Can you break a BMS by using a lower voltage?

The reason I ask is because i adjusted my charger to output 41.2v (10s) and charged my board a couple of times. However now when i charge with my 42v charger the bms only outputs 40.8v. I tested the charger and it outputs 42.3v

oh that’s interesting…

Which BMS is it? one of the Battery Supports ones?

This cheapo one. I broke one before, but that was totally my fault.

My guess is that it’s more likely that your battery just got out of balance because of undercharging and now can’t charge fully because its too far out of balance. Happened to me once when I replace 2 cells that where out of balance with the rest.

could be a few things, but really 1.2v/10 is only .12V per cell. you won’t be losing any real capacity. the cells are probably just wearing down a bit, so the bms is doing its job!

I’d be more worried if it were just spiting out the same voltage you put in.

I’m 100% sure this isn’t normal. These cells are nearly brand new. 9 of the cell groups are at 4.08v and the other one (#7) is at 4.17.

@Namasaki I guess I’ll try discharging the higher voltage cell group (above). It’s not that far off so I don’t think that’d be causing the problem. I don’t have anything to discharge it with so I’ll maybe ride and charge to see if it rebalances.

Oh one more thing guys. My BMS has P-/B-/C-, along with 11 balance wires (1 negative 10 positive). I got the balance leads hooked up correctly and I removed P- (bypassing BMS) and have C- to my charger, but I’m wondering if I even need B- hooked up (currently is) to battery negative since I’m bypassing. Wouldn’t it just charge through the balance leads since there’s a negative there as well?

It’s not that far off but it could be enough to keep you from going full charge.

sounds feasible since there is a ground wire on the balance port. You could try disconnecting it and charging it stopping when the pack hits 42v and then check for balance. I think if you manually balance down your high cells, you will be able to do a full charge with the bms.

I gotta go take my grandson to Gamestop. Be back in a bit.

@Namasaki @longhairedboy @saul

To loop back on this and for anyone curious, it seems like yes… you can break (some) BMS’s by using lower voltage.

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well I think thats more because you could not get a full charge with lower voltage.

from what i’m seeing with these cheap bms’, they can work on various size batteries, they’re all 12s, but you can just leave others unconnected because all the cell circuits are running in parallel.

Sure, but I used a 42.2v charger afterwords and the charger thought it was full at 40.8v, can’t get any higher.

Thats most likely because your cells are out of balance from undercharging. I have experienced the same thing when one of my packs where out of balance. In this situation, you’ll have to go in and balance cells manually to get it back on track. I don’t know if undercharging can break a bms, it seems highly unlikely, however it’s pretty evident that a bms can’t really do it’s job without a full charge.

Isn’t that the point of the BMS though? To bring your cells back to balance? My understanding is that while it’s charging and a module hits the cell voltage threshold (4.2v) it’ll charge the module with a trickle current while the other packs catch up. Right now one module is at 4.2v, one is at 4.17v and the others are at 4.1v.

I get what you’re saying about needing to manually balance, but i only charged it with 41.2v like 3 times, it’s weird to see it this out of balance and I feel like the BMS should be the one balancing the pack for me.

The balancing circuit has to be in series to work and to balance each cell or cell group. What would happen if you took cells that are connected in series and then connected them to the balance circuit in parallel. I don’t think that would work. I think it would cause a short circuit.

Yes it most certainly is but I think you need to do a full charge for it to work properly.

Right and that’s what I’m saying, I’m plugging in 42.2v now, and it stops at about 41v. Currently, all modules are at 4.1v besides 1 which is at 4.2v. I would’ve assumed it would slow charge that higher pack while charging the rest. It’s not really that out of balance.

I’ve tried several different chargers and found that the best charger to use with a bms is a lab power supply with CC/CV output. The way it works is amazing. It starts in CC mode with what ever amps I set the max to be and slowly brings up the voltage so that it stays just above the pack’s voltage until it reaches the max voltage that I set, in my case 42v. At that point it automatically switches to CV and starts reducing charge current as the battery approaches 42v. So once the charge current falls below the bms balance current, (126ma for Bestech) and the battery shows 42v, the power supply will continue to provide low current charge to to bring up the low cells while the bms continues to trim the high cells. Now it’s working similar to the way a hobby charger works.

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That does sound nice, but I like the sleek & simple solution that a laptop style charger gives you. I guess I’ve just yet to find a high quality, reliable, BMS in a tiny form factor.

Well, good luck with that search :grin: