E-bike battery pack repair - any advice appreciated.

Hi All, I’ve been sinking deeper and deeper into an ebike battery pack rebuild rabbit hole of late. My mother rides an ebike, it was her main way of getting about, but then she left it unused for ages during lockdown, and sadly when she went to use it again it wouldn’t charge up.

From the research I’ve done this seems a common problem, where some of the cells in the battery pack can get over-discharged when left unused, then the battery management system will prevent it being recharged.

Anyway so thinking I could somehow revive the pack I went ahead and dismantled the battery to find it comprised of 40 Samsung 18650 cells, arranged in a 10 series x 4 parallel configuration. I have a basic knowledge of electronics so broke out my multimeter and found one row of four cells seemed to be the culprit. While 36 of the 40 cells registered a reading around 3.4-3.6V four of them were down around 1.2V.

Having watched a bunch of videos about reviving 18650’s cells I ploughed ahead and tried a few methods to bring the four dead cells back to life and succeeded with 1 of them, but disappointingly the remaining 3 were toast. So next I sourced 3 matching Samsung 18650s, same chemistry, capacity, discharge rate etc.

Those new batteries arrived in the post today, so using a separate charger I charged them up to about 30% capacity and would like join them back to the original nickel strips but don’t have a spot welder. I know I could carefully solder them in place, but this will damage their capacity, so am trying to blag the use of a spot welder, but as a temporary fix I’ve used little magnets to hold the batteries in place while connected to the nickel strips.

It’s hard to tell if the magnets are actually providing a good contact, but in this set up I reconnected the battery’s battery management system then connected the charger but the battery still doesn’t seem to be charging. I’ve tried measuring the voltage across the pack and it’s reading 32.6V which seems about right for a flat battery, and I read 42V coming from the charger and going into the BMS, so I think power is getting to the BMS fine.

Does anyone have any experience or ideas what could be going on with this battery pack please? I’m running out of ideas! Cheers Kev

Edit: I did think the battery management system could be the culprit, so measured the voltage leaving the BMS and feeding the battery and that seems to be 32V, perhaps a bit low, but thought it might be balancing its output with what it’s detecting from the battery?

The voltage on the BMS output and the battery will be the same, minus any voltage drop along the wire between them.

In fact, you can test if the battery is charging by connecting a voltmeter to either end of the same wire between the BMS and the pack. If there’s any current flowing you’ll measure a small non-zero voltage due to the voltage drop. If it’s zero, nothing is happening.