Cell voltage diff. Is this normal ?

[missoo]

Hi,

Could you tell me if my cells voltage diff is normal ?
It seems a bit high ...
I've got a LifePo4 280A 12V cells.

Thanks.

 

[PreppenWolf]

Normal depends on some further information.

Did you top balance all the cells before building your packs?
Are all the cells the same brand, from the same batch?

If no to either question then yes, what you are seeing is normal if you throw batteries together.

If the answer to both questions above is yes, we must dig deeper.

 

[missoo]

Yes I top balanced the cells before building the pack.
After several months of using it, I noticed electric issues in my RV so I opened my pack and I noticed that a top balancing cable was untighten on one cell. So I tightened it and I top balanced the cells one more time.
The cells are from the same batch.

 

[PreppenWolf]

OK.

How deeply are you cycling the batteries?

Are the cables/connectors all the same length/type/material between the cells and the battery packs?

You can try swapping a cell postion (swap location of highest with the lowest) then track and see if it's BMS related not doing its job.

If you are constantly full cycling the batteries, it might be your BMS isn't capable of maintaining the balance (poor quality or bad settings), or you're not giving it enough time at full charge to allow it to re-top balance.

 

[missoo]

I charged my battery pack with 220V and I got a voltage diff of 227mV..
Then it went back directly to 40 mV voltage diff. Also the battery voltage dropped to 13,5 V without any loads. I don't understand why.

I'll try to swap cells positions.

I don't think I am full cycling the batteries. I don't have that much consumption in my RV. Just a fridge and ligths.

 

[PreppenWolf]

"I charged my battery pack with 220V and I got a voltage diff of 227mV..
Then it went back directly to 40 mV voltage diff. Also the battery voltage dropped to 13,5 V without any loads. I don't understand why."

I don't think I understand this properly. I'm guessing you're saying you used a DC power supply fed by 220VAC shore power. If so and you were at 227mV (0.227V) difference between cells, you have a serious problem.

 

[PreppenWolf]

If you properly top balanced your cells to 3.65V before assembling the battery, either you have bad cells or your BMS is broken/has wrong settings.

 

[missoo]

Yes sorry for my english... It is exactly what I meant. I used a DC power supply fed by 220VAC shore power.

I am wondering if cell 2 is not dammaged.... Could I dammage it as I used the pack with a balancing wire untightened on this cell ?.. I guess so.
I am using BMS settings recommanded by Overkillsolar.

 

[PreppenWolf]

If it was me, I would take the batteries apart. Re-top balance all the cells. Then set each cell aside for 24hrs (not connected to anything) and check the voltage of each cell after 24hrs. See if you have one diving down.

 

[sunshine_eggo]

If you properly top balanced your cells to 3.65V before assembling the battery, either you have bad cells or your BMS is broken/has wrong settings.

or the cells sit too much, and they have dissimilar self-discharge rates to where they lose their top balance in as little as 1-2 weeks.

 

[PreppenWolf]

or the cells sit too much, and they have dissimilar self-discharge rates to where they lose their top balance in as little as 1-2 weeks.

True. @OP do we know the brand/grade of these cells?