Battery repair

[Jeff151ebike]

Great help so far for a new ebiker. Have had a problem with a second battery on one of my ebikes. My 36v battery has not been fully charging. I would put it on the charger and it would show a green light as fully charged. Then when on the ebike only showed 3 lights of the5 total in my display. I took the battery apart and all are showing 4.12-4.15v. Except the end row of batteries. Each show 0 v. It is 4x10 pattern of batteries. Could it be something not hooked right or making connection? Or could it be just 4 bad cells on the end of the battery. They are 18650. 2000ma
I’ll try to post pics.
Thanks for the help

 

[tomjasz]

“Then when on the ebike only showed 3 lights of the5 total in my display.”


Those displays are indicators, not reliable measurements. Please report actual voltage reading with a multimeter.

Hopefully that is a homebuilt pack?

 

[Chalo]

I took the battery apart and all are showing 4.12-4.15v. Except the end row of batteries. Each show 0 v.

If the pack sat for a long time without being charged, the BMS may have fully discharged that cell group to keep itself operating. If that's what happened, you can only replace them with cells of equal or better capacity.

Do not try to put lithium ion cells that have been discharged to zero volts back into service.

 

[Jeff151ebike]

It is the pack that came with the ebike. More than likely the battery set for over a year before charging. I purchased the ebike 2nd hand. So I guess my mission will be to replace those 4 cells that show 0 volts. I have ordered a mini spot welder and nickel strips and will begin the project once they arrive. Thanks very much

 

[tomjasz]

Shitty Chinese batteries and an absolutely shit build. Man, I’ve seen homebuilt batteries that aren’t as sloppily built. What a piece of garbage!

 

[tomjasz]

What is the ebike brand and model? Help some other buyer avoid that crap. I’m sorry you got ripped off!

 

[amberwolf]

Except the end row of batteries. Each show 0 v.

Is it the most negative end row? Or the most positive?

If the former, then the BMS may simply be powered by that row, and killed them over the long time between charges.

If the latter, then it is more likely that there is something wrong with the BMS itself, where it's balance shunt resistor is stuck "on" and thus always draining that bank of cells, even when it shouldn't. (this could also happen in any other cell group position).

It is also possible it's simply one of the cells in the dead group that killed the whole group.


When you replace the group of cells with new ones, then you can test for this by measuring the voltage across the shunt resistor itself for each channel, with it just sitting there not charging, in any state other than right after charging.

If there is any voltage across the resistor for a channel, then it's balance shunt is on. If it's not in balancing mode (meaning, the cell voltage is above the balance point, often 4.2v, sometimes less), then that means the shunt is stuck on, and the BMS needs replacing (unless you want to try to fix it).