Lead is Dead (almost)

You may be wondering why I am saying lead is dead ?The answer to that is simple; lead acid battery makers dug their own graves by grossly misleading the general public. They know damn well none of these batteries will never meet the cycle-life claims out in the real world whwere PSoC use is a reality. Why? Because the real-world is not a white coat white glove laboratory and in the real world “sulfation-happens”.

Claims of 1000 cycles or 1200 cycles or 1600 cycles is as laughable as Jim Gaffigan beating Usain Bolt in the 100 meter dash. In a battery use survey conducted on sailboatowners.com, the largest ever of its kind with 1480 users surveyed, the vast majority of boat owners reported they rarely if ever get more than 150 cycles out of their lead acid batteries. 150 cycles!!!!! Many of these are batteries that have claims of 100 to 1200 cycles or more. Laughable is about the best way to describe it. Lead is dead because the manufacturers dug their own graves by misleading their customers. Perhaps if they had been more honest in setting reasonable cycle-life expectations the mass exodus from lead to LiFePo4 would not be quite as rampant?

The image below is but one example of why I can say lead acid battery cycle-life claims are laughable. I don’t and will never make a claim like this without data. We have been conducting capacity test on lead acid batteries for close to 20 years. While the equipment for capacity testing batteries has gotten significantly better, the quality of the lead acid batteries has not. This poor customer purchased a brand-new bank of Trojan SCS-225’s in the spring. His boat is on a mooring and he did not have solar. I explained to him the batteries may only last 1 to 2 seasons. He was very surprised that based on Trojan’s claim of 600 cycles to 50% depth or discharge. The results below are not atypical. The battery is at 63.4% state of health in just one season. By industry standards a lead acid battery is considered end-of-life when it can only deliver 80% of rated capacity. 20+/- years of testing marine use batteries to BCI Testing standards has taught us that claims of 1000+ cycles from lead acid are about as fairy tale as Tinker Bell..

These days LifePo4 has come down in-price enough to be at near parity with AGM & Gel in a $$ to usable Ah Comparison. When we add in $$$$ to Ah & include cycle-life, LFP is wiping the tears of the lead acid battery makers off the floor..All that said.. Let’s take a loot at current prices;

Lifeline GPL-31XT– AGM- 125Ah X 3 for a 375 AHBank=$1596.96- 187.5 USABLE Ah @50%DoD(at best *300 hundred cycles(real world usage)) =**8.52/Ah
*20+ years of read world experience including many hundreds of capacity tests..

KILOVAULT HLX+ 2400X  2 400Ah BANK= $3090.00 for 320 USABLE Ah @ 80%DOD-5000+/- cycles=$9.65./Ah

KILOVAULT HLX+ 2400  X 2 for a 400AH BANK= $3090.00 for 400 USABLE Ah @ 100%DOD 2000 cycles=$7.73/Ah

A t 100% DoD the cost per Ah is only a little more than AGM

What About Cost per cycle?

HLX2400 Cost per cycle@ 80%DoD=0.62

HLX2400Cost per cycle@ 100% DoD=$1.55

LiFeLineAGM Cost per cycle@ 50% DoD=$5.32.

Drop-In Pluses& MinusesPreface:

This article discusses 12V (nominal) LiFePo4 Drop-in batteries for use on boats.

This is the first article I’ve written since I suffered a major stroke on September 1 (nearly died). It took me six+ months of recovery to get to this point. I am writing this with new speech to text software(Dragon) which is not easy to master and I only have one finger to type with(my left arm/hand are still paralyzed.. My vision was also affected by the stroke so working at the computer for more than about 30-40 minutes a day is exhausting(though with hard work my endurance is slowly getting better). My brain is pretty worn out after 30-40 minutes, so an article like this has taken me more than120 hours to author. I’m getting it done but everything is just taking a lot longer. I am committed to MarineHowTo.com and this article is just the beginning of my recovery. I had started the outline of this article back in August before I had the stroke..


What do we recommend for drop in Batteries?

For the highest quality; Lithionics

We also love the KiloVault HLX+ batteries

A Tremendous value are the batteries by *SunfunKits.com

(*use coupon code; marineht for an additional 5% off at sunfunkits.com

Discuss what you read about in this article in the groups below:

LiFePo4 Drop-In Batteries For Boats

For general Marine electrical systems discussions; Boat Electrical Systems

I Actually Use LiFePo4

It’s important to know that I’ve been using lithium iron phosphate batteries on my own vessel since early 2010 I built my battery bank back in 2009 well before any of these drop-in batteries even existed. My LFP bank will be 13 years old on May 10 of 2022. The bank has in excess of 2200 cycles on it & most every single cycle has been to at least 80% depth of discharge with many many, many cycles (at least 100+ cycles) going to 0%. That battery bank can still deliver 100% of its 400Ah rated capacity so LFP batteries can last, where lead acid don’t.

Having been using LFP since 2010, you could not pay me to go back to lead acid..

Important:This article is not intended to pick on any manufacturer at all. It is intended only to make you a more educated buyer. Where it was possible brand names have been obscured..(I do call out one manufacturer but that is rare for me).

Drop-In LiFePO4 Batteries, don’t you just drop them in?

No you don’t! Any consideration of LFP batteries on a cruising boat must be treated as a system wide approach.Don’t just take my word for it. Boat US is one of the largest insurers in the US.

But, But, they say “drop-in replacement for lead acid?”

“Do Not Connect to analternator”?WOW!I’ve never seen a lead acid battery disallow connection to a non-smart alternator??Drop-in? Apparently not so much?

Terminology Used:

  • LiFePO4= Lithium Iron Phosphate also called, LiFe & LFP
  • BMS= Battery Management System
  • C-Rate- “C “= Capacity and the rate is usually depicted as 1C, .5C.,.33C, .02C etc. A 100Ah battery with a charge rate of .5C would be 50A charge current or 100Ah ÷.5 = 50A
  • Load Dump- A BMS Disconnect during charging which disconnects the battery.
  • VPC- Volts per cell
  • Pack voltage- voltage of the entire battery measured at the packs positive & negative terminals

What is a “drop-in battery?

A drop-in lithium iron phosphate battery is a self contained battery that comes in a standard lead acid case size e.g; group 24,27,31 group 4D, group  8D etc..These batteries are self contained and should always have a BMS built in.. Batteries that use an external BMS such as Vicron or some of the Mastervolt or Lithionics batteries are not considered drop-in batteries.

Ignore The Trolls & Safety

Internet trolls pollute the Internet like cigarette butts pollute city sidewalks and gutters. You know who I’m talking about, the undereducated know-it-all who enters any conversation regarding lithium iron phosphate batteries in a Facebook group or boating forum and   starts with; “The only people looking at lithium iron phosphate batteries are the ones who want their boat to blow up”. A great example of this can be found in the “parting shot” of the April /May 2022 issue of Professional Boat Builder Magazine.. Even though the parting shot is editorial in nature,Pro-Boat should be embarrassed by that piece and the complete lack of research on the authors part. Pro-Boat normally works with authors who are actually experts in their field such as Steve D’antonio or Nigel Calder but the author failed to do any research on this topic and put out a grossly under researched apiece..

Let’ cut out the BS right now. LiFePo4 and LiCoO2 (Boeing) are about as different as water and gasoline in terms of resistance to burning/exploding. While LiCoO2 and LifePo4 are both Li-Ion batteries, the chemistries are vastly different in terms of safety. Let’s not forget Flat-Earthers still exist too…

When you see these trolls politely ignore them &, as the internet goes, “don’t feed the trolls“..

Li-Ion= Rechargeable Li
Lithium = non-Rechargeable Li

Don’t just take my word for it here is the FAA

A Reader Challenge

Reader Challenge:I will continue to offer a challenge that I have been offering now for 10+ years on the Internet and that is; the first person to bring me an image of a lithium iron phosphate cell, properly installed, that erupted into flames or resulted in an explosion due to overcharging, I will pay them$50 cash for that image! In 10+ years not one person has been able to bring me such an image…This is because LiFePo4 is an extremely safe chemistry.

A Dramatic example of LiFepo4 Safety

The image below is but one example of the safety of lithium iron phosphate batteries. The cells below came out of a drop in battery where the solar controller failed & the 100V+/- array started feeding hundred+/- volts to the batteries. The BMS tried to protect the batteries but once the BMS shut off. in the solar array was still feeding 100V+/- to the chip in the BMS. Once the chip failed the 100V made it to the FET’s and they too failed allowing the full array voltage to get to the LFP Cells.

No fire, no explosion just swollen ruined cells & cell venting.

The Cells were overcharged so violently that it blew apart the metal case!


A 12 V (nominal) lithium iron phosphate battery is made from four 3.2 V cells wired in series. This is referred to as “4S”. This makes the battery a 12.8 V rated battery. The difference between lead acid and lithium iron phosphate is that each cell in a lead acid battery is a nominal 2 V cell but in a lithium iron phosphate battery each cell is 3.2 V. So, a 12 V lead acid battery requires six 2V cells and an LFP battery only requires four  3.2 V cells.


There are currently three different cell form factors being used in drop-in batteries. the first is called prismatic these are square blocks that nest together and most often require what is called cell compression,the Chinese call this a “fixture” . The “fixture” is used to prevent cell swelling / bulging during charging .

The second type of cell that is commonly used in drop-in batteries is called a cylindrical cell this can be anywhere from the size of a AA battery up to approximately the size of a D battery. Cylindrical cells are quite robust because they don’t need cell compression as the cylindrical form factor prevents swelling. The drawback to cylindrical cell batteries is that they typically require lots of spot welds to connect the cells in parallel cell blocks before they be are put into series series.

the third type of cell for drop-in batteries is called a pouch-cell we typically do not advise pouch cells for use on boats where vibration can be high. pouch cells can be easily ripped and often times they are just dropped into an and aluminum housing that can sometimes have sharp edges. pouch cell drop-ins are getting better and these days have less chance of ripping but prismatic or cylindrical is typically better for use in high vibration environments.



Pouch Cells


Clearly I’ve left out a lot of the important details of the manufacturing of a drop-in battery. In order to build a battery properly the cells must be impeccably matched before the the cell block is assembled. By impeccably matched I am talking about cell to cell Ah capacity and cell to cell internal resistance. If the cells are not carefully matched the BMS inside the battery may never be able to keep up with balancing.We have seen this in numerous instances with drop-in batteries.

What is a BMS?

BMS stands for battery management system. A battery management system is used to protect the individual cells inside the battery. Each 12.8 V (nominal 12 V) drop-in battery must use a battery management system. You could also call a BMS a cell protection system as it actually serves to protect the battery cells inside the drop-in battery. The BMS will protect the battery cells from such things as temperature voltage and current. The BMS also serves to balance the cells should they get out of balance. The BMS protects the battery by disconnecting the battery from the charge sources in the loads. Lead acid batteries do not do this.

Insane Market Growth  of Drop-In Batteries

In the last two years the proliferation of lithium ion phosphate(LFP) drop-in batteries has literally gone berserk. Drop-in technology has finally advanced far enough that I believe it’s now worth discussing in an article. I had previously avoided this topic because many of thethe early drop-in products were pretty poorly engineered, BMS’s had weak power handling etc..

Disclaimer: MarineHowto.com does not currently sell any lithium iron phosphate batteries. Health issues (a near deadly hemorrhagic stroke forced what is perhaps the permanent closure of my local electrical & on-line businesses.We are also not involved in any affiliate programs that could potentially sway our opinions! This site, like always, is for your education only. We want to ensure that you do not get ripped off in the field LFP drop-in batteries. What you get here is an expert opinion that is based on many years of experience in this field(since 2007). With no money potentially swaying these opinions, you’ get an unvarnished view.

While the internet is full of folks claiming to know what they are talking about sometimes it is easiest to just use a photo. The image below shows the ABYC marine safety standards that I work on. I have been involved with the lithium-ion subcommittee since it was first formed back in late 2013. I was personally invited onto this committee by the committee chair.

Beyond being a Marine electrician/engineer, I have been using lithium iron phosphate batteries on my own vessel since Spring 2010. And yes, that  bank (built well before drop-in LFP batteries even existed) is still going strong and it still delivers 100% of its original capacity when capacity tested.

I am also a voracious reader & research junkie;

As can be seen from the image below I have a massive collection of technical documents,research papers&white papers. Everything below is in regards to lithium ion phosphate batteries. I run a 32 inch monitor and I can’t even come close to fitting everything onto one page for a screen grab. This LiFePO4 folder alone has 377 PDF documents on lithium iron phosphate batteries. Yes, I have read them all…

LFP drop-in batteries have come a very, very long way in the last few years but this does not mean there are no sleazy manufacturers left out there. How do you avoid 98% of the poor LFP products? Easy, don’t buy directly from China on your own. If you don’t know what I mean by this I would urge you to spend some time on Will Prowse’s YouTube channel but, please don’t focus on his reviews(in a marine application sense), instead focus on how many failures he’s had cutting open & examining drop-in lithium iron phosphate batteries! Please remember that a manufacturer who is sending Will Prowse a battery often knows darn well who he is.. They still fail to send him well-built / well executed batteries many of them lacking cold-weather protection (You can’t charge LFP below freezing) even though they frequently lie and tell him the battery has it.. If guys like Will Prowse can’t pick quality batteries out of China how can you expect the average Joe to wade through all the murky information and get good LFP drop-in batteries directly from China? Hey, I’m not complaining, Will has indirectly sent us a lot of paying customers! These customers have had a number of issues with batteries,cells or BMS’ he’d reviewed. We’ve made a lot of money testing these batteries in our lab, only to tell the customer they had  been sent “B-grade or reject cells etc… This sort of stuff, has been sold as  “A”grade” but, the customer got “B” grade or reject products. This scammery runs rampant on Aliexpress, Alibababa, eBay, Bangood Amazon etc. etc. so be very careful when ordering direct from China because you’re on your own once you do..!

Purchasing- Rule#1

Rule number one for purchasing  lithium iron phosphate drop-in batteries is that you always want to buy from a well established US or North American company!(This site is read world wide but is still a US based company. (Insert Germany UK, Norway, Sweeden etc. for USA), even if that company is having the batteries manufactured in China. Hint: All lithium iron phosphate cells are manufactured in China. Some US manufacturers such as Battleborn choose to import the components from China and assemble them here in the US. This allows them to better control assembly quality. You want, and need a presence here on North American soil (insert your country here)  to ensure that the company can stand behind the product and you are protected by US (your country) consumer laws..

Most of the “bad” images you will see below are what happens when aDIY attempts to become the importer of LFP batteries.These sorts of issues are almost non-existent  where a decent US company is behind the importation design & manufacturer selection process.

Disreputable sellers will lie and think nothing of it. This LFP cell is 100% made in China……

Always Check with your Insurer First!

The image below is from Markel, one of the largest insurers of boats in the USA.

And here’s an insurance questionnaire/form. Lie on this document and guess what ? You’re not covered!

Important:This insurer wants to see a US company, on US soil so they  have someone to go after should an accident occur. This is why they demand a company based in the US.

Batteries such as Kilovault (MA), Lithionics (FL, Battleborn (NV), Dakota (ND), Relion (SC), Mastervolt (WI), Victron (ME), Trojan (CA), Lifeline (CA) etc. would meet this criteria.

Batteries that come directly from China such as SOK, AmpereTime, Chins, AO Lithium etc. Would not meet this criteria.

There are still many insurers who allow LFP but please check with them first!

What About the ABYC:

If your Drop-in battery cannot meet the underlined criteria (Insert “Strong-Opinion”) you  may want to keep looking:


What About the ISO/ European Safety Standards:

ISO/TS 23625
ISO is Pretty Similar to the ABYC only this standard is active now (ABYC E-13 coming very soon)…

“Rod, isn’t the ABYC is a “voluntary” standard?

Absolutely, but here’s the rub. Every Marina in the United States, and most in Canada, require insurance. If you’re vessel is insured the insurance company has standards they expect. In North America those standards are the ABYC standards. They use Marine surveyors to ensure the boat is safe and up to their underwriting standards. Marine surveyors are currently using ABYC TE-13  as a guide for LFP installations and are actively calling out installations that don’t meet the nature of the TE-13 document.We get emails about this routinely.Our answer is, as always, do as your insurer requests as finding another policy especially if your boat is older can prove to be very difficult..

ABYC TE-13 is currently a technical edition (TE)meaning a technical white-paper defining safe installations. TE-13 will be converting to a full-blown standard called E-13 perhaps as soon as July 2022(could be earlier too). I cannot speak to any of the specifics at this point because I am under an NDA (non-disclosure agreement). Throughout this article I will give some “strong opinionson what is safe and what is not. You can take from those opinions what you what you want. My opinions will be based on what the final document may look like.

Be Very Careful With the Marketing!

“Marine Grade”

I’m not intending to pick on this manufacturer but it is a good example of where marketing and standards can clash..  Recently a customer emailed us about the new XXX Brand “Marine” batteries, a brand we had never worked with. He is insured by Markel. A couple of days later he sent us this email:

One would think a battery marketed heavily as “Marine Grade” would actually meet the Marine standards?

  YouTube Reviews

I really do like like Will Prowse and I think he’s doing the general public a tremendous service in cutting open all these drop-in batteries and exposing all the dirty little secrets. We too have cut open a slew of these batteries I just don’t do video or video editing well..  The problem I have with Will is that he does not operate in the marine environment and the marine environment is a different set of circumstances & standards than it is for RV or off-grid cabins. For example, I don’t know a single RV that has a 12V bow thruster that can pull over 600A(no load rating) at 12 V and 1600A+for in-rush LRA/FLA rating. An in-rush like this is capable of ruining some FET BMS boards especially after repeated thrusts.

We have had too high a number of readers & a few customers(who self installed) destroy or damage drop-in batteries with their bow thrusters and windlass motors! The good news is that FET BMS’s are getting bigger and more robust all the time so these issues (wimpy FET BMS’s) may soon be behind us…As you can imagine, it gets very, very expensive when you ruin a battery by applying too much in-rush current.. Be careful taking Wills advice at face value for a Marine application is all I’m saying.

Having been asked over and over what “Drop-In” batteries Compass Marine Inc. prefers, likes or That we believe are well designed, the following two brands are where we stand today.

Lithioncs $$$$$$$-Amazing quality drop-in batteries. Built like no other LiFePo4 batteries!

KiloVault $$– These 100Ah, 150Ah and 300Ah 12V batteries are very, very well built for the price point and include one of the most robust BMS’s of any drop-in product we know of. They also use extremely high quality aluminum prismatic cells and each battery has Bluetooth built in for external communication & “visual” TE-13 compliance. Even the busbars inside these batteries are made of Nickel plated copper. Having torn piles of LiFePO4 batteries apart I can say without a doubt these represent one of the best values there is in a 12V drop-in battery.

Is Your Vessel a Good Match for Drop-In Batteries?

Use this flow chart to find out

An upgrade to lithium iron phosphate batteries can always be done in stages. We typically advise starting with the charging system first, as your on-board items become antiquated or become failure prone. We suggest upgrading any sort of charge equipment eg; solar controllers, alternators or alternator regulators with devices that can be fully programmed for lithium iron phosphate batteries in the future.Doing this as the items begin to become failure-prone means that in the future when you’re ready to upgrade to lithium iron phosphate batteries your system will be ready for it.

Let’s Look at Drop-In Battery Specifics:

The popularity of drop-in LFP has literally exploded in the last 2 years. This is good for LFP batteries as a whole, but can also potentially be bad, if the right drop-in’s are not chosen to properly match your vessel.. There are things that need to be considered beyond just “dropping them in“. The term “drop-in replacement” is a very misleading moniker, as these batteries are far from a “drop-in” replacement for lead acid.

Drop-In batteries are most often sold in standard lead acid case sizes eg: Group 27, 31, 4D, 8D etc.. One of the drawbacks to a drop-in battery is that most of them lack any external communication between the internal sealed BMS and the vessel. Currently the 320Ah Lithionics GTX12V320A is the only drop-in marketed battery that offers external communication between charge sources and the battery.

Most of the drop-in batteries have been Chinese in origin, and this is not necessarily a bad thing, if you’re buying from a reputable manufacturer. A large number of the US available brands eg; Trojan, Lifeline, Relion, Kilovault etc. are buying from a hand-full of premium Chinese factories. The difference between straight up US sticker applicators & Trojan, Lifeline Relion, Kilovault and pure US based sticker applicators is that Trojan, Lifeline Relion  & Kilovault designed the batteries and have them built to their specs just like Apple computer does.

Where drop-in LFP batteries often fail the purchaser is in marine specific engineering. To understand why, we simply need to look at the reason these batteries were originally created. Drop-in form factor LFP batteries were originally designed for telephone pole mounting where light weight and “drop-in” replacements for lead acid were critically necessary for the solar powered street lighting & cell repeater industry. The demand for this type of battery, especially in third world countries, is absolutely staggering.

Drop-in batteries were not invented for the use  you think they were

I know many boat owners tend to assume we are a large market, but we are not, and no, many of these drop-in manufacturers are not specifically building marine batteries for us, though they certainly are marketing to us. The application of a “marine” sticker, and perhaps even a well marketed brand name on the plastic box, does not always denote a product that is well engineered or specifically engineered for use on a cruising boat.

Unfortunately, for our industry, many of the “A” graded LFP cells used in the plethora of Chinese drop-ins, are sold into the street lighting or EV market industries. Please understand that the term”A” grade is really a meaningless-term in China. “A”grade really means EV grade but the Chinese have figured out that people think “A”grade actually means something…It may actually mean “A”bhorrent grade” depending on who you buy from

For boaters buying drop-in batteries direct from China this can mean the low-grade “orphaned” or “rejected” cells wind up in batteries that may look exactly the same but are sold on Ali-xxxx, , eBay or through other less reputable sources.. Once the cells are sealed in its glued together plastic case you the buyer have no way to know what quality cells you got.Below is an example;

Direct from China FAIL! This battery came directly from Aliexpress:

You may ask yourself why I say take Will Prowse reviews with a grain of salt? This image explains exactly why I say that. I have yet to see Will’s reviews include a discharge capacity test of each individual cell in the battery. I know darn well why he does not do this …. Time.. We use the exact same computerized capacity tester Will does, plus a few others, but this stuff takes time, lots of time. The problem with this Aliexpress battery is that the cells had a 8.3% variance in their Ah capacity! This battery tested at 100% capacity on cycle #1. On the recharge is where the problems showed up. As a result there was no way this particular BMS could ever keep the cells in balance. The bottom line is this battery was built from reject /grade B cells that never should have been in a battery that was marked or sold as “A” grade! We’ve done lots of testing for viewers of Will’s that confirmed the cells or battery they got from Aliexpress were not actually A grade or EV grade cells.There are not many companies capable of doing this testing, so this has meant quite an in-flux for us in the last few years.

Sadly Ali-xxxxx has literally become the numero-uno dumping ground for reject LFP cells and batteries from the Chinese factories. They get away with this because they know the vast majority of buyers have no way properly to test them once the battery case is glued together. Frustrating? You bet it is! Always buy your LFP batteries from reputable companies not directly from China unless you love to gamble.

The internal BMS in this battery was completely incapable of balancing the cells & one high cell kept tripping the BMS on high voltage. 3.65V is the maximum safe cell voltage for a LiFePo4 cell (the image above has two cells in the danger zone). We spent about two hours reprogramming the BMS after multiple emails back & forth with the manufacturer who refused to give us the BMS passcode so we could custom program it to make it work better for poorly matched cells. We then tried to charge it and to get the cells balanced for three straight days. Nothing we did could balance the cells so this battery would not trip the BMS on cell high-voltage.Please note the recommended charge voltage was 14.6V and even at 14.2V we have two cells well into the danger zone.

The customer finally gave us the okay, after emailing with the manufacturer,  to open the battery and test each cell individually. This required the complete destruction of the battery case because it had been glued together with what appeared to be superglue. The only way to get open was to cut it open. When we finally capacity tested the cells it was discovered that there was an 8.3% variance in cell to cell  Ah capacity on these four 100 ah cells.There was no way this BMS was going to keep up with that variance, balancing wise.

Once the manufacturer had the testing data they then backpedaled on the warranty and refused to take battery back because the battery had been cut-open(which they had said was ok). The customer was out all the money he spent on shipping plus the battery costs direct from China(His take was that as an Aliexpress buyer you have no rights and no recourse). He was also-out  about $380 in labor for our shops testing costs. In the end he wound up buying two KiloVault batteries and has had zero issues since..

Drop-In LiFePo4 – Important Points to Consider:

 BMS Current Handling 

The current rating of the internal switch that protects the battery is quite often too small for the task on many cruising boats. Drop-in LFP batteries routinely use multiple tiny little MOSFET switches as the batteries BMS protection ON/OFF switching. Unfortunately these FET’s often can’t handle the typical loads imparted by many cruising boats. On board devices such as bow thrusters (400A +), windlass’ (100A to 300A+, large inverters 150A to 300A +, electric winches 75A to 300A +, electric cook tops, massive alternators, chargers or large inverter-chargers are very very common on-board cruising boats these days. These are exactly the devices many boat owners are hoping to see a gain in performance from when switching to LiFePO4.

This is what a 120A rated FET based BMS looks like it is pretty for a typical 100Ah drop in battery. This is what an internal FET based BMS boards typically looks like with the heat sinks removed. The blue wires connect to the neg end of the cell string and the black wires are connected to the external negative battery post.fet bms’S DISCONNECT ON THE NEG SIDE OF THE BATTERY This one uses two 10GA wires for its 120A continuous rating. All 120A has to pass through those two 10AWG wires, the printed circuit board and the FET’s. The hotter FET’s run the shorter the MTBF (mean time between failure) is. This Particular BMS ,a JiaBaida, uses 32 FETS(the board has double sided FET’s.  We have cut open drop-in batteries with 100A rated BMS’s ( that use only 10 FET’s.(see image below this one)..

What a FET BMS Looks Like

The Miniscule BMS below came out of a customers AliExpress  drop-in that was sold as a 100A continuous BMS. It was also sold as Hot and cold temp protection. As can be seen the temp sense port(only one port not two.) is not even soldered to the BMS’s PCB. Hard to have hot and cold BMS protection when the sensor ports are not even installed. These are the “Lies” we talk about when flying solo and ordering directly from China..Oh and this was wired to the batteries neg terminal with 12AWG wire…! 12AWG/100A!

Do You know the quality of the FET’s ?

What if I have High-Current DC Devices?

If you own a vessel with high load devices, do yourself a favor and look at the contactor ratings (the BMS protection switch) that “marine specific” companies such as Lithionics/OPE-Li3, & Mastervolt use for their”marine specific” LFP batteries. What you’ll often see is a 500A continuous rated Gigavac, Blue Sea ML-RBS, Tyco EV-200, or in the case of Lithionics, military grade 500A contactor/relays are being used as BMS protection switches.

Compare that to some of the drop-in batteries being sold out there which can have relatively low-current handling capability due to the use of often under-designed( for a cruising boat)FET based switches. The manufacturers building “marine specific” batteries eg; LFP Mastervolt OPE-Li3, &Victron know what a typical cruising boat needs in terms of current handling and they engineer this into the product. Does this mean you need to use a “marine specific battery? No, it does not it just means you need to choose your drop-ins very carefully for your vessel.

A Dead FET BMS (windlass in-rush)

Below is the BMS “switch” used by Mastervolt on their MLI series LiFePO4 batteries. It can handle bow thrusters, large windlass motors, massive inverter-chargers, massive alternators. etc.. The ML-RBS  switch is rated for 500A continuous, 700A for 5 full minutes and 1450A for as long as 30 seconds. While many smaller boats can often get by with a FET based BMS, not all boats will, so please consider the max continuous discharge and recommended charge ratings of the battery you are purchasing. This rating is not usually limited by the cells but rather the internal BMS’s current handling capabilities.

Let’s take a look at the BMS switch Mastervolt  uses. (Lithionics uses a similar switch on their external BMS batteries.

You read that correctly; 500A continuous or 1450A for as long as 30 seconds!!!

Sadly, when buying direct from China,you can still find diminutive 50A continuous rated FET switching BMS’s installed inside a 300Ah LFP battery. As a comparison a 150Ah KiloVault can handle 150A continuously or 200A for as long as 30 minutes. While a 50A rated BMS may be fine for small boats,  if you have large on-board DC loads, or want to charge a 300Ah battery quickly, then a battery like this is going to be a less than ideal battery for marine use. So, you still really need to do your homework to make sure the batteries & the internal BMS are a fit for your vessel.

Building a Drop-In Bank to handle Large DC Loads

When it comes to FET based BMS batteries we typically advise smaller individual batteries, wired  in parallel. This is done to share the load across the FET based BMS’s. For example three 100Ah / 1C rated LFP drop-ins can theoretically handle a 300A discharge, if the parallel wiring is perfect and all batteries share the load equally(rarely happens that way.) A 300Ah 8D format drop-in, like the one addressed below, can really only handle a 100A (0.33C) discharge. When in doubt with FET based BMS systems smaller batteries in parallel are usually a better solution than one large battery with a low current rated BMS.

You Need to Read The Specs Carefully!

Purchase FAIL!

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

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

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


The specification also claims 2000 100% SOC to 0% SOC cycles, not too dis-believable for LFP. However they then claim 20,000 cycles at the end of the spec! This is at least double what any reputable LFP maker claims! It then claims “fully charged in 60 minutes“.

Fully charged in 60 minutes” Holy $hite, that’s fast, but just to be safe, lets do the math..

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

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

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

You read that math correctly. The only way we you can get this battery to charge in 60 minutes is if you only discharge to 83.4% SOC……. So much for those deep-cycling &  “fast charging” LiFePo4 batteries? My point here is to help you learn to dig deeper into the specs so you can learn to spot bogus claims.

If you’re less than educated on a subject, drop-in battery makers will try to sell you anything you want to believe. Educate yourself and do the research

Another Purchase FAIL!

The image below is a prime example of how boat owners, without enough knowledge, can get burned buying LiFePO4 batteries. We were consulted by an owner who purchased a 300A drop-in battery from what he thought was a “reputable manufacturer“. During the transaction, he had no consultation with the manufacturer and no questions were asked by the re-seller. He just ordered it based on it’s “8D drop-in” format, the claim that it was an exact drop-in replacement for his lead acid Lifeline 8D battery, and the 300Ah capacity rating.

He felt comfortable because it was, what he considered, a “reputable manufacturer” and they are based here in the USA. He quickly destroyed three alternators and the BMS kept disconnecting when he was inverting with his large 3kW inverter/charger. The BMS disconnecting while charging also damaged his inverter/charger. When I pulled up the spec sheet on the 300Ah drop-in battery he’d purchased, the problem became crystal clear. It is highlighted in yellow below….

You are reading that correctly, this massive 8D form factor LiFePO4 battery was only capable of a 100A discharge and a max short duration charge of 100A. To keep the BMS cool, and the cells balanced, the manufacturer has a “recommended” charge rate of just 15A to 50A for a 300Ah battery. This 15-50Ais not a limitation of the cells inside the battery it is a limitation due to the FET based BMS that is used to protect this battery.

PURCHASE FAIL! For this particular application this drop-in was a horrible fit. A fault of the battery? No, not at all. This was a shared failure in the marketing, the retail chain, and of the owner. I partly blame the owner here because he failed to do the research and fully comprehend the specifications of what he was actually buying. Of course who can blame him when these batteries are boisterously marked as “drop-in replacements“. As can be seen from this example these are ABSOLUTELY NOT drop-in replacements for a lead acid 8D battery as his Lifeline 8D’s never once disconnected themselves from the alternator or the inverter…

What EVERY Drop-In Battery Spec sheet SHOULD Look Like

*Highlighted specifications are the critically important ones.

High Voltage Disconnect (HVD) Cut-Off Protection: This is critical to know because it is the voltage at which the BMS will disconnect the battery from the vessel.This is usually specified on a per cell basis. So a 3.65V disconnect on a 12V battery would be 3.65V X 4 cells=  14.6V BMS Disconnect

Minimum Absorption Voltage (to Initiate Cell Balancing): This is important because you need/want to activate balancing with each charge cycle. You also want to avoid pushing to the maximum charge voltage every cycle if you wish to maximize cycle life.

Maximum Absorption Time: Again, This one is critical to cell longevity. If it suggests a maximum absorpyion duration of 30 minutes you had better make sure all your chargers can be programmed not to exceed this..

High Internal BMS Temperature Charge Cut-Off- This is one you have little control over other than to not push your BMS near the charge-current limits. It is always best to charge at no more than the “recomended charge-current. 95% of the load-dump damage we’ve seen is not due to high voltage cut-offs but rather from a BMS disconnecting due to BMS Temp. Be Sure your manufacturer specifies This!
High Internal BMS Temperature Discharge Cut-Off:  same as above but for discharging.

Delay until Peak Discharge Overcurrent Protection Cut-Off: This spec, from a reputable manufacturer ,will almost always be followed by the millisecond rating (ms)before disconnect eg:320A 8ms This would mean the BMS will disconnect if it sees 320A for 0.008 seconds or more. This is why knowing the in-rush draw of all DC Motors is critical before installing LiFePo4 drop-ins.

Recommended Continuous Charge Current: Always follow this guidance not the “max charge current.The reason for this lower number is to keep BMS temp down and to allow balancing to keep up. Many of these BMs’s only have 20-390mA of balance current to work with! If the celllsget out of balance & you are fast charging the BMS may never be able to keep up!!

Maximum Parallel Configuration : (Identical Model Batteries): Do not exceed this number!

Maximum Series Configuration:(Identical Model Batteries):Do not exceed this number!

If you are unfamiliar with what the specifications mean or why they are critical you may want to reconsider drop-in batteries until you have completed the research phase.


Many of the very cheaply sourced drop-ins are using 18650, 26650 or 32650 cylindrical cells inside the battery case. In a worst case, a 100Ah LFP battery, built from 18650 cells, would need a grand total of 364 cells with two connections per cell.

Hows that math work?

18650 Cell = 1.1Ah (typical Ah rating for an 18650 LFP cell)

91 Cells Make Up Each 3.2V cell

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

91 X 4 = 364  18650 Cells

Positive & Negative Connections Inside The Battery = 728

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

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

A Look at the BAD of LFP:(most of these images are “direct fromChina” purchases).

Do you suppose this Rube Goldberg level Ali-xxxx LiFePO4 drop-in battery manufacturer, and I use the term "manufacturer" sarcastically here, has paid to have this battery vibration tested?
                                 Image courtesy MHT Reader

No  Not Kidding!

                                                     Image courtesy MHT Reader

Heck the guy assembling these cells, most likely in his mom’s spare bedroom, can’t even solder well or use a spot welder with any level of quality or precision. Vibration testing? Only if they are flat out lying about it. Purchasing LFP anything on Ali-xxxx is a very strong buyer beware!

This LFP”starting battery” has NO BMS Protection!! Balancing only!!

                                                                     Image courtesy MHT Reader

Starting a 44HP Westerkeke takes…….640A!!!!

What’s Wrong here?

OK, I’ll Help Out.. Look closely at the series connection below!!!!

An 18650 cylindrical-cell battery failure

How did we discover the spot weld failures? The zipper like discharge graph was a dead giveaway..After a discussion with the manufacturer we had to tell the customer to stop using his bank immediately…It had also lost significant capacity from over charging.His lead-acid charger that held 14.6V way too long. We were testing them for capacity when we discovered the spot weld failures (brand purposely obscured). In the screen grabs below you can see how varied the voltage was on discharge.We had wanted to run the discharge at 40A but the zippered graph was even  worse at 40A so we ran the capacity test at 10A..

Data point =12.582V

Data point = 12.702V

What happens when you cram multiple small wires into one terminal and ask them to carry 100A +/-Hint: You get terminal melt down..

And here’s what they look like when you hit them with thermal imaging.

This is the level of “quality” you may find inside that beautiful plastic case

Because you can always trust the sticker on the outside of the battery

More Direct from China misleading BS


A FET  BMS has aluminum heat sinks because it needs cooling. Stuffing it in-between foam blocks is well…… not a wise solution.

But, some dude on the Internet said LiFePO4 is 100% safe…..

No battery chemistry is 100% safe, especially when you over charge it.(However no-fire , no-flames & no explosion just cell swelling and heating… FWIW this “starting battery” has zero BMS Protection!!

                                                                        Image courtesy MHT Reader

Do you know what horrors are hiding in that”direct from China” plastic box?

The Battery Below(image5) is typical of the Quality you’ll often find on Amazon or Aliexpress. So, what happened?

1-BMS is Catastrophic protection only. If a BMS allows cells to charge beyond 14.6/3.65VPCWalk away!!

2- BMS Allowed cells to hit 15.6V /3.9VPCbefore disconnecting!max safe cell voltage is 14.6V//3.65VPC

3- Cells were not in a case that provides cell compression to limit cell swelling

4-Cells balooned and split the battery case open- Battery ruined.

Aluminum Prismatic Cell ruined by over-charging

*image5Cells ballooned/swelled  due to over charging. This BMS was catastrophic protection only and allows the cells to hit 15.6V before the BMS disconnects (this is enough to ruin the cells and totally brick the battery)

This what proper cell compression for prismatic cells looks like looks like! Has your chosen manufacturer included this?

The “Bad”images above are a reality of what you can often find buying your batteries directly from China without a reputable company insulating you from these horrors…

One last warning about buying direct from China

What Better Quality Looks Like

Compare the above cell block from Ali-xxxx to the photo below of a Lithionics g-31 drop-in battery. This battery uses impeccably matched aluminum encased 5c LiFePO4 cells. The cells are fixed in place by an injection molded jig that protects them from movement and vibration failures. The busbars are high grade nickel plated copper and self locking flange/wizz nuts are used to hold the cells to the busbars. The BMS used in this battery is certainly FET based but it is made here in the USA, of Mil-Spec components, and is designed to handle starting engines. There is a huge variance in the quality of LiFePO4 drop-in batteries. Yes, this battery is more expensive than a 100% Chinese made drop-in, and they are only sold after a consult to ensure they are the correct fit for the vessel. Bare minimum alternatives to the Lithionics would be the KiloVault HLX Series. A lot of folks swear by Battleborn but the Battleborn batteries to not yet meet ABYC requirements for E-13.E-13 will be official in about 60 days from this article being published.

Internal Wiring 

It is not uncommon to open a 100Ah drop-in battery, rated at 1C, and find a single 10GA or 12GA wire feeding the main positive and negative terminals. When someone finds a 10GA or 12GA wire rated for 100A, under any safety standard, please let me know?

This is how Lithionics does it on their 4D 320Ah drop-in.

BMS Shortcomings -Lack of low or high-temp Protection

Some of the drop-in batteries may lack  BMS temp protection altogether . Drop-in batteries should have both low and high temperature protection (a requirement for both ABYC and ISO) but many don’t. Far too many drop-in batteries lack low temp protection and a large number of manufacturers who claim it has low temp protection are actually lying about it. If  You live up North,buyer beware!

Non-Communicable BMS 

This one is perhaps the most frustrating aspect on-board a cruising boat. For a trolling motor, who cares? It’s not powering anything critical. For a house battery, on a cruising boat that ventures off-shore, and is powering critical navigation and safety equipment, this can create a dangerous situation. A non-communicable BMS is one that can not communicate externally with the vessels charge and load systems, or even you the owner. It has no means of externally communicating or sending/sounding warning alarms or activating relays/triggers to properly and safely disconnect charge sources or give ample warning of an impending BMS disconnect. Some batteries are now featuring Bluetooth monitoring but this still requires you the owner to be watching it.

Let’s take a look at one of the worlds most respected marine standards for shipping etc., Bureau Veritas.

As can be seen, under Bureau Veritas standards external communication between the battery and the rest of the systems such as charging is a requirement. For why see below.


LFP batteries are charged using a CC/CV profile. This means constant-current/constant-voltage

Bulk = Constant-Current(charge source working flat out  or as hard as it can)
Absorption = Constant voltage( voltage is held steady for a short time or until current declines to the manufacturers spec.
Absorption Duration = Once the batteries have achieved the absorption voltage the time the batteries spend  at this voltage must be limited. Many lead acid charge sources spend far too long in absorption and this is not healthy for LFP



Email from MHT Reader:


The alternator for the Volvo MD2030 with 300 Amps LiFePo4 14.6 max lasted a few hours. I believe BMS was switching on to off  I to keep theLiFePo4 voltage to safe measure? Boat service replace alternator and it happens the second time? I now read your story on lfp and it explain to me why.”

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

BMS load dump illustration
What a voltage transient looks like:

The load dump transient captured in the above image is from an ISO test of a 12 V automotive alternator. Of important note is how quickly this transient surpasses 90V. The transient surpasses 90V in just 0.01 seconds!

   A) The alternator diodes, unless avalanche style, (rare in many existing marine alternators) but all Balmar alternators now use them, can be blown and the alternator can be rendered non-operable. Two years ago we  did exactly this. Using the alternator test bench here at CMI the 90A  alternator was running at full bore charging an LFP battery. The “system” I set up had a .3A dummy load on, light bulb, to simulate a depth sounder. With the alternator running at full bore I disconnected the battery, just as an internal sealed BMS can do for BMS temp, cell diff-voltage or cell high voltage. Poof went the alternator diodes and the light bulb was burned out instantaneously! Worse yet the voltage transient I recorded on the “load bus” (think your navigation electronics) using a Fluke 289 was 87.2V. Ouch. Even if your alternator uses avalanche diodes, like Balmar’s do, the voltage at which they begin to protect the alternator is far too high for the vessels load bus equipment so you still need a way to protect against a load dump.

   B) If the boat is wired, as is typical with drop in batteries, the voltage transient caused by the open circuited alternator will now directly feed the DC mains and potentially destroy your navigation equipment.

TIP: At a bare minimum, every drop-in LFP battery bank, that can be charged via an alternator, should be installed with an Alternator Protection Module!

The Balmar Alternator Protection Module is an inexpensive insurance policy against a BMS load dump destroying your alternator. If you can afford to purchase drop-in LFP batteries you can also afford to protect your alternator from a BMS load dump.

A well designed marine specific BMS would open a relay that can de-power your charging sources on the input side, thus shutting the charge sources down correctly and safely with no risk of a damaging voltage transient. For a large inverter/charger it would de-power the AC input side, for an alternator it would de-power the field wire or regulator B+, for solar it would open a relay in the PV feed etc. etc.. With a drop-in battery, that features a sealed BMS, you have no way to do any of this. Only Lithionics Drop-In Batteries have this Capability.

But Rod, I plan to program all my sources below the BMS disconnect voltage.

Sounds like a good plan right? Well, lets examine the reasons a BMS can disconnect

This image sums up why programming a lower charge voltage cannot always protect against a BMS disconnect.

Are  BMS Load Dumps Real?

I’ll  let Balmar Explain this;

How about a fairly knowledgeable owner who bought a very beefy alternator and still killed it due to BMS load dumps.

Drop in batteries with the exception of Lithionics do not have a way to properly shut down the alternator before the battery disconnects. For this reason we need work arounds..

The number one reason we see batteries shut down(when everything is programmed correctly) is almost always due to BMS temp related issues not necessarily  high cell voltage..

A good technique to mitigate load dumps is to keep a buffer “load” on the charge bus at all times (Buffer load = lead acid battery on the systems charge bus see FET Isolator wiring below). With FET isolators we like to see them at least double the rating of the alternator eg; a 200A ARGOFET for a 100A alternator. The cooler FETs run the longer they last.   And yes, we have seen FET isolators fail…

Load Dump Work-Around’s

Using Low Volt-drop FET Isolators

Using Low Volt drop FET Isolators


There are many benefits to using DC to DC chargers. One of those benefits is that the charge profiles can be custom configured to charge lithium iron phosphate batteries where your factory alternator or legacy lead-acid charge equipment cannot be programmed for this. The Victron and Sterling power DC to DC chargers can also absorb a load-dump from a BMS disconnect where your factory alternator cannot.

However, caution needs to be used when sizing DC to DC chargers. A DC to DC charger should be sized at a maximum of 50% of the factory alternators rated output. This means if you have 100 amp factory alternator the maximum DC to DC charger you should use is 50A. This will help keep the alternator cool and keep it from burning itself up. Currently there are only two DC to DC chargers we recommend and those are Sterling Power and Victron.

The only drawback to using DC to DC chargers is that you give up charging your lithium ion phosphate batteries quickly. Seeing as that is one of the major benefits of  LFP batteries we would strongly advise considering an externally regulated alternator with an external regulator  such as the BamaMC-618 or Wakespeed WS-500these regulators can be programmed for LFP and have an alternator temperature sensor to protect the alternator from heat damage. This will also result in considerably faster charging!

Caution:I don’t often do this as I hate to ever advise against any manufacturer but there is one manufacturer that we would urge a very strong caution on and that is Renogy. We’ve not seen a manufacturer with this many failure prone products in 30+ years in this market. Their failure rate is far too high for us to even consider making a recommendation. Consider yourself warned about buying Renogy.

You can read about  DC to DC Chargers HERE.

WARNING:Do not size a DC to Dc charger at any more than 50% of your stock alternators rated output!

The drawback to using DC to DC chargers is that it results in SLOW LFP charging

Using a Victron Orion TR Smart DC TO DC Charger (Be sure it is not the Orion TR- it must be the TR SMART)


The short answer is we do not advise this for charging lithium iron phosphate batteries directly.You can however use your stock alternator if i it is behind a DC to DC charger that serves to protect it and that provides the proper charge profile for the lithium iron phosphate batteries.


1-A stock alternator rarely has the correct charging voltages for lithium iron phosphate batteries.
2-They can over absorb the batteries resulting in over-charge damage
3-The absorption voltage is very often too high (see below)which  can lead to BMS load-dumps
4-Stock alternators do not FLOAT, they only do bulk and absorption.
5-Alternator heat damage

Do You know the voltage set point of your stock alternator?

Please understand that we have been an alternator manufacturer for more than 15 years so we understand internal vs. external regulators and how these alternators are built. We also have access to data, such as you’ll see below.This is data the average DIY would never have access to.

The max charge voltage for any drop in LFP battery is 14.6V(some are much lower). Below is a sampling of common internal voltage regulators. Pay attention the voltage set points!

 IF your BMS Disconnects at 14.6V / 3.65VPC a reg with a 14.6V set-point is likely to cause you BMS disconnect issues.

These are the most common regulators found in the very popular Delco 10/12Si series Alternators

There is also the Potential for Alternator heat damage!

LFP batteries have a tendency to enjoy eating alternators for lunch. The internal resistance of LFP batteries is extremely low resulting  in very long bulk-charging times. As a result alternators can burn themselves up trying to charge these batteries. I’ve said this many times before on the site and I will say it once again, there is no such thing as a small frame alternator that is continuous duty, I don’t care who built it!  Because Compass Marine inc. is a manufacturer of marine alternators so we get to see these failures regularly. We are not an n=1 data point like the “dude on the internet” who says your stock alt will be fine charging LFP. We have seen far too many alternators completely melted down by LFP batteries to ignore this information..

If you insist on using your stock alternator we would strongly recommend that you put it behind a DC to DC charger (50% smaller than your alternator amperage rating. This will help limit the amount of work the alternator is doing and protect it from a meltdown.Doing this means you can continue to use your stock alternator.

If you expect to charge lithium iron phosphate directly from the stock alternator without a DC to DC Charger in-between, we advise not changing a thing. Do not increase the wire size  to the battery bank ,do not move the volt sense a wire do not touch the factory wiring .zDoing so can result in an alternator meltdown. The typicalfactory wiring on these alternators is horrible and results in a lot of voltage drop. That in and of itself can help protect your alternator from melting down.

Why does LFP cause heat damage?

It is very simple your alternator never catches a break!

Our assembly bench on a typical day..

Don’t be this guy!

ImageCourtesy MHT reader

This stock Yanmar/Hitachi alternator was destroyed in a matter of weeks with just three 100 amp hour drop-in batteries.

Another burned up stator from charging LFP

Oh crap another one!

Jesus H…..! Another LFP Cooked alternator

But Ample built good alternators? Yes, they did but during this vintage Ample Power did not believe in using an alt temp sensor on their regulators.. When LifePO4 came around…Toast!

Series Wired Systems ?

In a parallel wired bank one battery BMS dropping out only creates problems when it re-engages into a different SOC than the rest of the bank by causing a large in-rush. With a series bank (for 24 V 36v or 48V a single BMS taking itself off-line spells disaster at sea and takes out the entire bank. I know a Drop-in owner who hit a granite bridge abutment in his electric boat using a 48V series bank of drop-in batteries. It did a few thousand in damage to the boat, and his pride, but it could have been much worse. The owner had zero warning the battery was about to disconnect itself before he lost all propulsion power. This failure occurred going under a drawbridge in a very strong tidal current. This is but one reason why the ABYC & ISO European standards make sense. Kilovault will soon be out with a communication system for series wired batteries so they stay in balance.We do not advise series wired drop-in batteries unless the BMS’s can communicate with one another. Parallel batteries stand a much better chance of remaining in-balance, series batteries do not unless the BMS boards can communicate with one another.


Please don’t assume every drop-in battery BMS will manage your battery for maximizing cycle life, it may not do that! The BMS in far too many drop-in batteries is designed for catastrophic level protection only. Catastrophic protection means the BMS is only there to protect the cells from thermal run-away conditions. They can have BMS disconnect voltages exceeding 15V for a 12V nominal battery. The max safe cell voltage for an LFP cell is 3.65V X4 cells = 14.6V max. It is up to you, the owner, to ensure the battery never exceeds a safe operating envelope  even if the BMS allows for this. Well built drop-in batteries use an internal BMS that actually protects the battery from a maximizing cycle life perspective. Batteries built to maximize cycle life will have much more conservative HVC and LVC voltage levels.

TheBMS on this battery is built to maximize cycle life:

Never purchase a drop in battery that allows the cells to exceed 14.6V/3.65VPC or that disconnects below 10.0V/ 2.5VPC

#9 Understanding Cycle Life Claims – When an LFP cell manufacturer rates a cell at 2000 100% DoD cycles this is; charge to target voltage, stop immediately once you hit that voltage, discharge to the low voltage threshold, repeat, repeat, repeat. If this target voltage for cycle life testing is 14.6V they charge to 14.6V, stop immediately and discharge. These cells, at this rating, are not held at a the target voltage for cycle-life testing. In other-words you may not get the claimed cycles using a lead acid charger that holds an absorption cycle timer orcharges differently than the way the cells were tested.


LFP batteries are charged using a CC/CV profile. This means constant-current/constant-voltage

Bulk = Constant-Current(charge source working as hard as it can see burned up alternators above)
Absorption = Constant voltage( voltage is held steady for a short time or until current declines to the manufacturers spec.
Absorption Duration = Once the batteries have achieved the absorption voltage the time the batteries spend  at this voltage must be limited. Many lead acid charge sources spend far too long in absorption and this is not healthy for LFP.

Do you know what this means?

max charge voltage 14.6V

max charge current 20% of installed Ah Capacity

When at 14.6V all charging must stop when accepted charge current has dropped to 0.02C or 2% of installed Ah capacity

Can Your existing charge system do this?

Pay attention to the details!

When installing these LFP batteries in parallel the max charge voltage is just 13.8V-14.2V   (it’s 14.6V for a single battery”details”)
Max charge current is 50% of installed Ah capacity or .5C.
When at 13.8V – 14.2V and charge current has fallen to 5% of installed Ah Capacity all charging MUST STOP

Can Your chargers do this?
Can Your charge sources be programmed for these parameters?

*Drop-In Charge Voltages – Follow the manufacturers guidance!

Some drop-in batteries are not using cells that are impeccably matched. Lithionics would be an exception to this rule but they are obviously a lot more costly.

Unfortunately, with most “drop-in” batteries you don’t really know what is inside, or how well matched the cells or cell blocks are. You’re essentially shooting darts with a blind fold on. Lithionics and Kilovault are in a very small group of manufacturers that take quality & cell matching to the level it should be. Lithionics can actually supply a performance test sheet for every cell in their drop-in batteries.  Battheborn matches individual cell modules(they use cylindrical cells) but not each cell in a module.

Series Solar Warning!

Over the last few years on boats one of the trends that can be a little terrifying has been that solar panel array voltages have been creeping up and up.. Many boat owners want to install their solar panels in series and then run them through an MPPT controller to maximize the energy capture of the array.

This is all well and good until there is an issue and the MPPT controller fails. Imagine what happens if you’re MPPT controller fails and starts passing PV voltage through to the batteries? If your array is over 60V & these are lead acid batteries they will eventually explode. If they are lithium iron phosphate drop-in batteries you will toast your BMS! Once the BMS is been fried by the solar array voltage you have no BMS protection & the solar array will continue feeding dangerous voltage to the batteries until they are destroyed. You can imagine what will happen if this continues to go on after in an MPPT failure. In case you’re wondering yes, these failures have happened and lithium iron phosphate batteries have been destroyed due to this. These failures almost never occur in tier-top tier supplier MPPT’s.

How do you avoid this?

#1-observe the maximum number of series batteries you can wire for. With most brands limit this is 24 V or 48 V. This voltage is typically the maximum SAFE voltage the battery bank BMS CAN handle. So, your PV array should not exceed this voltage.

#2 if you wish if you wish to use series-solar on your vessel you will be safer to split the array into smaller series strings that remain below the batteries maximum series allowable voltage and give them each their own MPPT solar controller.

#3 Use only top tier MPPT suppliers (eg; VICTRON, OUTBACK, MIDNITE, MORNINGSTAR ). These controllers use isolated input/output and  are designed not to fault in a manner that passes full PV voltage through to the batteries..

*Pack Voltage vs. Cell Voltage:

Pack voltage tells you nothing about cell voltage as can be seen below!

Know your loads before you buy!

The critical load data you need to know is the in-rush current for all DC Motors .This includes a windlass, electric winches or a bow thruster. You also want know your inverters Pre-charge in-rush.  Unfortunately most DC Clamp meters cannot properly capture DC in-rush current. We own three DC clamp meters that claim to do in-rush but all except the Fluke meters fail miserably. The image below is one of our Fluke 376 meters capturing the in-rush current for a Lewmar V2 Windlass. This customer ruined his FET BMS (seen in an image above in this article) byusing his “direct from China” drop-in battery to power his windlass. Warranty? Ha-ha now that’s funny….

The image above is a prime example of how drop-in battery bank went wrong for this customer. he wanted to lighten the load in the bow of his sailboat so he installed a single drop-in battery to power his windlass.What he failed to understand was the BMS’s current handling rating . In just a few short weeks he destroyed his drop-in battery with his windlass when he failed to account for what the peak in-rush current handling of the BMS., Warranty? Not covered!

1) Balance current-The sealed internal BMS’s in most drop-in batteries don’t have a lot of balance current to work with, usually mA level currents for balancing. We have even seen some BMS specs suggesting they can only balance the cells at a maximum 10 – 30mA or just 0.010A to 0.03A.If you’re running a 200 or 300Ah @12V battery the cells had better be well matched or the BMS may not be able to keep up….Again, Only buy from reputable Vendors!

2) Ballancing –Ballancing Does not usually start until the cells are exceeding 14.4V or 3.6V per cell. Some are slightly higher and some slightly lower, just depends upon what you bought. Where the cells begin balancing MUST always be specified!If you don’t see this spec ask the manufacturer.. This means that in order to ensure the cells stay in balance they need to get to a balance level at each 100% SoC charge cycle. The reason drop-in makers suggest such high voltages is because balancing is typically done at the top-of charge with a FET based BMS.  KiloVault batteries begin to balance at 14.0V(pack) or 3.5VPC.This is is excellent for cell longevity and iswhy Kilovault can claim 5000 cycles @ 80% DoD….

3) Absorption Duration –The manufacturers, for obvious reasons, want a short absorption voltage duration, some as short as just 2 minutes but many demand less than 30 minutes. With mA level balancing current, two minutes is not a lot of time to re-balance cells so they depend upon the battery getting to the balance voltage with each excursion to 100% SoC. If it does not get to a balancing voltage, the battery cells can become out of balance and the FET BMS may never be able to catch up with out of balance cells..


#1 Externally communicable BMS; at a bare minimum Bluetooth FOR ABYC  “VISUAL”COMPLIANCE.

#2 BMS current handling(in-rush data): You’ll need an internal BMS  capable of handling the amperage’s found on your  boat. If you’re vessel has large DC loads such as electric motors powering winches a windlass or a bow thruster you need to carefully confirm that the drop-in batteries you’re buying can handle these loads. The proper measurement of these motors is not the wattage rating it is the in-rush-current.

 #3 UL Testing -Bare Minimum=Individual cells that have passed UL testing

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

#5 Verification of internal cell matching. Currently Lithionics is the only drop-in battery manufacturer I know of that can physically send you the cell matching testing data for each cell in a battery. With only the batteries serial number, Lithionics can print this report and send it to you. This is the type of data that every drop-in battery maker should be able to provide.

#7 Internal wiring gauge & temp rating specifications

#8 External BMS alerts that can externally warn of a trend towards a disconnect.(Both an ABYC and ISO Requirement)

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



Legitimizing LFP

It’s not just the Chinese who realize they can grab market-share with LFP. After having their “deep cycle“clocks cleaned by LFP, both Trojan and Lifeline battery, two major lead acid players, have entered the LFP market. Having actual lead acid battery manufacturers in this marketplace actually lends credibility to LFP. These manufacturers can no longer ignore it as they have dug their own graves by misleading customers about cycle-life. The manufacturers lifeline and Trojan have partnered with in China are two of the finest drop-in battery manufacturers there are, these are, not elcheapo’s. In the end this is good for the market! Xantrex also now has a battery. Xantrex is no slouch as their parent company is Schneider Electric..

Victron is Also in the Drop-in Market

LFP WARRANTIES ARE Nothing more than lawyer Speak!!

LiFePo4 Marketing:

Each XXXXX brand Battery is Protected from over-heating, over charging…”

Warranty exclusion reality:


Exclusion:Damage due to over-charging

vs. the Marketing ; 

“Each XXXXX brand Battery is Protected from over-heating, over charging…”If you’re wondering how a battery that has a BMS that “protects from over-charging Can be “over-charged” it is pretty simple.”OVER CHARGING CAN BE FROM “OVER ABSORBING”! Lead acid chargers are notorious for over-absorbing !

Of the price-point drop-in batteries, Battleborn is quasi putting their money where their mouth is. They back the battery with a 10 year “manufacturing defect” warranty. (KiloVault is 7.5 years) Please understand that Battleborn is not a 10 year warranty that covers any sort of cycle life. This warranty only covers manufacturing defects. We see it repeated over and over that Battleborn(10 year) or Dakota(11 year) have the best warranty in the industry but that warranty only covers a defect in manufacturing! Defects in manufacturing typically show up pretty quickly. Lithionics for example actually puts a cycle-life warranty on their batteries(currently the only manufacturer we know of that does this-Let us know if you find others).

That said, kudo’s to Battleborn,Dakota, KiloVault and others who put a decent “manufacturing defects” warranty on their batteries. The internal build quality of the Battleborn, is  decent compared to many batteries at this price level,but the Kilovault we believe is better and is less money. We have/cut open a slew of Chinese LFP imports and what’s inside can be HORRIFYING! The only reasonably priced non USA assembled battery we have cut into that we find to be extremely well built are the KiloVault batteries.

Will Your Manufacturer even be around to honor a 5+ year warranty?

The two brands below no longer exist…..

Have you done enough research on a manufacturer?

Could this be a reason why one of the brands from above disappeared?


Float Charging

Float charging is a relic that’s left over from lead acid battery charging. Lead acid batteries directly benefit from being held at 100% SoC. LFP do not benefit from this.. Float charging is not necessary for lithium iron phosphate batteries. The only reason any lithium ion phosphate battery manufacturer even suggest a float voltage is to satisfy end users who want to continue  using legacy/antiquated lead-acid charging equipment . In no way does float charging benefit your LFP batteries. The act of holding LFP batteries at or near 100% SOC can only serve to slowly harm them and eat away at cycle-life. An LFP cell can achieve 100% SOC at just a bit over 3.4 VPC (13.6Vpack  voltage) if you’re battery manufacture suggests anything over 13.6V for float you may want to reconsider that and set it below 13.6V .You can always set it lower but should not go higher.

There are charger manufacturers out there who actually understand charging LFP batteries.Victron  is about the best known. Victron has a specific setting in their custom menu that allows you to set a “storage” voltage this is a voltage the charger drops to after a short float has been done. It can be custom programmed to allow the batteries to self discharge down to about 50% SoC before the charger kicks back in and maintains the “storage voltage.”the only chargers or inverter/chargers we currently recommend for lithium iron phosphate batteries are Victron.

Don’t take my word for it, here is Battleborn….

What About Storage?

As mentioned above lithium ion phosphate batteries do not prefer to be sitting at or near 100% state of charge for long periods of time. This is why you will see, from nearly every single legitimate drop-in battery manufacturer, a recommendation for storing the batteries at or near 50% state of charge or less

Below are snapshots from lithium ion phosphate drop-in battery manuals or specification sheets .

What about “Hybrid ” Systems (Lead & LFP in parallel)?

In one word NO!Sure you can find someone on YouTube to tell you what you want to hear, but this is not always what you should hear…

Reader Challenge:The First reader to bring us (in writing) a US based LiFePo4 Manufacturer/reseller that allows you to place lead and LiFePo4 in Parallel wins $25.00!!Here is where the Eoropean IzSO standards land

ISO/TS 23625

Even Direct from China  Manufacturers disallow it.

From LFP Manuals/spec Sheets

What About Over-Current Protection?

Lithium iron phosphate batteries can throw a ton of current into a dead short but the fuse protecting the wire must have a suitable AIC rating. AIC stands for amperage interrupt current. AIC is different than the fuses trip rating. AIC is the maximum safe-current the fuse or breaker can trip under without having an unsafe-failure. For example if a battery has too much amperage, in a dead short ,Circuit breakers can actually weld-shut before they can trip. This is why AIC matters. The bottom line is that class T fuses are what should typically be used when protecting lithium iron phosphate batteries.


The UL image below depicts a drop-in battery with a FET BMS that is “short circuit protected”. As can be seen this single drop-in battery can stilldeliver over 5500A into a dead short! Now imagine if you have two or three of these batteries in parallel or four+. The short circuit current of a FET BMS is always considerably higher than one would assume it is When in doubt use Class T fuses for LFP.

The reason I’m showing an image of the mega or AMG fuse below is because of the growing popularity of the Victron Lynx distribution systems. These are excellent systems, we love them,however, caution must be used when connecting them directly to a battery bank. In North America over-current protection  needs to  follow the ABYC’s AIC guidance. Mega/AMG fuses are fine so long as they are downstream of A fuse or breaker that is properly AIC rated to handle the batteries short-circuit current. In other words, MEGA/AMG fuses should not be used to directly connect to a lithium iron phosphate as the primary over-current protection as the can only interrupt up to 2000A safely..

Below are the specifications for a blue Sea systems Cass T fuse. Notice the fully encased metal body and the 20,000 A interrupt capacity. Also note that these fuses are rated at 20,000A at 125 V. Tthe higher the voltage the tougher it is to meet in AIC rating. Compare this to a typical ANL fuse which only has a 6000 amp AIC rating at 32V. a class T fuse would have a significantly higher AIC rating@12 V if it was tested at this point because it meets 20,000 AIC it 125 V there was no sense in spending the money to tested at a lower voltage.

I would be sloppy if I failed to mention that any installed fuse should not be sized to carry more than 80% of its rating. This also goes for circuit breakers. This is especially true when installing inverters & charge equipment and especially alternators. For alternators a fuse of at least 140% of the alternators rating should be used. Thankfully , Blue Sea Systems is finally addressing this and putting it in their literature.

In summary, do your homework, purchase carefully, avoid direct from China imports when you can,install your system safely, use good quality charge equipment and you will be happy for many, many years and thousands of cycles.

Good luck and happy boating!



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