Bms or bad cells?

[bear79]

On ebike battery 36v the battery somehow downgraded to 33v.
i have removed the packs and charge them with a 3.7 pulse charger.
i have succeded to measure at 36v all the packs.
i solder them again and seems to working_charging so far..

Does it mean that the bms faulty downgrade the batteries below 36 or what?
Thanks you are awesome forum.

 

[caleb7]

"downgraded" means it's fully charged at 33v? A 36v battery can be anything between 42-30v depending on power left.

If it's not charging above 33v it can be totally out of balance or a cell group may have damaged cell .

 

[bear79]

"downgraded" means it's fully charged at 33v? A 36v battery can be anything between 42-30v depending on power left.

If it's not charging above 33v it can be totally out of balance or a cell group may have damaged cell .

Thanks for relply.
downgraded means that the bms shouldnt stops the batteries going below 3.5 v.
When i opened the battery all the packs has it 3.3 v
Is it correct?

 

[caleb7]

The low voltage cutoff of BMS can be quite low, i think my "Heltec BMS" does this at 2.8V. The controller will cut earlier than this.

If we're talking about li-ion, 3.3V is probably not near any cutoff. It looks everything is normal then.

 

[bear79]

Then why when i opened the battery and charge the packs the bms awaked somehow.
icharged at full and saws me 41.8v.
i will try to test it on road to see if it has the right amp to make the road it should.
Anyway thanks.
Is it so easy the bms to be faylty or to battery?

 

[caleb7]

It's first time you say BMS didn't work, but worked after the 'manual' charge.

If one of the groups drainedbelow a certain voltage (can be due to a dead cell) , BMS may avoid charging as a safeguard (from a fire, etc. ) .

 

[docw009]

It seems like you had a battery that wasn't charging because of low cell voltage.. There are 10 cell groups and if any are too low, the BMS will not allow charge. You charged them all to 3.7V, and now the battery works, charging to 41.8V. If it continues to work like this, then you may be OK.

You said the cells were at 3.3V. Normally, the BMS will use 2.9-3.0V as the lower limit, Maybe you did not check all ten cell groups. If any of the cells went significantly under 2.5 volts, there is a risk of cell damage.