One cell in 4S pack discharging much faster

[Fpoet]

I have two DIY 4S LFP 280Ah packs in parallel (560Ah) in my van, each with a 120A Overkill BMS.

These are Eve cells purchased from Basen. I did top balance the cells when I received them, but only tested the capacity at 40A, as I believed I would very rarely be drawing more than that (I have propane for cooking).

It has been working great so far, with the cells staying together. The idea of this project was to last several days, and the batteries are doing the job. I even have to disconnect the chargers so that they don’t stay at full charge all the time.

Now with all that power, I decided to try a 1500W portable induction cooktop. Before I turn it on, all the cells are at 3.33V with a delta around 0.005V on the BMS app. When I do turn on the cooktop, the first cell of each pack drops much more than the others, one around 2.9-3V and the other at 3.1V. When I turn it off, the two cells voltage increases to almost reach the others.

It could be that these cells have lower IR. But since this is happening to the first cell of each pack, I am thinking it could be something in my setup... Any idea as to what could be causing this?

 

[Short_Shot]

Check your bus bar connections?

 

[HRTKD]

I agree that connections should be checked first. I saw a post within the past couple of days that indicated that cell 1 always gets hit hard. I don't understand why. I'm just throwing out that what you're seeing may not be uncommon.

 

[jwelter99]

Also try moving cells around to see if it follows the cell or not. It could be bus bars or other factors.

You mention "first cell" of each pack - I assume this cell is the negative connection to the battery?

 

[Fpoet]

Also try moving cells around to see if it follows the cell or not. It could be bus bars or other factors.

You mention "first cell" of each pack - I assume this cell is the negative connection to the battery?

Thanks for the reply. Yes, by the first cell, I mean the one that is the negative connection (through the BMS).

I will do more troubleshooting when I get home in a few weeks.

 

[Fpoet]

Update

I got home, opened the batteries and cleaned the offending connections and busbars with a scotch bright pad and acetone. One of the braided busbars was still giving me trouble, so I replaced it with a spare one that I had. I reconnected everything and tested each battery separately and then in parallel with the induction cooktop at 120A. The cells now stay pretty much together in each battery and the two batteries split the load equally (with a difference of about 3-5 A). I am happy with that.

Thanks for the help!

 

[HRTKD]

Glad you got it working! Thanks for the update.