One cell under voltage, one over voltage

[DLTooley]

Strangely, I have one cell triggering an under voltage disconnect and another triggering an over voltage disconnect.

I presume the error has something to do with the average between the two cells, but that’s pretty vague.

The bms is a jbd, 4 cell 280ah battery about 1.5 years old. I haven’t moved in a week and air is dry. It’s getting below freezing, but temperature on the battery is reading at 10c.

The two out of wack cells are varying wildly I think maybe reapproaching appropriate values, but it hasn’t been long. Cells are adjacent, 2 and 3. 1 and 4 are stable.

 

[DLTooley]

Cells were roughly at 2.6v and 4.0v. They are now at roughly 2.9 and 3.6 after a half hour. The high cell has been one consistently but modestly out of balance, ~60 milivolts.

 

[DLTooley]

They started diverging again after 15 minutes under a mild load. Load was switched off before disconnect. At 20 minutes they may have ‘stabilized at 2.37 and 4.12. They now appear to be leveling again, with no load.

 

[DLTooley]

They seemed to pop back into relative balance fairly quickly. 3.19 and 3.3. They are both still oscillating, but slowly moving back to equalization.

 

[DLTooley]

Now they are diverging again, another 15-20 minutes. No load, and no charging as it’s night time.

I’m guessing the bms is bad?

 

[Steve_S]

You just stated ONE trigger for such behaviours, it is Temp ! They will behave considerably differently @ 10C versus 20-25C Temp. This is something I tested a while back (when only 4 Packs online). The colder the cells are the more they will be affected by charge & discharge as well as retention. I even noted differences between packs when even 2C temp difference between packs... stacked on shelves the lowest pack closest to ground was coldest of course and with 2C temp diff between each shelf it was easy to watch play out and the changes when charging and each battery getting warmer how that also affected the current being taken in. Also the warmer batteries would discharge higher than the colder ones.

I chased a stoned monkey for a bit with the cells while working the issue. The cells had more divergence between them when colder and the "weaklings" surfaced by being more obvious within a pack. I figured this out when I was playing with temps, as I control the heat within the powerhouse, so I tested various temps from 5C and up and the results were eye opening ! Goodness Gracious quite the swing !!

I also tested each cell with a YR1035+ cell tester at different temps to see what the IR is and what happens and Holy Cow I was surprised to see how much the IR can change as the temps change and not surprised to see the "runner" cells have greater variance on their IR.

I'd suggest you do the obvious and check all the connections for tightness first, never hurts. Then consider warming up the battery to 20-25C Temp and make sure it is that warm all through and test with load & charge and watch the behaviours of the different cells as compared to when at 10C. Note that it takes time to warm them up internally.

Depending on your BMS if it has Passive or Active Balancing, that can also affect the results and how the pack will behave at differing temps. Do remember that the cells are tested @ Optimal Temp which is 25C and their performance will be different whether they are hotter or colder.

Hope it helps, Good Luck.

 

[DLTooley]

I’m down in warmer Yuma now, no further issues.

The only other change I’ve made is that the BMS is ratcheting down the battery capacity when I don’t charge to 100% every day. I did reset that. I’m currently charging to 13.7v on the Victron with a two hour absorb time.

The battery box I had was just big enough to insulate with the shipping foam from the original cell shipping. As such to do a proper heating pad I’ll have to do a rebuild.

For now, I’ll just stay in Yuma!