Top balancing gone wrong ?

[Jonboothx]

Hi All

I’ve top balanced my cells with a 30v 10a desktop PSU. I set the voltage to 3.64 and waited a few days for the output ampage to get to 0.01

I changed the standard cables for 12mm battery leads and connected both to my first cell

I’m not sure what I’ve done wrong, my volts are now (about an hour after reconnecting the cells in series)

Cell 1: 3.581
Cell 2: 3.545
Cell 3: 3.563
Cell 4: 3.542

Is this normal ? Am I expecting a bit much ? Expected them all to be the same or similar.

Thanks for any help

Jon

 

[Any name you wish]

Your delta is 39/1000 of a volt. I think you did fine.

 

[MisterSandals]

Cell 1: 3.581
Cell 2: 3.545
Cell 3: 3.563
Cell 4: 3.542

Is this normal ?

If you are referring to discharging to 3.5xx volts, yes this is very normal.
Most of my drop to 3.35V within an hour. Some folks have 3.5V like yours.

Label your cells and add these numbers to your notes.

 

[Jonboothx]

If you are referring to discharging to 3.5xx volts, yes this is very normal.
Most of my drop to 3.35V within an hour. Some folks have 3.5V like yours.

Label your cells and add these numbers to your notes.

Thanks for the advice, Have now labelled the cells. it was more relating to the difference in voltage over the cells. Was expecting them all to be the same or very similar

 

[MisterSandals]

it was more relating to the difference in voltage over the cells

3 or 4 hundredths of a volt can be explained by measuring error, connection contact differences,... its really a small difference with commodity electronics.

 

[A.Justice]

The way the bar graph of cell voltages is spread on the app makes the voltage deviation appear much more severe. In reality, your cells are VERY close in voltage, and I would be happy so far if they were my purchase.

 

[RCinFLA]

You are fine. After taking charge to 3.6v with current tapering off to low levels, the voltages you are showing mean there is less than 0.01% in cells' state of charge variance.

Put a load on battery of a couple of amps for a minute and all cells should settle down to 3.43v to 3.45v after resting unloaded for a few minutes.
This discharges surface charge capacitance. This is 100% state of charge.

 

[AE4KR]

I’ve top balanced my cells with a 30v 10a desktop PSU. I set the voltage to 3.64 and waited a few days for the output ampage to get to 0.01...

Is this excessive absorption time for a 280 Ah cell? I thought 0.1C was the charge rate at which they were considered full.

Thinking out loud, maybe that's the rule of thumb if max charge voltage is set for 3.65....

 

[John Frum]

Is this excessive absorption time for a 280 Ah cell? I thought 0.1C was the charge rate at which they were considered full.

Thinking out loud, maybe that's the rule of thumb if max charge voltage is set for 3.65....

"5.3 Standard ChargeThe standard charge means charging the cell with charge current 0.5CA and constant voltage 3.65V at (25±2)°C, 0.05C cutoff." -- https://diysolarforum.com/resources/eve-lf280n.139/

 

[meetyg]

Did you measure voltage at the cells too, using a DMM ?

Sometimes subtle differences in the balance leads and/or terminals can lead to a small difference. As others mentioned, your voltages are fairly close. But if you want to be as precise as possible, I would compare the voltages at the cells. Also, did you get an Overkill BMS, or a JBD ? Overkill calibrates thier BMSs, JBD don't necessarily.
You can still calibrate the BMS using a UART to USB adapter and the JBDTOOL for Windows, if you want to.

 

[Sky-HHI]

I changed the standard cables for 12mm battery leads and connected both to my first cell

When I top balance my cells , I place them in parallel, then connect my PSU leads ,one on the first cell ( + ) and the other to the ( - ) terminal of the last cell. I do not think it is good to connect both + and - to the first cell in parallel, this may explain the cell differences.

 

[Jonboothx]

Did you measure voltage at the cells too, using a DMM ?

Sometimes subtle differences in the balance leads and/or terminals can lead to a small difference. As others mentioned, your voltages are fairly close. But if you want to be as precise as possible, I would compare the voltages at the cells. Also, did you get an Overkill BMS, or a JBD ? Overkill calibrates thier BMSs, JBD don't necessarily.
You can still calibrate the BMS using a UART to USB adapter and the JBDTOOL for Windows, if you want to.

Hi Meetyg, it’s overkill and I did measure the cells with a multimeter, I’ve since top balanced again at 3.64 across the cells (eg ve+ on first cell and ve- on last cell) and the difference is now 0.006 !

 

[Jonboothx]

When I top balance my cells , I place them in parallel, then connect my PSU leads ,one on the first cell ( + ) and the other to the ( - ) terminal of the last cell. I do not think it is good to connect both + and - to the first cell in parallel, this may explain the cell differences.

Sky-hi, that exactly what I did wrong, I’ve since done it as you described again and got it to 0.006v delta.. thanks fella

 

[Sky-HHI]

Glad I could help. Stay charged.

 

[meetyg]

Hi Meetyg, it’s overkill and I did measure the cells with a multimeter, I’ve since top balanced again at 3.64 across the cells (eg ve+ on first cell and ve- on last cell) and the difference is now 0.006 !

6mv is great. Glad it worked out for you.

 

[Just John]

Hi All

I’ve top balanced my cells with a 30v 10a desktop PSU. I set the voltage to 3.64 and waited a few days for the output ampage to get to 0.01

I changed the standard cables for 12mm battery leads and connected both to my first cell

I’m not sure what I’ve done wrong, my volts are now (about an hour after reconnecting the cells in series)

Cell 1: 3.581
Cell 2: 3.545
Cell 3: 3.563
Cell 4: 3.542

Is this normal ? Am I expecting a bit much ? Expected them all to be the same or similar.

Thanks for any help

Jon

Totally normal and expected.
The difference in voltage they settled to can be explained by either different resistance when paralleled, or different self discharge rate. Either way, those are good values.

 

[Just John]

Is this excessive absorption time for a 280 Ah cell? I thought 0.1C was the charge rate at which they were considered full.

Probably. You just want the cells fully saturated. If they settle to 3.45v or above within 24 hours, you have achieved that objective. I usually use 1 amp as a tail current, but once or twice to top balance will do no harm.

Thinking out loud, maybe that's the rule of thumb if max charge voltage is set for 3.65....

Somewhere between 1 amp and 1/10 of an amp is what I would use for a top balance. Per the manufacturer, a full charge is different than what you would use for top balancing.