Hi everyone. I have a diy 10s3p 18650 li-ion battery build with a 10s Daly BMS in a charge-only configuration, but two of the groups don’t seem to be balancing.
On 31 December these were the cell voltages (after 48 hours of charging):
4.180, 4.186, 4.168, 4.181, 3.887, 4.182, 3.980, 4.182, 4.184, 4.190
The cell voltages today, 12 April, are (after being plugged in only a few hours after full charge):
4.191, 4.191, 4.208, 4.196, 3.699, 4.200, 3.908, 4.194, 4.199, 4.205
As you can see the difference between the groups has gotten bigger, even though I haven’t used my board much since December. What could be the reason for the big gaps for groups 5 and 7? Is my BMS broken? It’s starting to cause some of the other groups to charge above 4.2V, and group 5 is probably at the most risk when discharging.
It might be worth mentioning that my power supply only supplies about 41V (although when I measured it again today it was supplying 42.6V)
Most likely, you either have a bad cell or broken welds in those two groups which is causing those groups to drain much faster than the rest.
Almost certainly not.
Neither of these is anything to worry about. Likely, your multimeter isnt even very accurate to the 0.001 decimal place. Most aren’t. And being 0.008V above 4.2 isn’t going to hurt anything. Your BMS will enter over-voltage protection if any of your cells measure 4.25V or above.
You might be misunderstanding how cell balancing works on most BMSs. They do not charge up the cell groups which are too low, they slowly discharge the cells groups that are too high. This balancing function is very slow, and is intended to keep a healthy pack healthy for longer, not to save a damaged or unhealthy pack.
It seems very likely that groups 5 and 7 of your pack are damaged or unhealthy, and are discharging faster than the other groups. No BMS can save them from that. You can try manually charging up those two groups individually with a 4.2V power supply through the balance plug, and see if they stay in balance after that. But more than likely they will simply fall out of balance again until you open up the pack and find the issue which is causing the imbalance.
Sorry to be the bearer of bad news, but I hope that helped!
I really hope that isn’t the case, but the fact that they have been consistently off for such a long time leads me to believe that they might not be.
I think I’ll give this a go and see if it fixes the issue. I don’t have a variable power supply, so I might need to supply 5V and keep an eye on it. Or maybe I could tear open a small power bank and connect it’s charging circuit to the faulty groups.
I would do a cycle on the bench to see how your pack is doing.
Its something you should do after a few hundred cycles imo. (preferably the very first cycle too so you have something to benchmark against)
charge pack to full, all cells should be balanced.
do a controlled discharge for a specific current, lets say 10A for the hell of it.
discharge until empty (one cell reaches 3.0V)
Compare your results to your first cycle
Intresting metrics;
Overall Ah for a full discharge, how much capacity has been lost since first cycle?
Are cells discharging evenly?
did any cells get hotter than the rest? (thermal imaging useful here)
Voltage sag during draw, does it seem reasonable?
The above can be done for the charge cycle as well.
Granted these tests are more easly done if you have the right equipment, an electronic load f.e. is very useful.
Glad I read this before this weekend so I didn’t have to try and deconstruct my pack more just to charge up a low cell group. I don’t think I’d realize I can just use the balance wires to charge the group tbh I don’t expect the pack to last much longer since that group is likely just going bad due to age. Just want to prolong the life a bit longer until I get a new battery in
Also, if you have a charger for individual liion cells (such as ones used for vaping) you can hook up to those terminals and it will charge the groups in your pack just fine.
