parallel battery packs with one bms

[fruitman]

hi i am new here and i am just getting into building ebikes. i have two battery packs that are for scooters and one of them has a busted bms. they are identical battery packs, both are 10s3p 36v 7.8ah. i was thinking of just wiring them together to share the same bms so they can charge and discharge together. i have been doing my homework on this website and i came across this post by dnmum

"you can use just one of the BMSs that is installed on one of the packs. tie the B- and B+ terminals of the two packs together, connect the two packs through the sense wires so each cell is parallel with the same cell in the other pack, and then use the P- connection for the motor and the P+ is the red wire from the top of the two packs tied in parallel. then they both operate together as one pack, charging and discharging."

it seems like this should be easy for me to accomplish but i really don't want to mess anything up and ruin my battery packs.

i am looking for some extra clarification and maybe a bit of hand holding..

i have attached a photo of the bottom of my busted pack. the battery packs are the xiaomi m365 battery packs.

along the bottom of the pack it has metal pads labeled b0 b1 b2... b10. are those the sense wires that i would need to connect to the other battery?

what else do i need to do exactly? what are the p- p+ b- b+ exactly? im having trouble picturing this all in my head..

thanks!

https://imgur.com/a/uOnmGUc

 

[amberwolf]

First, are you sure the BMS is dead, or is it just protecting against charging or discharging a pack with cells taht are too high or too low a voltage? You'd ahve to measure the voltage of each cell / group to find this out.



If the cells are all ok, then in theory, as long as all teh cells are already exactly the same voltage, in each pack, in each group, then all you have to do is:

Disconnect the wires (at teh BMS end, not the cell end) on the dead-BMS pack. Tape each wire off as you do it so you don't short anything, and cut ONLY ONE wire at a time.

Reconnect those wires to the working-BMS pack exactly like the cells already attached to that BMS. Start at the most negative wire, and work your way up to the most positive.


If the cells aren't the same voltages, you'll first have to charge each one up to be the same, or discharge high ones down to match low ones.

If any cells are too low or too high, then you shouldn't use those cells, and may have to do a more complex rebuild of the packs.

 

[dogman dan]

That post refers to charging two functioning bms, connecting one to the other as he said. this allows one charger to charge both. But each bms still balances and does the other functions.

For one bms to control both, you need to fully parallel the packs. This means parallel connecting the big discharge wires, and, parallel connecting the smaller wires that lead to the bms.


For sure, begin by individually charging any cell groups that are crazy lower than the others. Your bms may be shut off because one group is too deeply discharged. What I mean is look first at the cells, then start thinking it might be the bms broken.