Can I be lazy when I top ballance my batteries?

If you don't want to take your battery back apart, you can use the adjustable power supply and just do one cell at a time with it set to a couple amps and 3.6 volts. Just let it top out each cell to 3.6 volts, and wait until the current falls to less than 1/2 amp or so. After you do that to all 4 cells. Let them rest a couple hours and they should all measure very close in the 3.4 volt per cell range. Since each cell is getting the full power supply current, this actually does not take much longer than doing the 4 cells in parallel with the same power supply. 4 amps into each cell, one at a time, or 4 amps split across the 4 cells, just 1 amp each cell, takes 4 times longer, but does all 4 at once. In both cases, the idea is getting all 4 cells to true full charge "top balanced".

I had to do it like this with my Chevy Bolt packs since they have welded buss bars. I independently charged each cell to get them all within about 5 millivolts before I had my BMS.

Gazoo said:

The battery voltage doesn't mean anything when it comes to parallel top balancing.

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Voltage means more at 3.65 than it does at 3.4. Perhaps that is what you were implying?

Ampster said:

Voltage means more at 3.65 than it does at 3.4. Perhaps that is what you were implying?

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No. I meant to imply the importance of top balancing with relation to the SOC and cell voltage when the cells are first received from the supplier! That's why I added " The SOC of the batteries could be vastly different when you receive them even if all the voltages are equal" to my post.

News of the day.
I moved the charge voltage to 13.5. When I went to bed, the batteries were at 13.47 with about 4 amps going into each battery. Ballance looked good.
Now, the batteries are at 13.5, and all of them went out of balance. The worst set is .121 volts out. With the highest voltage in that bank at 3.34

I'm gonna keep them at this level until they either get worse, or the bms does their magic.

I'll keep you informed.

MNotLyon said:

News of the day.
I moved the charge voltage to 13.5. When I went to bed, the batteries were at 13.47 with about 4 amps going into each battery. Ballance looked good.
Now, the batteries are at 13.5, and all of them went out of balance. The worst set is .121 volts out. With the highest voltage in that bank at 3.34

I'm gonna keep them at this level until they either get worse, or the bms does their magic.

I'll keep you informed.

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All of the batteries or all of the cells?

What type of BMS? If it is a passive resistor balance type, the voltage may still be too low and it is not doing anything. Even for LFP, many do not start pulling balance current until a cell is over 3.4 volts.

MNotLyon said:

I'm gonna keep them at this level until they either get worse, or the bms does their magic

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Depending on the type of BMS, the size of your battery and the amount of imbalance it may take a while. If you don't increase voltage or Amps and balancing is active it should not get worse.

GXMnow said:

What type of BMS? If it is a passive resistor balance type, the voltage may still be too low and it is not doing anything. Even for LFP, many do not start pulling balance current until a cell is over 3.4 volts.

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Each battery has an Overkill Solar BMS. I have turned off "balance only while charging". I have noticed that they don't appear to balance when there is a draw on the battery, which is most of the time for my application, so I'm not expecting this to go quickly. They do ballance while charging, and while setting idle though.

MNotLyon said:

News of the day.
I moved the charge voltage to 13.5. When I went to bed, the batteries were at 13.47 with about 4 amps going into each battery. Ballance looked good.
Now, the batteries are at 13.5, and all of them went out of balance. The worst set is .121 volts out. With the highest voltage in that bank at 3.34
I'm gonna keep them at this level until they either get worse, or the bms does their magic.
I'll keep you informed.

Click to expand...

You're fighting the original state of charges being different and there's no fixing that unless you wait a REALLY long time for a BMS to equalize it.
A few ways of going about doing an initial balance - which needs to be done either at the top or bottom state of charge:

- Disassemble and parallel the cells while charging/discharging to your top or bottom balance set point - I use 2.8 or 3.45 as my set points because they're deep in the charge/discharge 'knees' without actually being at the maximums. Dont bother just paralleling them unless you're will to wait a week or more, there's so little voltage differential it takes forever to move current around. If you charge or discharge a lot more current moves around and what would other take a week or two is done in a few hours.
- Balancing has to be at top or bottom otherwise it's useless, the voltage curve is so flat in the middle your SOC won't actually be balanced even if you're at a .005v differential.
- Take a 3.4-3.6 charging source individually to each cell and wait until the charge acceptance (amperage) goes to 0, dont move the voltage settings AT ALL when moving between the batteries. The nice thing about this way is you can start charging and ignore it, or even come back the next day. Since you've set the voltage below the maximum you dont have to worry about overcharging the cell.
- Or discharge the cells individually down to 2.8v using a resistor. I really good impromptu resistor is a 15 inch length of .032 stainless wire. At 3.2v it'll draw about 5 amps, if you need more amperage do multiple lengths. Some clip test leads and a 5 gallon bucket of water and you have yourself a load bank! The problem with this method is that you have to watch the voltages like a hawk because once it goes below 2.95 it'll drop like a rock and you can easily over discharge if you're not paying attention.

Maast said:

You're fighting the original state of charges being different and there's no fixing that unless you wait a REALLY long time for a BMS to equalize it.

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At this point, I'm just playing with charge parameters until my power supply shows up so I can do a proper top ballance.

Balancing during not-charging is often counter productive if the goal is to have a top balanced battery.

Update:
I set my charger to charge at about 10 amps, and used my battery bank for a few days while waiting on my power supply to show up.
I noticed little or no change to the balance of my batteries. The BMSs were set to only balance during charging above 3.4 volts per cell.

I balanced each of my batteries (12 volts) individually. I am now 2/3rds of the way through a capacity test. So far, I'm pleased.