Battery cells wont balance.

[Sturmey]

Hi. A family member has 7ah cyclotricity battery which is starting to perform poorly lately. It range has decreased and whereas before, it use to take about 3 hours to charge, now it charges in a little over 2 hours. He tried to balance battery by leaving charger on overnight but did not improve things. I checked battery capacity for him (with dummy load) and it appears to be at less than 70% of its stated capacity. It was noticeable with voltmeter that battery was only charging to 40.4 volts (despite charger at 42V) and while discharging on load, battery cuts out at 32.7 volts.
Anyhow, sometime later, with the battery partly discharged, we decided to open battery to check in individual cell voltages and noted a variation of between 3.63 to 3.8 volts in the cells. We also noticed that the BMS is very tiny and contains no bleed load resistors as shown below in photos. ( There is a large vacant area on the small BMS board) I very much suspect, in this case, that the BMS fitted is a 'non-balancing' type and there is no facility to automatically balance the battery.

 

[anotherkiwi]

How old is it?

 

[Sturmey]

Its nearly a year old. We are slightly hesitant about opening the battery as I think there is a years guarantee. I did caution that once we opened the battery, we may not be able to return. However, the battery is still usable and better than 60% even with the unbalance. I'm draining some of the high cells at the moment using a coil of wire as a load and hope to manually balance and the battery may improve. I do hope however the the unbalance did not damage any cells. However, I was surprised that such a poor BMS is fitted, as the battery hardware (casings etc) is good.

 

[wheeliepete]

Only a year, bugger! I'm guessing they are unbranded cells in the pack which is possibly why they have become unbalanced if your BMS lacks this feature. Branded cells are generally far better at maintaining balance on their own. If you can balance the battery successfully, fit a new BMS to keep the pack in good health. This one looks like it will fit your pack.
https://www.ebay.co.uk/itm/10S-36V-30A-Li-ion-Cell-18650-Battery-Balance-Protection-BMS-PCB-Board-W-Cable/232979535343?hash=item363ea995ef:g:vXoAAOSwNphcRrU-:rk:1:pf:0

 

[Nealh]

After you have manually balanced the groups, see what happens to the sitting voltage after you have charged up. If after 24 - 36hrs or so they remain good then the cells are probably ok, If some bleed down then they will account for the 30% or so capacity loss and are past their best. If that is the case a new BMS won't help much.

 

[Sturmey]

Thanks for your replies. The BMS also acts like a solid state relay in that it has an additional 2 thin white wires that go to a switch on the battery that turn on and off the power. (There is no handlebar display unit) The cells are also unbranded so I think I leave it sitting for a while and perhaps recheck the balance, rebandage the battery up again and hopefully he will get another summer out of it.

 

[Nealh]

Most of my batteries have extra 2 wires for main switch as will many batteries, I even added a switched BMS to my lipo's which are behaving better then me wasting time using a lipo charger.