Troubleshooting electric bike battery pack, Lithium Ion 36V 9Ah

[Chungalin]

This equipment doesn't match a more specific subforum, so I'm posting here.

I have a 36V 9Ah battery pack for an electric bike that is not charging well and has a lower voltage than nominal.

I've disassembled it and it consists of a matrix of 20x CGR18650CG Lithium Ion cells. It's arranged in a 10x4 grid, meaning: 10 packs in series of 4 cells in parallel (10 x 3.6V = 36V; 4 x 2250mAh = 9Ah).

I've measured each pack of 4 cells and the first ones (from first (+) to sixth) they all read about 4V (correct), while from seventh to tenth (-) they read between 1.5 and 1.9V. I've split the individual cells of this 4 suspicious groups, hoping to find one of them specially dead... but not. All cells in a group read the same low voltage, not higher, not lower than the whole parallel group.

The charger has a test point to each union between the 10 packs.

What can be wrong? I've seen several vids on YT that show how to revive LiIon cells, but that trick doesn't seem to apply to these.

[stj]

Re: Troubleshooting electric bike battery pack, Lithium Ion 36V 9Ah

read the voltage of all the suspect cells - write it down, then wait a few days and repeat the test.
one of the cells may have a bad internal resistance.

[Chungalin]

Re: Troubleshooting electric bike battery pack, Lithium Ion 36V 9Ah

Interesting point: the time factor. Thanks!

Then you mean that good cells with rise its voltage slowly by themselves while the bad one will keep low?

[stj]

Re: Troubleshooting electric bike battery pack, Lithium Ion 36V 9Ah

i was thinking more that a poor cell would slowly deplete itself through internal resistance.

[fzabkar]

Re: Troubleshooting electric bike battery pack, Lithium Ion 36V 9Ah

FYI, you can safely measure a low voltage battery's ESR with Bob Parker's ESR meter.

[Chungalin]

Re: Troubleshooting electric bike battery pack, Lithium Ion 36V 9Ah

I have EVB ESR meter but I’ve never used it for measuring batteries. I’ve read official website ( http://clientes.netvisao.pt/greenpal/information.htm ) and looks like it’s not safe for the meter to measure batteries ESR with normal probes. I don’t know if this warning applies to all kinds of batteries or just larger ones. I’ll ask to the maker/vendor.

[sam_sam_sam]

Re: Troubleshooting electric bike battery pack, Lithium Ion 36V 9Ah

I’ve measured each pack of 4 cells ---? while from seventh to tenth (-) they read between 1.5 and 1.9V. I’ve split the individual cells of this 4 suspicious groups, hoping to find one of them specially dead... but not. All cells in a group read the same low voltage, not higher, not lower than the whole parallel group.

You need to split the individual cells and very slowly charge them at no more than at 100 milliamps BUT only do this for cells that have a reading of 1.9 volts or more

The ones that read 1.5 only charge them with no more than 50 milliamps

Also watch the voltage when you first put them to charge if after an hour the voltage dose not go up ANY then they might not be good any more

One very important thing is keep a very close on the temperature of each cell if they start to get warm STOP charging them they are to far gone to revive them

I have done many time before some time they come back and some time they do not but they will not have full power any more

Please be careful when doing this