Cell Overcharging.

Here's the layout:

I have 32 3.2v, 100ah lifepo4 cells in a 2P 16S configuration. One 200ah Daly bms monitors 16 groups of 2 (this is a parallel first configuration).

When the battery is discharging, all the cells appear to be balanced with each other (10 to 20 mv variation). However, when the sun comes up, and I'm bulk charging at around 50 amps, one paralleled group rises up to 4.5 volts. This is strange as the 15 other groups are balanced with each other.

I shut off the solar power and given enough time, the overcharged group starts to become balanced with the rest. This seems to only be a problem when charging.

Could this possibly be an issue with the bms?

2P16S. We can refer to each parallel group as a cell.

Did you top balance the cells before battery build?

"Balance" relates to SoC, not voltage. Balance is only meaningful above about 3.40V and below about 3.1V.

4.5V means damaged cells. Electrolyte breaks down above 4.20V. Why doesn't the BMS shut it down?

Did you mean 3.5V?

Small voltage variations in the operating range is expected even if cells are imbalanced.

You either have imbalanced cells, failing cells or bad sensing connections.

Check that all connections are torqued to the proper value, all crimps are of high quality and with nothing interfering with the cell connections.

Does a 50A DIScharge result in that same cell sagging?

No, I didn't top balance the cells before the build.

Yes the group was at 4.5v

I haven't tested them at a 50A discharge.

I have no idea why the bms didn't shut down. Could it be a dud?

Atomicly said:

No, I didn't top balance the cells before the build.

Yes the group was at 4.5v

Click to expand...

Confirm all cell measurements with a voltmeter moving forward.

Atomicly said:

I haven't tested them at a 50A discharge.

Click to expand...

If this cell also sags under 50A to a similar degree, one or both are a dude.

Atomicly said:

I have no idea why the bms didn't shut down. Could it be a dud?

Click to expand...

Yes, unless there's a short time delay before it triggers protection, and you were inside of it.

You have 16 groups of 2 cells. Obviously you are able to see the individual group voltage by saying 1 group was at 4.5 vpc while the others are at balanced at what voltage per cell group?

As mentioned you best figure out why the bms didn't shut down, what do you have the bms to shutdown at vpc?

Could be a loose terminal connection giving false voltage readings. Clean and retorque all connections and try again.