Dropped battery, one cell dented. Trying to understand failure mode.

Hi there,

I have a 10s2p battery without hard case (just shrink wrap). I dropped it, and now the negative terminal of one of the cells in the last pair on the positive end of the battery is dented. Photo attached.

I first noticed an issue when the battery voltage was suddenly so low that my ebike wouldn't even turn on. Multimeter said 24v.

I charged it up again, and it got back up to 42v like it should, but over night it dropped back down to 38v.

I unwrapped the battery, noticed the dent.

I tested each pair, working my way up from the negative end to the positive end. Except for the last parallel pair (positive end), every pair reads 4.14v +/- 0.01v; the last pair reads 0v. So it seems the cells are well balanced, but the last pair is defective.

Here's what I think happened, I'd just like you all to confirm:
- The dented cell is internally shorted out
- It self-drains to zero
- It's parallel neighbor then discharges into the dented cell until it also reaches zero
- When I charge it up to 42v, I'm only charging the non-dented cell in the pair, and as soon as I take it off the charger, they dented one discharges the non-dented one again.

Is that about right?

Follow-up questions:
- Do I only need to replace the dented cell? Or is the other cell in the pair damaged too by virtue of being discharged to zero?
- Is the battery safe to let sit like this, with the final cell pair at 0v? It should just be a 9s2p battery now, right?
- Why didn't I notice any kind of heat / get a battery fire if one cell is shorted out. Wouldn't that cause really high current?

Thanks!

The cell group in question acts, electrically speaking, like a single cell. The dented cell is as you state, likely internally shorted and has drained both cells in this group to zero.
Both cells in this group are now dead on account of being pulled to 0v.
Do not charge it....this is when the flames start.
Your only option is to remove these cells and replace with matching cells.
And no... It isn't now a 9s2p battery! But yes it is safe to sit with the cell group at 0v..
Don't charge it!

Thanks!

Definitely won't charge it again. I was just asking whether it's as harmeless as a 9s2p battery for sitting around doing nothing until I get the cells replaced.

Same thing happened to me(The link). If you have time I would suggest removing the damaged cells. I got really lucky and mine was not shorted, I still discharged it fully and gotten rid of it.