LiFePo4 Battery imbalance

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bezda asked Feb 18 2022 at 1:41 PM

LiFePo4 Battery imbalance

Hi, one of the cells on my battery goes into constant imbalace. It takes about 4-5 full repeated charging cycles for the cell to get balanced. After that when battery gets slowly discharged dow to 50 % (at ~2 amps per hour) and I charge it again, the cell gets disbalanced again.

Is that a standard behavior, or is that a fulty cell and it should be replaced under warranty? I have another battery connected in series which doesn't have this behavior and cells just stay balanced for dozens of charge cycles.

Lithium Battery BMS

6

Stefanie (Victron Energy Staff) ♦♦ commented · Feb 18 2022 at 1:54 PM

Hi @bezda,

what type/brand of battery is it? What are your chargers settings, especially absorption time?
What is the cell voltage when unbalanced compared to the other cells?

It's definitely not normal behaviour, but difficult to say if it's a faulty cell.

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Alexandra ♦ Stefanie (Victron Energy Staff) ♦♦ commented · Feb 18 2022 at 3:30 PM

Or charging faster than recommended by the manufacturer.

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bezda Stefanie (Victron Energy Staff) ♦♦ commented · Feb 18 2022 at 3:55 PM

Hi @Stefanie, it's Victron LiFePo4 160Ah, charging is controlled by Lynx Smart BMS, via CerboGX and MultiPlus. Absorption time is set to 15 minutes.

Towards the end of charging cycle the cell is 3.38V vs 3.55V in other cells. You can see screenshots of my last charge cycle, including voltage/current graph from VRM here - https://photos.app.goo.gl/hQPuvWe5bT2nJhz49

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snoobler bezda commented · Feb 18 2022 at 5:48 PM

As someone who has DIY'd hundreds of batteries of various chemistries, that is almost certainly faulty cell behavior caused by excessive self-discharge in a failing cell. It's conceivable that there are issues with the cell connections, sensing leads or the BMS itself, but those are unlikely AND also defects in the battery construction that should be covered by warranty.

If you observe all 4 cells in the 3.50-3.60V range at charge 1 and then the same cell is lagging in the 3.35-3.45V range on charge 2 following a discharge, you almost certainly have a defective cell or other related battery defect.

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Stefanie (Victron Energy Staff) ♦♦ bezda commented · Feb 18 2022 at 4:38 PM

15 minutes could be too short to completely balance the cells. It also depends on the discharge rate. Victron recommends at least one full charge in 31 days for a system with slow discharge rates. Full charge in this case also means at least 2 hours of absorption at 14.20V/28.40V.

So this would be the first thing to try and see if it reduces the imbalance to like 30-50mV and the cell to 3.50V minimum.

If that doesn't help, you could try to charge only the battery with the low cell at a much lower current of like 2A during the entire absorption phase or even longer (keep the battery and charger BMS controlled). Goal is to get the one cell above 3.50V where balancing should start. Since you have two of them in series, it would mean to isolate the one battery from the other for the above procedure if possible and of course you would need a 12V charger in that case.

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bezda Stefanie (Victron Energy Staff) ♦♦ commented · Feb 19 2022 at 5:22 PM

@Stefanie I have no problems getting the cells to get balanced, problem is it takes 4 charges to do so and then one discharge to get the cells disbalanced again

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Mark answered · Feb 22 2022 at 9:51 AM

Based on the information provided, my thoughts are that these batteries are not being charged correctly and as such haven't had a proper opportunity to resolve any imbalance. This has also likely accumulated over some time.

The VictronConnect screen images (apart from the very last) show that the battery did not receive a proper charge (sufficient to balance the cells) for over 44 days.

The VRM plot for the ~6 day period shown also supports this, over those multiple charge cycles the absorption voltage was only ever reached once, and that was during the very last charge cycle. In addition it does't look like the absorption voltage was maintained for very long (I can't tell from the time scale, but I assume only 15 min based on the absorption period set).

The Victron Smart Lithium batteries are quite effective at resolving accumulated cell imbalance and then maintaining it, but they need to be given an opportunity to work. As already pointed out, the balancing function is only active above a certain voltage threshold and this needs to be maintained for a sufficient duration.

This is what a full charge cycle (according to Victron specs for their Smart Lithium batteries) should look like in a similar system - note that the absorption voltage (28.4V) is continuously maintained for 2h, with the charge current fully tapering off.

In a system controlled by a Lynx Smart BMS that unit is the brains, it will only trigger an absorption cycle if either of the 2 trigger criteria are satisfied - 'Repeated absorption interval' or 'SoC threshold'. So if you are doing light discharge/recharge cycles on a daily basis it will not recharge to absorption voltage every day and that is not necessary, only when either (or both) of those conditions are satisfied will it increase the CVL from 27.0V to 28.4V.

If you share a screen image of your Lynx Smart BMS charge settings that would also be helpful, but in general I recommend to increase the absorption time to the Victron recommendation of 2h (like already suggested) and give the batteries a proper full charge cycle, then see how it goes for a few cycles after that.

Note that for really badly out of balance cells, sometimes an even longer absorption period (like 8 to 12h) is necessary as a once off measure for the balancer to transfer the required amount of energy throughout the cells, you just need to monitor how well the cell voltages balance up.

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Answer 1 · 2022-02-20T10:49:42Z

Paul B answered · Feb 20 2022 at 10:49 AM

More than likley a faulty cell. Do a cell resistance test and compare to the other cells.

Answer 2 · 2022-02-20T11:18:06Z

klim8skeptic answered · Feb 20 2022 at 11:18 AM

I have no problems getting the cells to get balanced, problem is it takes 4 charges to do so and then one discharge to get the cells disbalanced again

Failure to follow manufacturers charging recommendations regarding absorption times is not an automatic reason to deny warranty...

Failure to follow manufacturers charging recommendations will drag out the fault finding regarding your low cell voltage problem.

Balancing of cell voltages is cumulative. Victron recommends 2 hours of absorption. The OP thinks that 15 minutes is enough?? But concedes that 4 charge cycles will balance the cell pack voltages??

Poeple that have special batteries, that apply special charging regimes, inevitability have special problems.

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snoobler commented · Feb 22 2022 at 12:38 AM

@klim8skeptic

I suspect you don't have a lot of experience with LFP cells or batteries, nor do you appear to have experience specifically with Victron requirements. Even Victron indicates their batteries need only be in absorption for TWO HOURS PER MONTH. 15 minutes a day more than meets this requirement - even 5 minutes per day meets that requirement.

Bottom line is that healthy well-matched cells will not go out of balance on a single discharge unless there is something else wrong with the battery (sense lead issues, internal connections, BMS fault, etc.).

The OP is not failing to follow manufacturer's charging recommendations, so your snark is inappropriate.

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klim8skeptic ♦ snoobler commented · Feb 22 2022 at 3:58 AM

Did you look how the OP has been charging the battery?

6 incomplete charge cycles, only 1 that hit the absorb voltage for 15 minutes? All in a 5 day period.

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LiFePo4 Battery imbalance

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