Battery woes again

[isla]

My last wisper 575 battery died on me so I bought another 2nd hand one and have taken good care to recharge it after each cycle which is usually only to the local shops. Yesterday I decided to go a bike ride and after 8 miles my battery died despite it still showing 3 bars on the display so it would seem the bms kicked in and turned it off. Today my battery was full ( as it was going to be ) at 39.9v , after 5 miles it went to 38.4, I got another 3 miles and it died at 37.1v. I have now opened it up and checked each cell on the bms pins. Confirmed with chat gpt one cell is very low. …is my understanding correct? It has suggested I try to balance this by using a bench supply which I have one, setting the voltage at 4.1 and the amp at 0.2 using the probes on the bench supply and placing them in pins 1&2 to try to bring the cell up? Is this the correct and easy way to do it as I’m not wanting to buy another battery, I’d rather sell the bike first. Nothing but a headache. Thx

 

[saneagle]

My last wisper 575 battery died on me so I bought another 2nd hand one and have taken good care to recharge it after each cycle which is usually only to the local shops. Yesterday I decided to go a bike ride and after 8 miles my battery died despite it still showing 3 bars on the display so it would seem the bms kicked in and turned it off. Today my battery was full ( as it was going to be ) at 39.9v , after 5 miles it went to 38.4, I got another 3 miles and it died at 37.1v. I have now opened it up and checked each cell on the bms pins. Confirmed with chat gpt one cell is very low. …is my understanding correct? It has suggested I try to balance this by using a bench supply which I have one, setting the voltage at 4.1 and the amp at 0.2 using the probes on the bench supply and placing them in pins 1&2 to try to bring the cell up? Is this the correct and easy way to do it as I’m not wanting to buy another battery, I’d rather sell the bike first. Nothing but a headache. ThxView attachment 63737

Yes, that should sort it, though you should charge at 0.2C, not 0.2 MPS. The cells are probably around 2500 mAh, so that would be 0.5 amps.. The rest is a bit out of balance too, but you should be able to sort it with the BMS balancing system. Just leave the battery on charge for a long time after the light goes green. The balancing system operates when any individual cell reaches 4.2v. It drains them at about 100 mA to let the others catch up. You can use that system to do any balancing, but if one cell is quite low, like yours, it takes a long time. Yours is about 70% down, so needs .7x 2500mAh = 1750mAh, so 17.5 hours to balance.

 

[isla]

Yes, that should sort it, though you should charge at 0.2C, not 0.2 MPS. The cells are probably around 2500 mAh, so that would be 0.5 amps.. The rest is a bit out of balance too, but you should be able to sort it with the BMS balancing system. Just leave the battery on charge for a long time after the light goes green. The balancing system operates when any individual cell reaches 4.2v. It drains them at about 100 mA to let the others catch up. You can use that system to do any balancing, but if one cell is quite low, like yours, it takes a long time. Yours is about 70% down, so needs .7x 2500mAh = 1750mAh, so 17.5 hours to balance.

Thankyou for the quick response so should I plug it in and leave it for 24hrs or should I place the prongs in the bms pins first? I’d rather take the 1st option if it was possible.

 

[matthewslack]

Thankyou for the quick response so should I plug it in and leave it for 24hrs or should I place the prongs in the bms pins first? I’d rather take the 1st option if it was possible.

Don't try to bring individual cell groups up to full voltage unless you are sure you fully understand the risks.

If you leave it unattended with a power source that can go above 4.2V you risk fire or explosion.

With that first cell group voltage it is highly unlikely that 24 hours on charge will fix it.

 

[saneagle]

Thankyou for the quick response so should I plug it in and leave it for 24hrs or should I place the prongs in the bms pins first? I’d rather take the 1st option if it was possible.

Both methods will sort out the low cell. Your suggestion will be faster.