LiFePO4 Batteries - Thoughts & Musings Photo Gallery by Compass Marine How To at pbase.com

LiFePO4 Batteries - Thoughts & Musings

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07-DEC-2017

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What's In The Box..?

DISCLAIMER:

I have had so many requests to open this article I have decided to remove the password protection.

Please understand this article is still not entirely complete or done, but there is still a lot of information in it now. An article like this takes lots of time to write photograph and proof read and I still have a very busy marine electrical business to run.

Please be patient. If you see typos feel free to point them out so I can edit and proofread faster.

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.

******WARNING******

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 folks contact me who have ruined LiFePo4 batteries. 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. DO YOUR RESEARCH! Trust me, it does NOT end with this article. This article is BARELY scratching the surface.

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.

So 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.

#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.

#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 many 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 cells but no fire, just thousands and thousands of dollars in hard earned money GONE.

******IMPORTANT******

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.

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.

EDIT: Balqon is apparently out of business. Winston cells are available in the US branded as Voltronix. Contact Voltronix if you want more information on their cells. Other options are Sinopoly or CALB cells.

What's In the Box...

~~LiFePO4:~~

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 - 13.4V.

If you want to do other than a 4S configuration the cells are ideally connected in parallel first then in series. This 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 monitoring. The nomenclature system you will most often see for 12V house banks is as follows.

4S = Four Series

2P4S = Two Parallel / Four Series

3P4S = Three Parallel / Four Series

4P4S = Four Parallel / Four Series

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. It 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.. I much prefer the simplicity of a redundant lead acid reserve bank. Rewiring an LFP bank at sea is not a task you want to undertake...

As prismatic cells get larger the concern over vibration tends to grow. The most common reported failures for "unknown odd reasons" has by far been in 700Ah and larger cells. At least one manufacturer, Sinopoly, suggests not using cells larger than 200Ah on a boat. Their competitors don't claim this and still hold that all their cells pass the same testing.

What I do know is that none of the commercially available prismatic cells from the likes of Winston, Voltronix, CALB, Sinopoly, Hi-Power, GBS etc. are tested to any specific industry standardized or Mil-Spec vibration testing. All vibration testing is done in-house to their own in-house standards.

Getting a straight answer on vibration testing, from any of the Chinese manufacturers, is like walking up hill on ice with leather sole shoes. I have tried and their info is non-existent on this subject.

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

DIY Builds:
This is a real cost saver but is NOT for the faint of heart or the limited skill DIY'er.. It is not recommended for the average boater. In the DIY build you source the cells, 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:
Lithionics/Bruce Schwab, Mastervolt & Victron build LFP systems for marine specific applications. These systems are well engineered, well executed yet also expensive. If you want LFP and don't have ththe systems Bruce sells, and the Li3 Lithionics system is quite beautiful.

Drop-In:
I really even hate to mention this type of Li battery 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. If it seems to good to be true, it probably is! Caveat emptor on Li Drop-In's. It a Li battery does not have any sort of external communications capability, it is a "drop in".

I URGE STRONG CAUTION ON ANY "DROP-IN" LFP BATTERY!!!!

When in doubt go with the Ocean Planet Energy Li3 Lithium, Mastervolt or Victron. All of these companies have well thought out marine specific systems.

EDIT 10-27-2015 - Recently Genasun decided to leave the LFP marine market, when good inventory runs out. This was reportedly due issues in the supply of the CALB LFP cells not meeting Genasun specifications for consistency. According to Bruce Schwab,the Genasun factory representative, the Coulombic efficiencies of the cells, even with measured identical capacities, were not matching well enough to meet Genasun's very high standards.. This can make for a balancing nightmare.

Bruce Schwab the owner of Ocean Planet Energy worked closely with Lithionics to develop a new LFP system, suitable for cruising boats, and to fill the hole Genasun left, and it is now finally shipping. For more info on the Ocean Planet Energy Li3 Lithium Batteries see Bruce's web site.

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

LiFePO4 vs. Lead - Pro's & Con's

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

LiFePO4 Pro's:

*CYCLE LIFE:
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. 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.

*CHARGING SPEED:
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.....

*WEIGHT/SPACE:
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...

*STEADY VOLTAGE:
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.

*CHARGING EFFICIENCY:
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 currently one of, if not 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.

FACT: Lithium Iron Phosphate cells/batteries can enter thermal runaway and catch fire. Once on fire these fires are very difficult to stop. While it's much more difficult to cause thermal runaway in LFP it's not impossible.

ALL BATTERIES ARE DANGEROUS!!

Let us not forget that. As Li goes LiFePO4 is currently the safest widely available option. 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 the vast majority of marine systems use 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)..

01-NOV-2014

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)

LiFePO4 Con's

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

LiFePO4 Con's:

#1 OVER CHARGING:
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 a BMS system to ensure over charging can not happen.

#2 OVER DISCHARGING:
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"..

#3 CHARGING BELOW FREEZING:
LFP cells should not be charged at temps below 32F. They can be used below these temps but NOT charged. Lead acid could really care less. Another way to DENT YOUR WALLET!

#4 CELL BALANCING:
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 taking massive amounts of current and the next one cell has gone full/empty and hockey sticks before the other cells.

If one cell becomes out of balance it can hit full before the rest of the cells does and this one cell can be ruined. Cell balancing is most important & most critical when pushing high charging voltages, which IMHO and experience is 150% UNNECESSARY. 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 a cell going off early just what the overall pack voltage is.. A good BMS (battery management system) will cut charging well before any damage can be done.

#5 FLOAT CHARGING:
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 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 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.

#6 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 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. The algorithms in these chargers also lengthen the absorption duration based on the bulk duration. LFP will be in bulk for a long time hence these chargers will hold these insane voltages for far, far too long.

DO NOT EVER use these settings to charge prismatic LFP cells. You will eventually DESTROY your batteries. Here's a hint you DON'T FLOAT LFP BATTERIES. The sheer fact is the people who create these potentially dangerous charge profiles very often have no experience with LFP banks, but, they DO want to SELL you a charger....

On the other side of the coin the Chinese are eager to get into your pants, like a 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 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 want you to charge to 14.6V then STOP CHARGING once you hit 14.6V and .05C - .1C current.

The reality is this is not 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?

Please do the research, do your own testing, as I have, you decide, it is YOUR bank and YOUR wallet. I suggest strongly looking at Genasuns's MAX allowable charge voltage. It is 14.2V. This is with a fully gourmet factory engineered system. For a DIY, max out your charge voltage at a 14.0V, and keep the duration to less than 15 minutes at 14.0V, and you will be a happy camper...

You really need to source chargers, controllers and alternator regulators that allow 100% CUSTOM PROGRAMING not some piece of junk wanna-be Li, menu-fed, pre-determined & "selectable" charge profiles made up by a charger maker, who's reading a misleading Chinese spec sheet, and does not understand what it even means. Wow, that was a mouth full...

The previously mentioned ProMariner ProNautic P and Sterling ProCharge Ultra chargers are excellent chargers, for lead acid, because they DO have a custom program feature for voltages only. You could use the custom profile but DO NOT use their "Lithium" charge profile. Where these chargers fall flat, for LFP, is the duration at constant voltage. These chargers lack the ability to shorten the absorption voltage duration so you as a user would need to manually manage that. Where these chargers also fall flat, as most do, is in the lack of a dedicated voltage sense lead. Also the largest charger they make is just 60A. Kind of small for an LFP bank. For a shore charger, a fully programmable unit is better, and by fully programmable, I mean absorption duration AND voltage.

#7 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 all 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".

#8 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.

#9 LEARNING CURVE / HUMANS:
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. 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..

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

How simple? Charge to full and stop... Run the bank down to 80% DOD and recharge. 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)

#11 SHELF LIFE:
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. 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..

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.

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 /2- = 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 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 let alone a full 1C. 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 manyt 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. Big, big difference....~~

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.

The BMS:

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.

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

I like 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. A Battery MANAGEMENT System is what you want. It is automated and protects your bank even when you are being oblivious to what is going on. A BMS / Battery MONITORING System 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 the important areas below.

WARNING LEVELS:

HVC = High Voltage Cut Off: (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. Generally speaking if this is a warning level event they are often pack level voltages. Remember this is a WARNING LEVEL alarm or cut. A BMS systems for LFP cells will 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.

IMPORTANT:

Please bear in mind that the BMS is not there to manage charging or chargers 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 Cut Off: (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.

ALARMS:

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.

EMERGENCY LEVELS:

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 LIKE OR 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 small resistors to divert current from the cells actively hitting high voltage levels to those cells that are not yet at high voltage levels. 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 RARELY done correctly.

In reality I believe shunt balancing needs to be done, and executed, with a good sound design and executed, in my humble opinion, at LOW CURRENTS. BMS systems such as the Clean Power Auto House Power BMS do not allow shunt balancing until after the HVC has cut away charging. "But, but cell balancing won't work automatically then?" Absolutely!

EDIT: 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 do 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 programed 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)

Shunting is 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. 500+ 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 500 cycle most cycles to 80% DOD. They have only been balanced once, 500+ 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 fractional C 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 800+ 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 750 cycles (took a very long time and lots of work to do that) and has exhibited virtually no capacity loss, no notable changes in internal resistance and no cell drift. In the first 50 cycles I actually saw a minor bump in capacity. Why? Likely due to the fact that I have chosen safe & sane charging levels and performed a good initial top balance. The cells really seem to like it.... This is the way "I" do it, you can always choose how you want to do it...

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

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

Funny Story:

I was conversing with a gentlemen who 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 that he was going to ruin his cells; "You have to have a BMS that balances!", "You must have cell balancing!", "Hey guys here's a fire waiting to happen!". One particularly obnoxious electrical engineer 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...

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

~~Laying Out The BMS~~

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

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 here? I no longer consult long distance on LFP design because owners refused to follow design criteria, Genasun stopped Selling to DIY's and then dropped LPF altogether, and now Clean Power Auto has pulled out of the DIY space. If you've done your research I suspect you know why this trend exists.

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

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

Additional items shown:

#1 Two 70A Cole Hersee RC-700112-DN SPDT 70A Relays. One for Alternator HVC and one for Solar HVC. (Ignore the relays in this image they were for layout and mock up only and are SPST not SPDT)

#2 Blue Sea Terminal Strip

#3 Piezo alarm buzzer 12V.

~~#4 Momentary re-set switch.~~

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.

BOTTOM BALANCE:
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 once 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.

TOP BALANCE:
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 750 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.

Balancing The Cells

As I mentioned I first charged these cells, INDIVIDUALLY, to 3.75VPC and X current taper. Today I would stop at 3.65V. 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 exactly 20A then stopped charging and moved onto the next cell.

WARNING: It is fairly important that you top each cell up prior to wiring them in parallel, unless you have a large power supply or do them in series using a large inverter/charger. They should ideally 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.

It is really a net myth that simply wiring the cells in parallel will serve to get them balanced. While they may be at the same "voltage" they may not be at the same SOC. This is because once voltages become equal or at parity, the current has little to no voltage to move any meaningful current between the cells. You need to actively charge in parallel above the resting voltage in order to get the cells to balance and only when the current has declined to next to nil will you know the cells are all at the same SOC. With a small power supply this can take lots of time.

The resting parallel conundrum:

As the voltages converge, after being wired in parallel, the movement of current between cells slows to a crawl, it is simple Ohm's law. We are talking 0.0001A level current movements and at this rate "balancing" takes forever.

This I why I top balance the cells until my bench top power supply is flickering between 0.00A and 0.1A of current. Once this is achieved I will discharge them to about 50-60% SOC, in parallel, and if I have the time they can sit there for long periods. You don't want to charge to 100% SOC then let them sit there as this can serve to degrade the cells. While this level of degradation is slow it also depends upon the cell temp. If your cells are at 90F in Florida, and you let them sit at 100% SOC, they will degrade faster than sitting at 100% SOC at 50F in Maine. Bottom line, don't let your batteries linger at or near 100% SOC for very long.

TIP: 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 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 prismatic cells 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 I wired these cells in parallel I now had 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 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...~~

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