Battery Repair

Standard procedure is to open up the pack, measure the pack voltages. If one cell is low, use an external power supply to bring it up to the same voltage as the rest (with a current limit so it charges slowly). That may help, although if it happens again it could be those cell(s) are weak and need replacing.

Watch for 'smarts':

- the BMS may have noticed something is awry and made a note of it which doesn't clear once the fault is fixed. I know Makita batteries can permanently refuse to charge if an overdischarge is detected ~3 times.

- the BMS may become unhappy if power is removed from it. Therefore if changing cells you may need to arrange to keep the BMS powered while the cells are swapped out.

Dig around (eg Youtube) for specifics for your kind of battery as it may give clues as to the cause of the lights, and whether there are any pitfalls to be aware of.

Theo

Yes this method has been done with our pepper and Nao robot battery packs, using a bench power supply to get them over the minium voltage. But the best option for us was just to replace all the individual cells inside the pack (18650 or similar)

I've repaired em but my experience is once a cell goes the whole pack is useless You can buy un regulated packs from Hobbyking and have a crack at a repair, but that is a BIG battery 15 x 5AH cells probbaly

(I meant 'measure the cell voltages' not pack voltages)

Often the reason for a cell being down is because the BMS is powered from the first cell in the pack. That cell slowly discharges while the pack is out of the tool, while the others don't. That causes an imbalance.

It is possible that recharging that first cell is enough to bring things into balance again and the pack is as good as new.

But if it's been doing that for a long time it could also be that the first cell has degraded due to being over-discharged and so has a lower capacity when recharged. That results in the BMS declaring the pack is flat long before the other cells are empty. In which case replacing the cell is the way to go. But ideally you need to ensure the cell is replaced with the same model, because the specs can vary quite a lot from one model to another (ie it's not just about capacity).

The nuclear option is to replace all the cells, so they're all fresh and from the same batch. In which case you may need to keep the BMS powered while doing so, so it doesn't 'forget' anything. I think some BMSes have their config in volatile RAM, and if it's gone they're a brick - there is likely info floating around for whether this applies to a specific battery.

Theo

"From trash to treasure: Teardown & Repair an Ego 56V 5.0Ah battery"