Replacement BMS not playing nice

[JCrowe69]

Hi All,
I burnt out the BMS of my 72v20AH battery, bought an identical replacement from the manufacturer, and carefully replaced it, the 20S wires were easy as they were just two plugs of 10 each, and all other wires were like for like. Now the battery will charge but not turn on when disconnected from the charger. But, if I plug the battery into the bike and also connect the charger everything works, does anyone have a clue what might be going on?
Cheers Jason

 

[Comrade]

If you unplug the battery from the charger and before you plug it into the bike, does it have voltage on the output?

 

[docw009]

Did you check the 20 group voltages to make sure none ore under voltage?

 

[amberwolf]

I burnt out the BMS of my 72v20AH battery,

What was the specific cause of the failure? What happened just before it failed; how was the system being used?

Most of these don't just burn out without being overloaded by the system they're providing power to, so your present BMS is probably working just fine, and protecting the cells against damage that could lead to a fire.

 

[JCrowe69]

Hi All, Thanks for your replies, I'll check Comrade and docw009 suggestions, amberwolf my son was riding it home (we live on top of a steep hill) and when he got home the battery was smoking, pulled the BMS apart and it looked like the mosfets had melted/caught fire, here's an image

 

[JCrowe69]

If you unplug the battery from the charger and before you plug it into the bike, does it have voltage on the output?

I just went and did some voltage measurements, this pack says it is charging at 84v
1. Power off disconnected from charger 0 Volts (Good!)
2. Power on disconnected from charger 5 Volts
3. Power off connected to charger approx 35 Volts
4. Power on connected to charger approx 44 Volts
I'm wondering now as docw009 mentioned maybe there is an issue with one of the 10S plugs or the wires connecting them to the cells, I haven't been this deep into a battery before (mechanical engineer) so was wondering what and how should I check this?
Cheers
Jason

 

[amberwolf]

Yeah, that's pretty toasted; even the wiring appears damaged, which implies a higher current than it is designed for was being pulled thru it by the system.

Sounds like the battery is not powerful enough to run the motor for your application.

Or, the controller failed, blew up inside, and shorted out the battery, causing it to fail like you see.

Or the motor itself overheated, failed shorted internally (or in it's cabling) and blew the controller up and then shorted the battery.

In either case, that would keep the battery from turning the system on, even if the battery is working ok.

Either of the last two cases would probably leave the motor hard to turn. If it's a hubmotor in the wheel it would be easy to tell, but if it's a chain or belt drive it may have a freewheel between motor and wheel, so the wheel would turn easy but the motor might not if you tried to turn it by hand.

If the motor spins easily, then the controller and motor are probably not the cause, and something is either wrong with the new BMS, or...not sure.



BTW, you might want to monitor motor temperature once this is fixed to see if it is handling it well, or if it too is overheating.

I just went and did some voltage measurements, this pack says it is charging at 84v
1. Power off disconnected from charger 0 Volts (Good!)
2. Power on disconnected from charger 5 Volts
3. Power off connected to charger approx 35 Volts
4. Power on connected to charger approx 44 Volts

Only the first one sounds correct.

All the others sound like the ghost voltage you can get when the BMS has turned off the port.

If it charged correctly, and all cell voltages are identical, the BMS must be connected to them correctly, and be able to read them all / balance them, etc.

Is this a programmable BMS? If so, maybe it isn't yet setup correctly for your system / cells.