Battery Repair/Rebuild

[pjwalmsley]

Hey, last fall my battery died. I believe there are a few bad cells.

It's a triangular A123 battery glued and soldered together by CellMan. Great pack while it worked, but something obviously happened.

Basically it charges to full capacity, but then drops by about 8 volts and looses a lot of current after a few miles. My sister dumped the bike on grass a few weeks before it began exhibiting issues. I took the battery cover off and built a connector to hook up a light. I've been charging the battery to full capacity, waiting a few minutes, unplugging the bms, attaching the drain, then checking voltages between cells. They seem to be sporadic. Am I doing this correctly? It seems like they should be mostly full with the exception of a few. Does this mean the BMS is bad? Does this sound like a bad cell or a bad bms? I should probably do it again to be sure.

What would you guys recommend I do? Is their any way to remove the cells? Can i solder tabs back on, or do they have to be welded?

 

[dnmun]

no idea of what you did so no way to comment.

 

[alsmith]

It could be your connection but you've given no details, I'm not sure who has the ability to diagnose accurately by guesswork.

Stop blaming your sister (+ apologise) or there'd be loads of fall off-stopped working posts- you don't think designers/builders are aware of people falling off?

If your electrical work wasn't properly done it could have come loose, could have caused sorting, maybe it puts a permanent drain on the cells it is connected to? Does your wiring go to one cell, some cells, all cells, bypass the BMS?

But this all guesswork- you haven't given much detail. From what you've said though I'd remove the connections you've made, while your removing them check carefully for any damage- loose connections etc. Does the problem stop, or still happen? From the very limited information (about changes) given that's the only logical guess I can make, but the problem could still be anywhere else- loose connections are often a cause.

Good luck with finding your problem.

 

[dogman dan]

Uh, we don't know if that fall damaged the pack or not. It's possible it did. Depends on the fall. It's quite possible, that more damage happened in storage, if you did not unplug the bms during the storage.

Step 1, try to determine if all wiring is connected, particularly to the bms. If not fix that and then try charging it again. Also make sure the charger plug is not broken. You can get a broken wire, which results in a partial charge.

If it seems to be a cell problem rather than just connections to the bms, then unplug the bms. Now you can start looking at the cells without any chance of any oddities being caused by the bms.

Try to charge each paralleled cell group one by one, and see which ones hold a charge and which don't. A cheap 50w RC charger is perfect for this task, but you can do it with a phone charger power supply. But you have to watch a 5v supply like that very close, so you don't overcharge a group.

If nothing else, you have some good cells left that are quite valuable, and you can charge them for other uses with a cheap RC charger. Well worth investing a few bucks on an RC charger. Once you determine which cell groups are ok, you could sell the good ones off easily, paying for the charger.