Revive LiIon pack?

M

mitchm33

100 µW
Joined
Sep 10, 2018
Messages
8
I have a 20S 25P Samsung 30Q pack with a zeva BMS.

I had the pack off the bike for about 3 months as I worked on the mechanical aspects (no BMS for that time). Today, as I was hooking up the battery to the rest of the system, I noticed that one of my parallel groups was at 0.00V on my BMS, the rest are around 3.8V. I took the pack back off found the suspect group and I got a few mV on the multimeter. As I've been researching what to do, the voltage are risen to 110mV over the last hour.

Do you think I can revive this group? As far as a reason for the failed group I have no clue.. All cells look fine.

This thing will be a massive pain to replace the 25 cells if I have to rip it apart so any advice would be helpful. I also have a this charger https://www.google.com/url?sa=t&rct...1%2C8+kW.pdf&usg=AOvVaw1kjyR0bXmOA-X_n1fT6IoY

Image of pack here: https://drive.google.com/file/d/12mNbvR7eFzbNrST6Wec2-hHZCIikRjO9/view?usp=sharing
 
Are you measuring voltage directly on the cell ends or at the bms end of the sense wires?
I'm at a loss to see how a 25 p block could completely discharge without heating something up and leaving some kind of trace. It would have to have been through one or more of the cells and at a very slow controlled rate.
I think you have no choice but to take that block out and individually check each cell.
 
Any cells that went below 2v are damaged and should be replaced, as they are a fire risk if recharged and used.

People have very very slowly recharged such cells, and reused them, but they will have higher internal resistance, more internal heating in both charging and discharging, worse voltage sag, and lower capacity, than they did before being damaged.

Your choice, but I wouldn't recommend it--I'd replace the entire group that went below 2v.


You'll also want to investigate *why* it happened, because you could have short across that group from a balance / sense wire (even without a BMS connected to them), or from a tab to a can, etc., and you don't want that to damage the replacement cells, too.

When voltage rises on a group, without doing any charging, it means there was a short across the group before, draining it, and now that short is gone. You want to find that short.

If it was a bad cell that leaks internally, which is possible, then it would continue to hold the rest of the cells down to it's level, unless it has been disconnected from them. If it was disconnected from them, it means there are problems with the parallel connections of the group itself, too.

If you do find a physical problem there, you shoudl check the entire pack for such problems, so nothing happens to any other group later on.
 
Thanks all for the advice.

I ended up pulling the 25 cells out as this will be a pack I use often and its just not worth the risk. All the cells are reading abut 200mV so there was definitely an issue. The scary thing to me is that I still can't find a trace of what the issue may have been.. How common is it there I would have a faulty cell? No sign of shorts, heat, or punctures.

The pack had only been charged up, and half way discharged and then sat for a few months. Maybe one cell just dropped below 2.5V and it went into shut down and dragged the rest down?

Thanks,
 
Correct - there was no BMS during the 3 months of downtime. all of the other parallel groups were fine though, so I'm still searching for answers as to what happened. I don't want to make the same mistake twice!
 
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