One cell voltage spike

[msgn]

Hello,

This is my first attempt in building a brand new 8-cells 24v system (150Ah). 8 cells in series.

Before attaching them to the charge controller/inverter/bms, I kept them in paralles for 6-7 days, then applied a 10A current from power supply, going from the first cell positive to the last cell negative terminal. When they arrived at home they were all well balanced already at 3.32v but I wanted to do everything right.

Yesterday we had a very sunny day here in central Italy and my small PV array produced 25-30 amps. The cells started to charge, but Cell-1 quickly spiked over 3.50, then 3.65 and then 3.69 when my BMS decided to wisely cut the current. Trying to use the pack with a moderate load (500-600 watt), no unbalancing occurred. Everything set back to a very balanced pack. But today it was sunny again and Cell-1 did the same as yesterday.

I tried swapping Cell-1 with Cell-2 in my series and the result is: it's always the same cell to be unbalanced. This excludes errors in BMS connection of terminals, etc. It looks like Cell-1 could be defective or very unbalanced.

What do you think it could be the problem? Is there anything I can do to make Cell-1 behave like the other perfectly balanced cells?

Thank you!

 

[Kornbread]

chrisski said:
What is 3.45? 95%?

I recently had a cell drift out of balance. Not a lot of difference voltage wise when settled. Could be as little as .005 volts. When charging and nearing 100%, would get .2 volts difference. In between there were differences all over the board. I suspected a loose connection, and sure enough it was.

I was expecting to top balance again, but after sitting 5 days on the Overkill BMS set to balance all the time when above 3.4, balance amperage unknown, appears within .015 again. I will do some stress testing this weekend to see how that goes.

Not easy to top balance a pack once assembled, but if I need to I can take the bus bars off and put a power supply.

From this thread https://diysolarforum.com/threads/gangfeng-280-cells.59951/page-2#post-744624

Yeah, somewhere around 95%. At 3.4'ish, we are into the knee of the curve. Charge all the way to 3.6x if you wish, but there's simply not much power in the knees at either end. What I see is ~51v to 53v is the meat and potatoes of where the bank likes to operate, and in this range, just a small difference in voltage can be a rather large difference in power. Not so much at either end and that's why, or at least I feel that is why we either top or bottom balance. And IIRC, an active balancer such as the jk, doesn't simply burn off power from the high cell, it pulls power out of the high cell and transfers that power into the low cell. Traditional top balancing is more of a one-way additive subtractive process.

It's my opinion and take it for that, but with 2+amps working from both ends, I think we have reached the age where traditional top/bottom balancing may not be absolutely necessary for a properly operating pack. Just my $.02 and take it for that.

With that said, I can imagine scenarios where there are multiple partly cloudy days/weeks failing to provide enough power for the battery pack to reach a good state of charge for balancing, that could cause some cells to get way out of line necessitating a much more drastic approach of disassemble and top balance. Especially if the pack had runners to begin with. Agree 100%, it's a real pain to disassemble/assemble a pack, not to mention dangerous, but we are all dealing with 'grade b' cells, some with huge tolerances, which may be so far out of line the only choice is to start with a top balance.

So yeah, in the meat n potatoes flat area of the curve, the cells, although they may be far from balanced, are generally all going to be very close in voltage. It is at the extremes where they show their true state of charge.

It's common to have runners in the pack hence why most top balance, especially considering the typical bms can only burn miliamps off the runner/runners.

 

[mikefitz]

Option 1
Lower the charge voltage to 24.0 and set a long absorbtion time to give the BMS a chance to balance.
Or
Option 2
Load the 'high cell' with a DC load, example an automotive filament headlamp bulb, for several second periods, as the cell approaches 'runaway'.
Perhaps combine both options 1 and 2.
Option3
Breakdown the battery and top balance to 3.65 volts

 

[msgn]

Thank you, I'm going to try with Option 2 and then the others.
What do you think could be a clear signal that the cell is defective and needs to be replaced?

 

[TomC4306]

What do you think could be a clear signal that the cell is defective and needs to be replaced?

You're not there yet.
Balance the cells.
Cycle the battery.
Observe behavior.

 

[TomC4306]

Lower the charge voltage to 24.0

What? 24 volts is empty.
I would try charging at 27.6 with a long absorption time.

 

[mikefitz]

Thanks TomC, have corrected error.

 

[msgn]

Update

None of the options kindly suggested by mikefitz and TomC worked. The cell continued spiking for 5-6 days. Then one day it slowed down in its voltage variancy. And yesterday it started behaving like the other cells. Today the same, under a shiny bright sun. Maybe it's finally equalized.

Thanks for the help.

 

[MisterSandals]

The cell continued spiking for 5-6 days.

When you mention “cell 1” is that the cell position (first cell on negative end of battery) or a cell designated/named “cell1”?

In other words, does the behavior follow the physical cell when moved or always happens in same position?

Can you post a pic of your battery that shows the wiring?


As an aside, this is why I label my cells with letters and take notes on things like this.

 

[msgn]

When you mention “cell 1” is that the cell position (first cell on negative end of battery) or a cell designated/named “cell1”?

It's explained in my first post:
I tried swapping Cell-1 with Cell-2 in my series and the result is: it's always the same cell to be unbalanced. This excludes errors in BMS connection of terminals, etc. It looks like Cell-1 could be defective or very unbalanced.

 

[squowse]

Hello,

This is my first attempt in building a brand new 8-cells 24v system (150Ah). 8 cells in series.

Before attaching them to the charge controller/inverter/bms, I kept them in paralles for 6-7 days, then applied a 10A current from power supply, going from the first cell positive to the last cell negative terminal. When they arrived at home they were all well balanced already at 3.32v but I wanted to do everything right.

Yesterday we had a very sunny day here in central Italy and my small PV array produced 25-30 amps. The cells started to charge, but Cell-1 quickly spiked over 3.50, then 3.65 and then 3.69 when my BMS decided to wisely cut the current. Trying to use the pack with a moderate load (500-600 watt), no unbalancing occurred. Everything set back to a very balanced pack. But today it was sunny again and Cell-1 did the same as yesterday.

I tried swapping Cell-1 with Cell-2 in my series and the result is: it's always the same cell to be unbalanced. This excludes errors in BMS connection of terminals, etc. It looks like Cell-1 could be defective or very unbalanced.

What do you think it could be the problem? Is there anything I can do to make Cell-1 behave like the other perfectly balanced cells?

Thank you!

What were the other cell voltages and what’s your charging voltage? You really need to set the charging voltage low enough that the cells dont trip out. I would use 27.8V. Theres very little capacity above this voltage.

 

[msgn]

As I said some hours ago, the problem is solved. Thank you!