My BMS kept being warm after unplugging the charger

I got concerned when I found out that my BMS kept being warm after unplugging the charger and waiting for a while. So i unplugged the balancing leads and waited for it to cool down. When it was cold I plugged it back in and it didnt warm up. Is this normal ??? I have my BMS wierd up only for charging. Does the BMS still balance even after its been unplugged?

I checked the pack with my Fluke multimeter: 4,19v 8,38v 12,57v 16,77v 20,96v 25,15v 29,34v 33,5v 37,7v 41,9v

This is the BMS

How Hot? It is normal to be how while balancing. No need to be plugged.

Those P packs’ voltage values are all over the place, which is probably why the bms is super warm.

edit: My bad, I read it wrong.

I know, but is it normal that its still hot 10 minutes after unplugging the charger even that I’m blowing on it with a fan. It only cooled down when I unplugged the balance cable.

Why do you say that? I am seeing 4.19-4.20V for every p-group.

Edit: There is one p-group with 4.16V

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Nvm, you were right. I read it wrong. My bad.

Yes. It is normal. Balancing can take hours. Unless the bms goes above 40° celsius i wouldn’t worry. You do the right thing noticing the heat. It could be troublesome. As soon as the pack is fully balanced your bms should remain cold after charging.

bms temp during balancing:


250f or 121c, and this is normal.

this is why it’s not good to place the bms directly on the battery. but i digress.

bms should NOT remain hot when charge stops.

as a test, take P group voltages, jot it down. then plug balance leads back on, connect to battery but DON’T charge it. leave it for…hours, come back and take P group voltage readings again. if there’s a difference, you got a bms problem.

i’ve had bms that had slow drains on them.

failure rate is low, but not uncommon.


Is 121°C really a normal figure during charging? With how many amps did you charged to reach that temperature? I had my BMS placed on one P group and it killed the P group in the course of one year of frequent charging. However I checked the pack several times by hand and didn’t noticed any heating during charging. My BMS was also wired for discharge and I noticed that the pack got hot twice in the area where the BMS was placed.

it always get this hot, and it’s using a 1.5a charger.

but the heat isn’t coming from the charger. the tiny resistors are draining an entire P group at like 250mAh. so, a 4p group is 12Ah, being discharged at 250mAh with a tiny resistor - so that’s working full time trying to drain a giant charge.

anyway, yea it gets hot.

with yours, maybe your pack is perfectly balanced so it doesn’t kick into balance charge? i mean, you said it’s discharging too so it may be keeping it balanced at all times. this is the potential issue with bypassing the bms.

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Thanks! Now I understand better. I think my BMS just controls overcharge and under voltage, it shut down my pack because the P group came out of wack completely. But having the potential that something can heat up to 120°C in my enclosure makes me anxious. Could every BMS get that hot or are there better ones in this regard? Now I know it’s a good idea to check my packs and BMS’s on the end of the discharge!

bms balance through bleeding charge with resistors - resistors convert energy into heat - so i think all bms’ will get hot.

so yea, there’s something that is way above boiling point hanging out inside the enclosure, kinda scary.

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