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Best 8 cells out of 9

liviu.gelea

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Apr 2, 2023
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Bucharest
Hello. After founding a very detailed response to a question i was interested in about the "settling" voltage on lifepo4 batteries I thought this was a good place to ask for some help.
I bought 9 32700 cells I'm plannign to turn into a battery bank (4s2p). I bought 9 since I didn't trust them much and they were pretty cheap. In fact I bought 1 first and tested its capacity to above its rated 6000mAh ( it held above 6150mAh) and only then ordererd 8 more thinking I would exclude the worst one from the pack.
Now... since I bought them I didn't get the chance to test them all and kept them in storage for a year.
Recently I found that one of them had dropped to a mere 0.7v or started this way, who knows. I tried to "recover it" and charged it for a few hours at 2mAh until 2.8V, then using around 1A until full. Then tested it and it was 6139mAh. I so far tested a second one to 6245mAh and I'm in the process f charging all of them.
My question is what kind of tests I can do besides capacity to determine if it's safe to pair them inside a 4s2p bank and is it likey for a cell to be bad if it's the right capacity and has reasonable internal resistance?
For testing resistence I'm thinking I should do an AC test since I have a function generator and an oscilloscope besides a pretty good multimeter. Should I store them at full charge and worry if some drop more than others with time? should I just pair highest with lowest capacity in paralel to equalize the capacity of each 2p ?
Thanks.
 
Internal resistance (IR). Almost a certainty that the 0.7V cell has higher IR than the other cells.

Even if I didn't measure higher IR, that's my "spare" cell.

Self discharge (SD) is also valuable. Let them sit separately for 30 days in a climate controlled environment and note their respective voltages.

If you can count the charge current, re-charge to full, and you have a value that represents that which they lost over 30 days.
 
Thank you. Will do all 3 tests if I can get a system to measure internal resistance right.
Any thoughts on the idea I should pair them so that each pair has similar capacity?
 
Around here, 4S2P indicates two 4S batteries in parallel. I assume you mean 2P4S - 2 cells in parallel, 4 of those in series.

In any case, all series elements need to be as close as possible to the same capacity.

If you were doing two separate batteries, you would want Battery 1 cells to be as close as possible and Battery 2 cells to be as close as possible.

If 2P4S, you would want each 2P group to sum to the same capacity.

If you had a range of capacities from high to low:

1
2
3
4
5
6
7
8

You would pair cells:
1-8
2-7
3-6
4-5
 
Many thanks. Will do all of these tests:
capacity, internal resistance, self discharge and if every cell is ok will remove the one that was 0.7v, then pair highest with lowest capacity like you wonderfully suggested.
Many thanks!
 
A reasonable self leakage rate in % capacity per month is 0.3% to as much as 6% if stored in hot environment.

A cell series resistance is normally independent of shunt leakage resistance as they have different causes.

Without knowing what the initial state of charge was and how long a cell took to decay to 0.7v it is hard to say it is good or bad. You should not allow a cell to drop below 1v as it can start to grow lithium metal dendrites that can punch through separator causing cell shorts. This can also be the cause of high self discharge leakage rate.
 
Thanks RCinFLA, I hope the self discharge test will shed some light in this. I'm definitely not allowing the voltage to drop again, but while the cells were separate it was very time consuming to monitor all of them.
 
May want to invest in one of these. About $30-$35.
 

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