Author Topic: Lithium cell balancing  (Read 3117 times)

BruceM

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Lithium cell balancing
« on: July 04, 2021, 07:30:42 PM »
I've noticed that LiFePO4 cells direct from China have come down quite a bit.  So I've been researching how to best handle cell balancing for a 120VDC battery bank (39 cells in series operated from 119V to 132V. (3.4 to 3.05V)  I've got a few years to get ready for the switch.

I wondered if Starfire or anyone else might chime in on homebrew designs?  I am familiar with the various commercial  balancing products, but they generate a lot more EMI than I'd like.  The modest current, passive balancers are lowest EMI but still it is connecting a microcontroller with SPI interface to all the cells.

mike90045

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Re: Lithium cell balancing
« Reply #1 on: July 06, 2021, 06:16:34 AM »
I would prefer not to use balancers, but to order some extra batteries with the initial order, and then sort/bin the batteries, so you build a bank with all matched batteries, and top or bottom balance the bank.   (and hold in reserve the low capacity & high capacity cells )
Then just rely on the BMS to monitor cells in case one fails.

BruceM

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Re: Lithium cell balancing
« Reply #2 on: July 06, 2021, 07:48:56 AM »
Some folks do nothing but manual  monitoring and balancing every few weeks, even months for the first year or so.  I think I'd like something automated.  I'm not getting younger.


 I'm now thinking that some passive top balancing (fixed voltage load switch type) and modifying my big linear PV regulator to be fixed voltage might suffice.  The commercial ones are very cheap but the trip voltage is preset too high for my planned scheme.  The ones sold for bike packs are only 35ma of discharge, only when the batteries are full and still on the charger. That's not much time or current.  For my off grid PV system, it would be most of the day every day, so might work out even at 50ma.

I've never used lithium cells at all so lack the practical experience with them despite doing a lot of reading on BMS/balancer designs. 

My experience with my own linear 12V battery regulators is very positive; the batteries last a long time and I can swap in a new battery without trouble.   I could implement the same thing for lithium cells, though it might be much more than is needed, and with 38 or 39 regulators needed, it's quite an expense and build!






BruceM

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Re: Lithium cell balancing
« Reply #4 on: July 07, 2021, 07:58:37 AM »
OK on the basics but he is not mentioning battery balancing hardware, even though the photo of the 4 cell pack clearly shows a balancing cable hooked up for a battery balancer. 

Daly makes a decent 4 cell LiFePO4 basic one for cheap, I just don't care for the added EMI of that type with a microcontroller.  There are also plenty of  balancers with current and low voltage protection as well, for 12, 24, and 48V packs.  It more serious money for a balancer for 39 cells in series, and again comes with EMI issues I don't want.

Because my bank is full every day, most of the day, I think a simple passive top side shunt regulator or top voltage switched load resistor should suffice.  Because of the number (39) required, I'm trying to resist the urge to get fancy.


BruceM

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Re: Lithium cell balancing
« Reply #5 on: July 12, 2021, 11:28:35 PM »
I joined a DIY solar form to solicit experience with LiFePo4 cells.  It seems that claims of 10 year plus are fantasy. One retired EE circuit designer made an impressive 100V series string and balancer, his own inverter.  In three years with new balanced cells and modest DOD, he's had 3 of the 30 cells fail suddenly and without warning, as a complete short (dead), the first in a year. That's 10% failure rate in 3 years. And these were very expensive batteries with a very sophisticated BMS/balancer.   A friend of his who bought a set from the same supplier had the same experience.  Of course, the seller says it's never happened.  Uh huh.  The retired EE is planning on switching to refurbished forklift batteries at 48V.

It seems that LiFePo4 is not quite there yet in durability and reliability, once you get past the new adopter enthusiasm/delusions.


32 coupe

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Re: Lithium cell balancing
« Reply #6 on: July 13, 2021, 04:21:21 AM »

I wondered what was what when you first brought this subject up. I thought I'd wait to comment.

Chinese batterys ? Really ?

I don't think you can buy ANY good batterys anymore. (Yea, I'm sure I'll catch *** for that.)

I am in the marine industry and haven't had good luck with any of the major brand batterys.
About 2 years is the new norm for battery life. Wet, mat, gell....they are all junk.

I have owners ask about  lipo's etc. all the time. A while ago I asked a friend who I consider to
be an expert on batterys, chargers, solar etc about them and he says the problem is that their are
not really any  "standards" for most of the newer compounds in batterys and that the charge and
maintenance schedules are all over the place.

Unless one wants to go down the "virgin lead" road throw away batterys are here to stay.

I wish you could buy good batterys at a reasonable price today. I remember ( here we go !)
when the average car battery would last for years. My daily driver truck seems to eat a battery
on a 2 year cycle. Not cheap baterys either. The guys I work with seem to have the exact same
results. The last couple have gone dead suddenly. Start fine for months then BAM dead.

There is so much junk out there today you never know what you are getting. Especially from China.
I have gotten pretty cynical the last few years. Terrible service and no warranty seem to be the new
industry standard. That new battery with the fancy plastic case and bright shiney colors might just
as well be a gold plated t**d. And don't even bring up cost.....thru the roof !

Bruce, I thought you were using "cycled" fork lift batterys ? I'd stick with that and save your money.

Now if you run across those tall batterys used in back ups for com towers and crash carts grab them.
I haven't used them but have seen excellent reviews for them. I remember pricing some for a customer
a couple of years ago and he went with standard wet cells after seeing what those "virgin lead" batterys cost !





OK, rant over. Let the stones fly !

Gary




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BruceM

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Re: Lithium cell balancing
« Reply #7 on: July 13, 2021, 07:07:35 AM »
I'm using wet lead-calcium, "marine deep cycle" batteries, ten in series for 120VDC.  Because my DOD is so low, they last 4.5 to 5 years.
They only need a bit of water once a year, and float current is under 0.1 amp. 

I have zero interest in the fork lift batteries; 100Ah batteries are plenty for me, I'm only using  20AH.

My battery bank replacement cost is now up to $1000., every 4.5 years.  That's not bad, a fraction of what my neighbors with 48V L16 banks pay, and longer lived as well.



 

mikenash

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Re: Lithium cell balancing
« Reply #8 on: July 13, 2021, 08:21:04 PM »
Hey Starfire I wonder if you would like to chime in here?

It's a topic that interests me too as I need to deal with a situation where solar-charged batteries are often unused & unattended for weeks/months, and where they can be charged at sub-zero temps on a crisp, sunny winter's morning

I know you know what you're talking about; and I guess you have a couple of years under your belt using the new LIFEPOs

Cheers, Mike

BruceM

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Re: Lithium cell balancing
« Reply #9 on: July 13, 2021, 09:02:34 PM »
Mike, you just can't charge most LFP/LiFePo4 batteries below 0C.  Most the better BMS hardware automatically disconnects the PV regulator input at that temperature, as it will wreck the batteries.  Another issue is that a BMS failure can also result in the destruction of your battery bank; a concern for unmonitored or remote situations.

So LFP/LiFePo4 wouldn't be a wise choice for your application, besides the cost of the batteries sitting there for the taking while you're away.

I'm not sure why Gary (32coupe) is having such bad battery experience.  New cars tend to eat batteries more today because of so much "always on with ignition off" electronics load.  Perhaps in boats as well.  Heat is another well known battery killer, and virtually no auto or marine battery charge regulators bother to do temperature compensation.  So batteries in hot climates are getting overcharged to death, and just sitting in the heat, the batteries are aging prematurely. 

I've gotten consistently good service on both single AGM and wet Marine type batteries for my big 120V battery.  I get 6.5 years life on the former and about 4 to 5 years on the latter.  I'm on my 3rd AGM and  my 4th of the marines.  My neighbor has the same setup and gets the same service.  I bought my latest set 6 months ago, a bit early at 4 years but he was at 5 years so we got a good bulk buy deal on 20 at our nearest Car Quest.  We both use temperature compensated charge controllers and a battery balancing system on the large battery string.

Bruce



32 coupe

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Re: Lithium cell balancing
« Reply #10 on: July 14, 2021, 05:50:15 PM »

I know heat does play into the life span.
It gets HOT in florida !

 
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MachineNLectricMan

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Re: Lithium cell balancing
« Reply #11 on: October 31, 2021, 12:29:46 AM »
I have a lot of experience with lead acid battery technology, to the point of in depth knowledge of their manufacture, and it is rare that a new technology comes along that is better, but I am convinced that the LiFePO4 is the way of the future. Lead acid technology is extremely sensitive to abuse and depth of discharge. There are some that will last twenty years if correctly managed, but one screw up and that is lost. Be prepared to spend 10k to 20k dollars to get a decent sized bank with the 25% max depth of discharge limit for this lifespan. There are (or were) only about three companies in the world that made true deep discharge batteries that are a spin off from the old submarine battery technology. Only two remain in North America, one is in Kalifornia, the other in Kanada. (spelling no accident). There were a few overseas. All other manufacturers offer deep cycle imposters which are basically modified engine start batteries labelled as deep cycle.

The LiFePO4 batteries however are not nearly as sensitive to depth of discharge as long as they are never discharged below 2.5v per cell, and most batteries are capacity rated with this limit as fully discharged. They need to have electronic managing to prevent this but one control can manage the entire bank. There is the 0 degree temperature limit for charging (but not discharging), but really...... how hard is it to provide minimal heating for a battery bank? If well insulated in winter, most banks will be self heating anyway. These batteries are less than 1/4 price of the same size equivalent of lead acid (at the 25% DOD limit!). The LiFePO4 technology does not have the thermal runaway problems that the other lithium technologies have. This is one of the reasons Tesla has recently chosen this technology for future cars. You pretty much have to intentionally heat them artificially to get a thermal runaway.

Balancing? I am humored to see all of the electronic "contraption" circuits people have come up with for this. Turns out that there is a DIY or "redneck" solution that is extremely simply. The only difference between lead acid and LiFePO4 is that lead acid technology "self balances". Both battery technologies actually required balancing! When charging lead acid batteries, after a certain voltage is reached the electro-chemical reactions begin to cause water to be broken into Hydrogen and Oxy.. This process allows current to leak through the charged cells that charges the cells that are lagging behind. In LiFePO4 technology this must be provided artificially, but does allow voltage mode charging the same as finish charging lead acid batteries. It also turns out that 4 LiFePO4 cells charge at the same voltage as 6 lead acid cells which allows the same charger to be used!

OK how do we do this? Turns out that low voltage zener diodes have a very wide "delta zener" range, which all established electronics designers are warily aware of.  A 3.3v zener gives just the right amount of bypass current at 3.6 volts to emulate lead acid self balancing. And just like the contraption circuits using mosfets, etc out there, the zeners are connected across the terminals of each cell so that the zener current direction is shorting the cell. (diode band towards the positive). After fully charging, disconnect them as they will leak discharge some of your battery capacity. Use 3 or 5 watt 3.3v zeners. Also, with the size of the battery banks used in off grid, the zeners will act as self fuses if anything goes wrong and you overheat one. I.E. The diode will fail shorted then mechanically melt and open.

Keep in mind that discharging below 2.5v or charging over 3.7v per cell will cause dendrites to begin to form internally which will cause a major cell failure if that condition continues very long. Same with charging below 0 C..

In my opinion, the price of these will continue to drop and they will become almost "disposable". The caveat is only china is making them. Tesla is currently getting them from china, but Tesla is also in the process of build a US plant to make these! Question is, can anyone in the public get these from Tesla, and will Tesla's prices be reasonable and affordable? China has very dicey quality issues, some times you can get good stuff, sometimes really bad stuff and you never know which on any particular order.  Also, individuals have no market leverage when ordering from china and may have difficulty in correcting any quality complaints.

mikenash

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Re: Lithium cell balancing
« Reply #12 on: October 31, 2021, 06:15:42 PM »
Still watching this topic with interest, thanks

Fred8

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Re: Lithium cell balancing
« Reply #13 on: November 02, 2021, 01:29:59 PM »
Lead acid technology is extremely sensitive to abuse and depth of discharge. There are some that will last twenty years if correctly managed, but one screw up and that is lost.

There is a lot of BS written to promote Lipo ( all lithiums)  and disparage LA and this is some of it.
Lipo is FAR more unforgiving than Lead and sorry, anyone that said otherwise has no experience with the 2 technology's or is flat out lying/ Biased.

Run a Lipo dead flat and try and recover it. Extremely unlikely and if you can it will have bugger all capacity.  Run an LA dead flat and you'll recover it easily and the capacity fall off will be very small IF you only do it once.  Then again, once is all you would get to do it with lipo, no question of that.

Same token, over charge a Lipo and see what happens with that when it swells up and maybe lets the magic smoke escape.  I have BOILED  LA and for some hours and put a load on it to take the overcharge off and they batteries were fine. Not saying there were completely unscathed but they ran as normal  afterwards and only required a bit of a top up.  Not like you can determine a time how long any battery will last to say how much it's life was shortened but with Lipo, one way or the other too far and its game over, dead and buried.


 
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Be prepared to spend 10k to 20k dollars to get a decent sized bank with the 25% max depth of discharge limit for this lifespan.

I cannot comment on pricing where you are but that's ridiculously over the top for where I am unless you are talking about some pie in the sky technology or an unnecessarily large pack.

The bit about 25% Discharge is  regularly recited complete and utter BUNK!

 LA traction ( forklift) batteries are WARRANTED to  not below 80% DOD.  That is , for complete clarity, if you have a 100Ah battery you can take 80 Ah OUT of it  EVERY DAY and that's what the manufacturers warrant it for. 
This is easy to verify, just go to any traction battery manufacturers website and look it up. This 25 and 50% DOD for LA is ignorant Rubbish.
This isn't car batterys we are talking about or some other unfit for purpose type. Then again, even decent car starting batterys wouldn't' be terribly phased  by 30%  discharge especially if they were deep cycle which will handle 50% or better DOD. 

I really don't know why people continue to spread this complete and utter BS about LA batteries nor make out the Lithiums are capable of things they are not. it's misinformation that only causes people to make wrong assumptions and prevents them getting what best suits their needs and is completely unhelpful.  They are batteries for hecks sake, not your children you are trying to get into College. They both have their strengths and weaknesses like anything but why try to make them out something they are not? I don't ever get it but see it all the time.

Where I am, warranted 48V, 720 Ah Fork packs can be had for around $4000, often less.  They are gauranteed for 5 years for solar use provided they are not discharged below 80% . The manufacturers I have spoken to love selling them for solar use.  The current pulled out of them is typically much less, they are not subject to banging and vibration, not discharged nearly as much or run till the shift is over no matter how low they are, usualy set up with auto watering systems, don't have the max charge belted into them in order for them to be ready in the morning or for the next shift and overall have a Much easier life.

One Guy I spoke to said he'd never had a claim with any he sold for off grid use, wished he could say that for their intended application!
Lipo may have some advantages over LA like weight and size which is irrelevant in a stationary application anyway but in Bang for the buck stakes, LA craps on  Lipo.  From what I have read, also does in reliability and longevity.  Read of plenty of people whom have babied the things and they have fallen over in under 4 years.

I have also read miles of BS about lipo taking much higher charge rates than LA.  Again, largely BS. You can't just pump limitless power into lipo at all, not without shortening it's life of course.  OTOH, while LA is Limited in charge rate, the amount of power that works out to on a 6 or 700AH pack is more than anyone is likely to have on their roof in panels unless they are running a solar farm.
Even running .2C is going to be 140A on a 700ah pack and they can take a lot more than that on bulk charge.  How much solar or how big is your generator again?  Also read repeatedly that lipo can be charged at a flat full rate till it comes up to Voltage. Yeah, but it's going to cut the life significantly. In reality they should be given a bult and then a top off charge at a reduced rate similar to LA.  IF you want best life that is.


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The LiFePO4 batteries however are not nearly as sensitive to depth of discharge as long as they are never discharged below 2.5v per cell,

And if they are run below 2.5, they WILL Be damaged and maybe unrecoverable depending how low they get.
Not a good idea to Run LA right down either but they WILL recover from it. read PLENTY of reports of people that have locked up their Lithiums through over discharge and can't get even months old packs back up.  Only takes a BMS failure and they are bricked.


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These batteries are less than 1/4 price of the same size equivalent of lead acid (at the 25% DOD limit!).

Again, maybe where you live when you work on BS figures like 25% DOD for the LA but I have YET to see any decent lipos cheaper on an AH basis than LA unless you are talking those Chinese Cheapies that are all the rage and have dozens and dozens of Documented failures on Forums. I have seen a lot of people that make this cheaper than LA Claim but they are inevitably quoting 2nd and 3rd rate batteries in chyna and not including delivery and all shipping costs.   And of course if a cell is bad or goes tits up, you are waiting months for a replacement IF the seller Honours any warranty  if there was one and you are undoubtedly up for more shipping and customs costs.
With a fork pack, go to your nearest company and get a standardised replacement off the shelf. Drop the failed cell off at the local Scrappy and you'll get 25-33% of the price of the new cell back.

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In my opinion, the price of these will continue to drop and they will become almost "disposable".

Another forum fan boi myth.
There is a HUGE demand for  batteries world wide especially the lithiums. They are bing put in everything these days.  Tesla for one can't get enough and the production of these EV's  is ramping up despite the ignorance of people thinking they are cleaner than FF vehicles increasing demand.
The production of raw materials for Litho type batteries is very limited and mainly carried out in  3rd world politically unstable hell hole countries that have little to no environmental rules and enforcement which makes it infinitely cheaper to process there than bring the materials into first world countries where enviro regulations make the cost far higher.  One little conflict in these countries and production ceases and prices skyrocket.

I have NEVER seen anything in short supply through high demand get CHEAPER  in price.  Does not happen.

If one takes case in point, tesla power walls keep going up in price, around $1000 last year and the year before.  Not aware of ANY battery coming down in price for that matter.
Lipo is as cheap as it's going to get right now.  Add in supply chain problems from Convid and offshoot factors and there is no way  these batteries are going to get cheaper in present chemistry and if something new comes along that will have the elevated introductory costs for a few years as well.

As far as Disposable, that's a good point.  LA here is worth presently about a 3rd to quarter of it's purchase price as scrap which is also a factor in being better bang for the buck over lipo .  There are limited lipo recycling facilities at this time, more in some countries than others but some places are already talking disposal fees for these batteries due to their Toxic nature.  LA can be recycled at any scrap yard, not many places relatively will take lipo.


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The caveat is only china is making them.

Bingo and as you allude to, the quality is HIGHLY suspect.  I see most of the fanbois on solar and RV forums are all over the cheap, 2nd quality cells and the outcomes with them are hit and miss at best. If you miss, it's a lot of money down the drain or a long downtime and more cost to get a replacement... which will then never balance with the rest of the pack due to age alone.

 
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Tesla is currently getting them from china, but Tesla is also in the process of build a US plant to make these! Question is, can anyone in the public get these from Tesla, and will Tesla's prices be reasonable and affordable?

Tesla and other car manufacturers are saying and have been for some time that supply of batteries is both their biggest limitation, headache and expense in the vehicles.  If they can't get enough to satisfy their own needs (Powerwalls were halted from production for many months because of Cell shortages) They certainly won't be offering them to the public to compete with their own products and if they did, they would certainly not be cheap.  NOTHING with Tesla ever is and they would put huge markup on anything with their name attached anyway.


 
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China has very dicey quality issues, some times you can get good stuff, sometimes really bad stuff and you never know which on any particular order.  Also, individuals have no market leverage when ordering from china and may have difficulty in correcting any quality complaints.

Agreed but this should have been your preface paragraph because it' is the relevant and over riding thing to everything else you said.
what good is cheap and whatever else benefit if it's unreliable or short lived?

All this " In the future" stuff is all well and good to say something will improve but it in effect never comes about because then something new comes along and it's back to square 1.
These Lipo technology's are a prime example. Every time they change the formulation  it's back to the beginning again. No one knows how they will hold up in 3-5-10 years time and what the last formulation did is irrelevant because this one is different and unknown.

La OTOH being old fashioned and untrendy and heavy and bulky and all that IS well proven, known and understood.  I know people personally that have had over 15 years out of proper traction batteries.  I asked a Client of mine about the  La " Maintenance Myth. He said he shuffles down the shed every 3 Months or so to put a bit of water in the automatic watering system and make sure the rats haven't  done anything. He says a guy comes out every 12 Months, checks the gravity of the cells, removes, greases and replaces the leads and wipes them down with a rag to get the dust off and make them look pretty again. 

He's been running his farm off grid for nearly 50 years and figures on 2-3 Inverters per set of batteries. He ran a Moffit Generator  and then went to panels in the very early 80's.

I hear lots of people going on about " Maintenance"  Like it's an endless chore yet there is nothing like the time needed on them that people spend taking the trash cans out and brining them back in every week but no one sees that as some great hardship like they make out checking battery's every moth or 3 is.

BruceM

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Re: Lithium cell balancing
« Reply #14 on: November 02, 2021, 06:10:30 PM »
I enjoyed your post greatly Fred8, because I also disagreed strongly with the pseudo-technical content and attitude of the prior poster.  So much so that I avoided responding; I thought it a troll.

Regarding lithium LFP batteries, I found in pursing information on other engineering forums, I ran into another older EE like myself how had designed and built his own inverter and BMS/cell balancer. His was based on the very latest LiFePO4 cells at great expense.  He used a nominal 90VDC series string, and much to our mutual surprise, we had both done original inverter designs similar to the old Trace SW sine series.   

He used an original passive balancing design for his LFP cells which allowed him to see a display of all cell voltages to monitor them.  Since it was his own, new design, he monitored it closely.

Long story short, he'd had one cell die of sudden short every year for 3 years, and was so pissed at the unreliability of his  very expensive premium matched cells that he was considering changing his power system to locally available 48V fork lift batteries.  ​

I do expect that LFP cells will become more reliable in time; already some folks are very pleased with the reliability and lifespan of the very, very expensive BattleBorn 12V LFP batteries in marine applications (where size is a concern and cost is not so important.) 

Lastly, I'll add that 3 or 5 W, 3.2V zeners do not provide a useful non-manual  balancing system- for many reasons, such as connecting and disconnecting them manually  is a farce, and of course most would like more than 1 amps of charge rate, which is about all you could manage with a 5W zener.  Manual balancing CAN be used for new matched capacity cells, but that's not going to last for years or past the first bad cell that must be replaced.  To suggest this as a wonderful solution while denigrating others made me suspect a  troll.  Likewise the absurd notion that only US made submarine lead acid batteries are good when I'm surrounded by off grid homes all reliably and affordably run on a variety of LA batteries.

Not one company making LFP battery packs does NOT use individual cell voltage monitoring (high/low) and balancing. They don't do that for fun; they do it because it's needed.  But it's also true that a failure of the BMS/cell management circuitry can also cost you the entire battery bank, since as Fred8 points out, you are unlikely to recover an LFP cell once it's been over or under the allowable range of voltage. That is a well acknowledged fact, and it's why cell voltage monitoring is built into LFP packs; typically charging is disconnected when any cell goes to max voltage, and discharging is disconnected when any cell goes low.  This to required save the cell(s) from destruction. 

I found an example of such a disaster on one forum; A new DIY, LFP enthusiast was using a MPPT PV regulator, with a high voltage PV series string. A very common setup.  The MPPT, Buck type PV charge regulator failed short circuit (the typical Mosfet and  failure mode), applying the high voltage to the BMS.  The BMS was not rated for that high of an input voltage, so it's solid state (MOSFET) cut off switch also failed, as short circuit.  The LFP prismatic cells were then destroyed by excessive voltage.  The cell balancing hardware was also destroyed by too high voltage.  So a complete and total loss of well over $10K worth of new LFP batteries, all a cascade of destruction from a MPPT failure.

I'm hoping that the quality demands of automotive manufacturers will force rapid improvement in LFP quality and reliability, and the price of bargain B grade cells today will purchase reliable cells in the future.  I accept that I may still live out my life on lead acid batteries, and at what is now $17/month battery bank replacement cost, I won't lose sleep over that.  I won't switch until it becomes clear that it's cheaper for me to do so.