Pack too unbalanced for BMS to balance?

[remolacha]

Hi. I'm new to this. I bought a 48V hub motor kit on AliExpress, built a 13s5p pack using the Vruzend snap-together kit. I rode the bike short distances (1-4 miles) for a few months and noticed that the simple battery gauge on the throttle was losing bars more and more quickly. Now the motor cuts out after a few seconds. Time to check the pack. I find a cell group at 3.78V when fully charged. The rest are all at 4.14V save one at 4.04. I removed the nickel strips on one side to test each cell voltage. One of the cells in the 3.78V group read zero volts due to the cap not being on tight enough. The remaining cells in the group read 3.78V. All the rest of the pack's cells were well-connected and read 4.14V, except for those in the the 4.04V group. So I replaced the loose cap in the 3.78V group and connected the pack to a Vruzend 13s charger. The charger was alternating green/red for about 5 minutes, which seemed like balancing behavior, then went to all green. 48 hours later the cell group voltages were unchanged. I've tried two Vruzend BMS's.

Is 3.78V too far out of whack to balance? Bad cells? Looking for advice on how best to proceed.

Thanks!
Josh

 

[john61ct]

Balance manually.

New cells?

I'd break the pack, test cells (capacity / ESR), replace the bad ones, then top balance and re-assemble.

There are much more effective / forgiving balancers out there if the BMS' methodology doesn't work for you.

 

[999zip999]

Where did you get the cells ?

 

[remolacha]

From Vruzend. LG MJ1 3500 mAh 18650s.

 

[999zip999]

Check to see how well the caps are connected on the end of the cells. As those are good cells.

 

[remolacha]

Thanks...I will follow the advice from you both. Cheers.

 

[john61ct]

Make sure you finish balancing every cycle to start, then you can scale back as you see how close they stay together without balancing.

 

[dogman dan]

Yes, try to individually charge the low group. It may be that the bms won't let it charge normally.

Replace any bad cells, immediately.