Will this balance eventually?

[time2roll]

Got them down to within .02v And the bms just ignores the low cell during balance. It just pushes the other three right back up to 3.6, is this like I suspected that the bms doesn't work properly?

You will not see balance on the low cell. You only see balance on the cell(s) that are too high.

Did the BMS bring the voltages to within .02 or did you manually discharge them?

 

[tiger37373]

You will not see balance on the low cell. You only see balance on the cell(s) that are too high.

Did the BMS bring the voltages to within .02 or did you manually discharge them?

I had the bms bring them down.

 

[tiger37373]

That is within 20 mv and is perfectly fine. The BMS should have a delta to balance setting and is probably set at 20. If you want to balance tighter you would have to reduce that number.

I don't see that particular setting. Here is what my current setting looks like.

 

[time2roll]

It is the "Balancing precision" set at 0.015 and is fine.

 

[time2roll]

I had the bms bring them down.

OK and when you charged the battery the three jumped back to 3.60 much faster than the slacker? This would be because they may be within a few millivolts but were coming down and the slacker is still coming up. So the voltages were close but the state of charge was still significantly different. Hopefully the low cell made some progress but it could be slow until it gets to 3.42+

Keep the balancing on. Could set the balance turn on voltage down to 3.350

 

[labeeman]

No, I didn't balance before assembly. The overkill cells are advertised as being top balanced before delivery. I do not have a secondary way to charge an individual cell, in any case. I would have to buy a small charger.

How are you charging the pack now?

 

[Bob B]

I don't see that particular setting. Here is what my current setting looks like.View attachment 51691

Yeah .... there seems to be a lot of iterations of this App ... but time2roll is correct.

 

[GSXR1000]

Got them down to within .02v And the bms just ignores the low cell during balance. It just pushes the other three right back up to 3.6, is this like I suspected that the bms doesn't work properly?

the bms can not charge one cell more then the others... it can only slightly discharge cells.
so yes it would not try to discharge the low cell.

what you are seeing is from 20% to 95% the cell voltage is very close regardless of cell balance... then once it hits the knee (sharp upward curve) then a few amps in make the voltage change drastically.... and the out of balance shows it face. then you discharge 20 amps and boom they look almost balanced again

 

[Hedges]

You will not see balance on the low cell. You only see balance on the cell(s) that are too high.

Did the BMS bring the voltages to within .02 or did you manually discharge them?

I had the bms bring them down.

Being within 0.02V while down low on the knee around 3.35V might be balanced, but not necessarily.
Being within 0.02V while up on the knee around 3.6V would be balanced.

Lowering a cell to reach the same voltage another cell was raised to won't make it balance, same state of charge.
All cells should approach the voltage from the same direction.
Charging a cell to the voltage others settled to won't make it balanced. It will settle lower.
Charging it to the voltage they were charged to should result in it settling to the same voltage.

And then there is something about how long held at a target charge voltage, how low the current tapers before charging is stopped. Ideally the same.

Each time you discharge high cells toward the laggard it will get closer.
Once you get a CV/CC power supply and use it to charge the low cell, you should be able to get them balanced.

 

[HRTKD]

At the time of your first screenshot there was no balancing taking place because there was no charging. A number of discharge/charge cycles will probably bring the cells closer together. But a top balance should have been performed in the first place. I did a top balance and my cells stay well balanced. They've been doing well for the past nine months.

 

[Larry619]

At this point you’re better off doing a legitimate top balance rather than relying on the BMS to balance it over a (long) period of time.

I have the overkill solar BMS which uses the same app as yours, I’m assuming it’s also the same BMS design but with a different name. Once one of my cells was about that far off and it never got close enough for me to be comfortable.

If you don’t want to take apart your entire pack, here is what I did.

1) I left everything intact except I turned off balancing in the BMS settings. Bring the entire pack to a high state of charge.

2) using an affordable Amazon adjustable power supply (0-30V 10A) I placed charging leads separately on each cell. In your case, each parallel 3.2V pair/group.

3) Set charge to 3.6V and low amps…maybe 1-2A. Eventually it will get to or close to 3.6V.

4) continue placing leads to each cell or paralled group and charge them all up to 3.6V.

After some rest, this should get them all relatively to the same state of charge.

 

[Larry619]

One more thing to add. Make sure all your busbars, contact surfaces, and bolts are clean and snug. I applied some NoOxID grease too.

I had one cell develop oxydation and it through off the charging and discharging characteristics completely. All cells showed charging at 3.3V while the one cell said 3.6V. Immediate testing using a voltmeter without current showed equal voltage to the other cells.

Clean, torque, and check everything.

 

[tiger37373]

Thanks, I plan to pull them out and charge up. Just got the needed power supply. So, now will do proper top balance, as I should have done to begin with.

 

[tiger37373]

At this point you’re better off doing a legitimate top balance rather than relying on the BMS to balance it over a (long) period of time.

I have the overkill solar BMS which uses the same app as yours, I’m assuming it’s also the same BMS design but with a different name. Once one of my cells was about that far off and it never got close enough for me to be comfortable.

If you don’t want to take apart your entire pack, here is what I did.

1) I left everything intact except I turned off balancing in the BMS settings. Bring the entire pack to a high state of charge.

2) using an affordable Amazon adjustable power supply (0-30V 10A) I placed charging leads separately on each cell. In your case, each parallel 3.2V pair/group.

3) Set charge to 3.6V and low amps…maybe 1-2A. Eventually it will get to or close to 3.6V.

4) continue placing leads to each cell or paralled group and charge them all up to 3.6V.

After some rest, this should get them all relatively to the same state of charge.

Curiously, I measured the voltage at each cell just now before I remove the packs they measure 4.50 with my meter. WTH does that mean?

 

[Hedges]

Curiously, I measured the voltage at each cell just now before I remove the packs they measure 4.50 with my meter. WTH does that mean?

Does not compute.
That would mean extremely overcharged and rapidly degrading.

Double/triple check. If you believe it, put back in pack with BMS and operate a load long enough to drain maybe 5% from pack, getting them down to a safe SoC.

But it seems hard to believe, given reasonable BMS readings.

Oh, "before" you remove packs. Are they still wired together? What does BMS say?

If they are above 3.65V, get them back down soon.

 

[tiger37373]

Does not compute.
That would mean extremely overcharged and rapidly degrading.

Double/triple check. If you believe it, put back in pack with BMS and operate a load long enough to drain maybe 5% from pack, getting them down to a safe SoC.

But it seems hard to believe, given reasonable BMS readings.

Oh, "before" you remove packs. Are they still wired together? What does BMS say?

If they are above 3.65V, get them back down soon.

 

[tiger37373]

It appears that my old meter has bit the dust, nothing to worry on the cells, thank goodness

 

[labeeman]

It appears that my old meter has bit the dust, nothing to worry on the cells, thank goodness

Check the battery on the meter they will read high if the battery is weak

 

[tiger37373]

Just a quick update. I pulled all the batteries and charged the cells, as I should have done initially. All is well at the end. After settling back into the system, they're all balanced across the cells and between the batteries. Happy for the future! Attached are a couple of the batteries this morning.

 

[Larry619]

Just a quick update. I pulled all the batteries and charged the cells, as I should have done initially. All is well at the end. After settling back into the system, they're all balanced across the cells and between the batteries. Happy for the future! Attached are a couple of the batteries this morning.
View attachment 52738View attachment 52739

I’m so happy to hear this all worked out for you. I was the same way with my first battery build and I thought I could skip the top balance. Like you, the teardown and a proper top balance was well worth effort.

Enjoy your new system!