BMS active balancer becomes unbalancer as cells age

[hwy17]

If the balance voltage is higher, just before or at target absorption voltage it's not going to start to unbalance the false runners early like before with a lower balance voltage. Until they are all up to the target voltage, and the closer they are to target voltage (further into the voltage knee of LFP) the more closely voltage is related to SOC, so less likely any cell that falsely showed a high charge voltage early, will still be an outlier.

+1

You really don't want them balancing whilst floating. Personally I don't think it's a big problem floating at the cell nominal 100% SOC voltage: roughly 3.375v

But if you just floated at 3.4375 then you could balance while floating. Is 3.4375 going to overcharge and plate your cells? If you hold it 24/7 for days, certainly. Is it going to plate them in a 6 hour daytime solar float? Maybe if the cells were absorbed all the way to 3.5+ and are jam packed when the float starts. But what if they were only absorbed to 3.4375 then there might be vacancy to accommodate that overfloat for 6 hours.

But don't read this comment like I'm trying to correct you or something, your statement is the consensus, mine is the outlier, I am just using yours as a springboard to get to put my alternative thoughts out here.

 

[ScrotusGobbleBottom]

resistance affects the entire series string

good point! Never thought of that

it's a bottom problem only.

understood. So bottom balancing would be am idea or not?

 

[hwy17]

So bottom balancing would be am idea or not?

It's an idea but the problem with it in solar applications is the battery might never regularly reach the bottom for rebalancing, but should be reaching the top semi regularly. You'd have to do a manual bottom balance every month or two weeks or however often.

I read somewhere, probably from someone on here, that bottom balancing was big in the early DIY EV community, because their bottom SOC was most important to know when they'd "run out of gas". But I think top balancing has taken over in DIY EVs now too.

 

[Checkthisout]

I thought the balancer was there to make all the cells act as if they are connected in parallel.

If that's the case, how would we treat such a battery in order to insure the cells are evenly charged before sun goes down and we begin to discharge them?

 

[finite]

ive noticed my 2 year old, diy lifepo4 battery getting out of balance, two cells are out of balance, one SOC is about 10% over and the other is 10% under. 8S eve 304ah on a jk bms with 2A active balancing set at 3.4V. seems like what is happening is when its charging, the weakest cell's voltage is surging, making the bms think that cell needs to be discharged, so it dumps its power into the lowest voltage cell, the one with the least voltage surge. then when charging is done and its floating, the two cells reverse voltage state, the cell the balancer thought needed discharging is now the lowest voltage and works to undue what it just spent 1-2 hrs doing during charge. basically the weakest cell gets dumped into the strongest cell during bulk/abs charge, and effect is reversed in float. if the float voltage is not close to 3.4V, then the effect is never reversed, and the cells get pushed further and further out of balance every day. it seems on a newer battery, if the cells are well matched, they all surge together and this effect does not happen. but as cells age unevenly, the surge/sag overcomes the balancers ability to measure cell SOC based on voltage

problem is i dont really feed comfortable floating the cells at 3.4V for 4-6hrs a day, it degrades cell capacity. ive tried lower charge voltages, but only makesa difference of 20min in charge time, still spends roughly 1-2hrs every day putting the battery out of balance. at this piont i would be better off i disable balancing and just do it manually every so often. if only there was a way to disable balancing during charging, and only enable it during float or low current to eliminate the effect of voltage surge/sag. im looking at external active balancer triggered by victron relay output

Got a pic of batt wiring?

 

[Alarchist]

But if you just floated at 3.4375 then you could balance while floating. Is 3.4375 going to overcharge and plate your cells? If you hold it 24/7 for days, certainly. Is it going to plate them in a 6 hour daytime solar float? Maybe if the cells were absorbed all the way to 3.5+ and are jam packed when the float starts. But what if they were only absorbed to 3.4375 then there might be vacancy to accommodate that overfloat for 6 hours.

But don't read this comment like I'm trying to correct you or something, your statement is the consensus, mine is the outlier, I am just using yours as a springboard to get to put my alternative thoughts out here.

If 3.4375v works for you crack on, how long have you been charging like this, does it seem to work ok, are you sure you're not plating, allowing unlimited current charge for 6 hours will still be able to do it at 3.4375, I wonder how much though? I've not tried it, or seen anyone else ever recommend it, so you're a bit of an outlier, not sure I will try it, but happy to see some data.

 

[hwy17]

If 3.4375v works for you crack on, how long have you been charging like this, does it seem to work ok, are you sure you're not plating, allowing unlimited current charge for 6 hours will still be able to do it at 3.4375, I wonder how much though? I've not tried it, or seen anyone else ever recommend it, so you're a bit of an outlier, not sure I will try it, but happy to see some data.

It is an outlier, I've only ran my DIY battery for a month now this way. There is at least one more experienced member here who floats even higher than that but I'm not sure if they balance in float which is the main idea I'm working on.

 

[Spunky]

2. The secondary effect of a cell with high resistance is that it's losing energy as heat because of the resistance.

I've noticed in my packs of 16 cells (not single cells) that heat of even as little as 2°C effects what the pack contributes to the bank.
Where I have two stacks of rack batteries 7 high and I see the cycle counts higher on the top packs. So much that I shuffled the top to bottom (I know, ruined the long term test).
As hwy17 points out that there could be heat accumulating in a cell (depending on the charge rate, sunny or hazy day). So I'm suggesting, that it may not be that the balancer is chasing the cell, but we're chasing another variable.
Attached pix: Pack 4 in the 1st bank and pack 11 in the 2nd bank are newer with differently calibrated temp sensors.

 

[rustyrotors]

i understand how raising the balance threshold voltage could help the situation, in that the SOC rise could overcome the voltage surge, but it not seem to help on this battery, which some screenshots to illustrate..

also if it were a connection issue, would that not show up on the cell wire resistances show in the BMS?

I think i found a solution by lowering balance voltage to float voltage 3.37V and floating at 27V/3.375V. previously i was only bulking, no abs, no float. The past few days i have been bulking to 28V/3.5V and floating at 27V/3.375V, and it seems to be working. i figure its spending more time floating vs bulking, so over time it should work its way back into balance, even though the two stages work against eachother

so here is the balance at the bottom from last week. cells #2 and #6 are lowest balance or lowest capacity, and cell #1 is highest balance:



The rest of the screeshots are from todays charge cycle, i added in an absorption stage with tail current to see what happens. the battery is better balanced from a week ago ever since i started balancing at 3.37V during float. But cells #2 and #6 are still too low out of balance, and you can still see the issue.. bulk charging past 3.4V, cells #2 and #6 are now creeping up to highest voltage, balancer is dumping power into cell #1:



Issue continued.. bulking past 3.45V, cells #2 and #6 are significantly higher and balancer is dumping them into cell #1:



No change at start of absorption at 3.5V, balancer dumping cells #2 and #6 into #1



almost at end of absorption with tail current of 10A (house pulling 5-10A):



floating at 3.375V, cells #2 and #6 now show the lowest balance, balancer finally doing its job:



I can see how holding it at 3.5V or 3.45V absorption would work, with balancer set to the same absorption voltage, so that current drops to a low value so the voltage surge is not effecting the SOC voltage. drawback is you are basically overcharging the cell to balance it

 

[Alarchist]

almost at end of absorption with tail current of 10A (house pulling 5-10A):


I can see how holding it at 3.5V or 3.45V absorption would work, with balancer set to the same absorption voltage, so that current drops to a low value so the voltage surge is not effecting the SOC voltage. drawback is you are basically overcharging the cell to balance it

I don't think balancing from 3.375v will constructively balance the cells as they're still not in the voltage knee yet, so any cell could show a higher or lower voltage but not have a higher or lower corresponding SOC.

If a cell is falsely showing a high voltage early and gets balanced dumping current into others then is the lowest at target voltage and needs balancing back up, then I don't see how a lower balance start voltage can help.

I just did a test today for fun and lowered my balance start voltage from 3.49v to 3.44v - Result: It totally messed up the balance as it started too early unbalancing my cell with a higher initial under charge voltage or overpotential voltage? So when the others came up close to my 28v absorption voltage the cell that was initially showing the highest voltage and dumping it's load into the others was now the lowest voltage. It didn't have enough time in the 1 hour absorb to come back up, my charge settings stop charge if a cell hits 3.505, so just gives small bursts as they saturate and balance further to keep at the 28v target, but this wasn't enough and the earlier overpotential cell was still dropping in voltage before having a chance to come back up before they went into float.


Two of my three batteries during absorption:

(JK BMS 1) has the cell that likes to show overpotential early, if you let the battery balance from 3.44v with 3.505 target it gets unbalanced and can't re-balance in the hour of absorption. See battery 2 (JK BMS 2) has no problem though, hovering below 10mv deviation.



Here's yesterdays charge, with balance voltage starting much closer to target voltage at 3.49v, BMS 1 you can see the overpotential cell hit 3.505v quickly, but it was not pre unbalanced with a low starting balance voltage, so was able to be balanced in the 1 hour absorption. You can see the trade-off is the BMS 2 battery which doesn't have a cell that likes to overpotential early takes longer to balance than the graph above, but can still do it in the 1 hour absorption.



I still need to open up the box with the problem cell for my battery and check the connections, with my thermal camera. I have a flexible busbar for the link between the two rows of cells rather than the current solid busbars, and hope it's busbar related, until then I'll keep the balance voltage at target voltage. Which as Andy's video I posted earlier shows, there's some good logic for only balancing at absorption voltage.