1st big cell balance almost complete... how often do you fully balance your cells?

[SajgZi]

Hey everyone, I have a 8S 260Ah battery bank made up of 8 large cells. I did the initial top balance when I put together the pack this spring. I've been charging to 3.55V per cell and but didn't have balancing set to start until 3.58v. The cells mostly stayed in balance but today they were 60mV apart (thick green line in the attached screenshot) so I decided to do a full top balance again. I kicked up my inverter voltage .1V at a time to 28.8V (3.60V per cell) while monitoring the cell voltages. Three cells were near 3.65V (see screenshot) or else I would've done a bigger jump. It took most of the day but the cells are pretty close again. Tomorrow I'll keep going higher to finish off. I figure I'll do this twice a year or so.

Does anyone else do a "semi-automatic" or full manual balance more frequently? I'm looking for some best guidance on how and when to balance.

Thanks!

 

[time2roll]

I have my BMS set to balance while charging and above 3.4 vpc. Not sure why you disabled that function.

 

[SajgZi]

I have my BMS set to balance while charging and above 3.4 vpc. Not sure why you disabled that function.

I edited the post. It was enabled but the voltage to start balancing was set to 3.58v which shouldn't have been hit if all cells stayed in balance

 

[sunshine_eggo]

I have my BMS set to balance while charging and above 3.4 vpc. Not sure why you disabled that function.

^^This so hard. If the cells are reasonably good, this should maintain balance indefinitely if they're cycled on a regular basis.

Not sure why you would ever set the balance voltage above the anticipated range. It's not like most passive balancers can burn off much current at all.

IMHO, the vast majority of DIY batteries made from commodity battery cells will NOT stay in balance without help.

 

[time2roll]

There might also be a spread of 10 to 15 mV before the balancing starts. This is a better function than using the higher start voltage.

 

[SajgZi]

There might also be a spread of 10 to 15 mV before the balancing starts. This is a better function than using the higher start voltage.

Yes, I adjusted it to 5 mV and I'll start balancing when cells are >3.55V

 

[time2roll]

If you charge to 3.55 and start balancing at 3.55 it will be lucky to balance for a few minutes and not be very effective unless your charging holds this voltage for an extended period
.
I suggest going lower to 3.40 or 3.45 max to start balance. Might give an hour or two. Still not much. And none if they are all within 5mV.

Your call.

 

[Brett V]

I have a 16 cell battery and balancing is set to kick in at 3.4 volts with a delta to balance of 8 mv. They only go above 3.4 volts when charging and float around 3.38'ish as I use them mainly for a UPS and not daily cycling. They sometimes hit 9 or 10 mv delta but usually hang abound 6 - 8 mv. I'm not even sure if that's considered good or bad but I've seen some screen shots on here that were way higher.

 

[sunshine_eggo]

Yes, I adjusted it to 5 mV and I'll start balancing when cells are >3.55V

Bad idea.

3400mV, charge only, 15mV spread. Set it and forget it. The 30mA you lose in balancing is nothing.

3550mV should be fine if you want to have this conversation again.

 

[SajgZi]

Well I finally sat down and wrote out all the voltages and now the cells are happily balanced basically twice a day - first when the solar tops off the battery and again at night after the sun goes down and the inverter brings the batteries back up. I thought the BMS wouldn't have enough balancing current to do much but at the top of the charge range there's plenty to work with. Thanks for the assistance and the kick in the butt to get me going the right direction!