Oddity when balancing 4S LFP pack connected to BMS…

[Supervstech]

I have a generic OVERKILL style JBD BMS. it never charges evenly, pack 4 always hits 3.6 before the others get out of 3.3… so I decided to hook a 5a CV charger to the low cells.
sounded simple, however, I hit an issue. Cell 2 charges fine up to 3.6, cell 1 fine up to 3.6 cell 3… when charging it, cell 2 dives in voltage, and everything starts dropping to 3.3ish… and the pack goes all wonky.

 

[Supervstech]

Here is what it looks like while not charging cell 3

 

[Supervstech]

Prior to charging 3, I had 1,2,&4 sitting at 3.55 as SOON as I start charging 3, the others dove to this point. Which is doubly odd, as 4 was holding around 3.5 for days prior to me attempting the balance.

 

[Bob B]

Have you compared the BMS readings to DVM readings?

It seems like it must be inaccurate BMS readings since it wouldn't be possible that all the cell voltages would change that much that fast without creating a fire.

 

[Supervstech]

Yeah, with a 160Ah pack, huge jumps seem to be BMS reading inaccuracies… I will check it all now with a meter.

 

[Supervstech]

Now with cell 3 charging i get this...

 

[Supervstech]

Volts read same as bms with my meter

Here is with cell 4 charging...

 

[Supervstech]

Cell 4 charging...

 

[Just John]

That is odd behavior. I know another user reported odd behavior and it turned out to be a cell with unusually high self discharge.
Perhaps make sure balancing is turned off, and try again. If the odd behavior is bad enough, it might be best to test each cell individually (i.e. remove all busbars and BMS leads and try charging).

 

[Supervstech]

Oof, disconnecting everything will be a pain.
It is all spot welded together.

 

[Supervstech]

Cell 4 almost full, pulling about .128A
As soon as it hits 3.6, I will charge cell 1 full, then 2 and post pics as I go.

 

[Just John]

Oof, disconnecting everything will be a pain.
It is all spot welded together.

Well, good connections, just makes it more difficult to figure out what the heck is going on.

 

[Bob B]

Maybe it's just because they are so close to the upper knee?

 

[Supervstech]

Maybe it's just because they are so close to the upper knee?

Thats just it.
I got three of them up into the 3.5 knee, then hooking up the last low cell, DROPPED all the other cells... weird.

 

[Supervstech]

Down to .48A

 

[Just John]

Thats just it.
I got three of them up into the 2.5 knee, then hooking up the last low cell, DROPPED all the other cells... weird.

That's what made me think balancing kicked in, although it shouldn't without the charge going through the BMS.

 

[Supervstech]

Like I said, I am ONLY charging a single cell at a time.
I connected #1 now. Once it is full I will post the page showing it and 4 full, then I will move to 3 fill it, and then 2... maybe it'll normalize...

 

[Supervstech]

That's what made me think balancing kicked in, although it shouldn't without the charge going through the BMS.

Yeah, if the balancing could handle THAT much current I think the balance leads would have fried... this is a 160Ah pack...

 

[Supervstech]

I am now thinking I didn't let the cells fill enough and the self discharge dropped them into the 3.36 range... I will fill them all the way to under .5a at 3.6V this time.

 

[Just John]

Yeah, if the balancing could handle THAT much current I think the balance leads would have fried... this is a 160Ah pack...

Yeah, was trying to think of a good logical explanation, but there really isn't one.