but $$$$$…
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, Mark Grasser DC Solutions or ElectroMaax)
#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 ElectroMaax 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 wire that does nothing but sense voltage, this wire carries no current on it, so you get an accurate reflection of battery terminal voltage, provided you wire it correctly.
IMPORTANT VOLTAGE REGULATOR FEATURES:
#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 Lead: 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. 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 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.
#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 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..
As an example I run a custom built 160A Mark Grasser DC Solutions alternator on our boat and she is current limited to 120A. She will drive at 120A all day long and rarely break 230F. This is exactly what I want to see. I do this by limiting its max potential using Balmar’s Belt Manager feature. This keeps the alternator from melting itself down and prolongs its useful life.. It also gives you a known value for charge current that won’t vary due to temp limiting or heat…
#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.
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 lame! Sadly the charging portion of most inverter/chargers is apparently after thought for most engineers who design them. The engineers who fail to provide dedicated voltage sense leads on battery chargers or inverter/chargers have failed you as a customer who is actually looking for 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 get it. 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 on LFP banks but also on large lead acid banks too. Of course because LFP has such a narrow voltage window, that it operates within, system voltage drop becomes a much larger issue, performance wise.
How voltage drop murders charging speed:
At the charger end it sees 14.0V and enters CV mode or constant voltage mode. It now begins limiting 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!
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 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!
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.
JUST SAY NO TO FLOAT CHARGING!!!!
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.
STORAGE SOC EXPERIMENT:
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©
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.
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!