question

bezda avatar image
bezda asked

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 BatteryBMS
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3 Answers
Mark avatar image
Mark answered ·

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.

1645521055327.png

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.


1645521055327.png (43.5 KiB)
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Paul B avatar image
Paul B answered ·

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

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klim8skeptic avatar image
klim8skeptic answered ·

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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