Bms problem or bad cells in 36v package?

[Sweden]

I have a 36v stand up escooter that has been cutting of faster and faster latest time.

I started with measuring the cellgroups right after it cut off. All of the ten groups had 4,0v-4,1v. I plugged in the charger and it charge for about 15 minutes. Measuring again and now I have 3,66v in one group and 4,1v in the rest. How could that be? I thought it would be the opposite. Do you think I have a problem in the 3,66v group or is it the BMS that causing the problem?

This is the first time I open up a battery so total beginner in this.

The issue is in the B2 group

Thanks!

 

[Diggs]

My thought is you have some bad cells in that group. I had something very similar happen on one of my battery builds long ago that used computer laptop batteries (not a good idea.....).

 

[docw009]

You must have measured it wrong the first time. B2-B1 is 3.66V? This is what an unbalance battery looks like, and many scooter/ebike packs end up like this, I do not think it is the BMS. It is bad cells.

I think the main cause is one of the three cells in the B2 group was weak. Causes the group to drain faster. It initially ends a little lower in voltage than the others after a recharge, With repeated run/charge cycles, the group never catches up and the spread gets larger. Now you have that group at 50% charge and your range is short.

In my opinion, move on and replace the battery. Safety issues.

 

[E-HP]

The problem with the majority of scooters out there, the 8" and below and most of the 10", is that the riders use the throttle as an on/off switch, which makes sense since there's not much noticeable modulation of the power anyway. Consequently, they are always run at full tilt. It's not until you're at the 11+ inch scooters, and some of the 10" wannabes where riders can or need to ride at partial throttle.
Seems counterintuitive, but the for 10" and below, those scooters need the best quality batteries, not the cheap junk they come with, in order to deal with WOT all the time, so batteries that can discharge well above the required current, in order to not be stressed. I'd replace the pack with the best cells possible that you can afford

 

[Sweden]

You must have measured it wrong the first time. B2-B1 is 3.66V? This is what an unbalance battery looks like, and many scooter/ebike packs end up like this, I do not think it is the BMS. It is bad cells.

I think the main cause is one of the three cells in the B2 group was weak. Causes the group to drain faster. It initially ends a little lower in voltage than the others after a recharge, With repeated run/charge cycles, the group never catches up and the spread gets larger. Now you have that group at 50% charge and your range is short.

In my opinion, move on and replace the battery. Safety issues.

Thanks for your answer!

Yes I do question my first measurement….. perhaps I measured b2 as the next group(group 3).

Ok. You think it’s a bad idea to replace the bad cells in group 2? According to the controll box it’s about two years old. But perhaps the quality of the rest of the cells is low? Can’t find much about ”CHY 18650” cells.

 

[E-HP]

It looks like the 2200mAh version of those cells (the ones in the pack are 2000mAh) go for under a dollar a cell. A good cell costs over 5 times that. It wasn't designed to last long.