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One cell low voltage in lifepo4

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Vepr308

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Feb 19, 2021
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I have 2 24v batteries that i added overkill solar bms to each about 6 months ago. It's been fine but for some reason, today i looked at the app and the one battery is showing 0%. When i click on the menu to see the individual cell voltage, 1 of them is showing .704 while the other 7 cells show around 3.43. I'm not really sure what the problem is or how to fix it. These were from big battery when they used to sell them without a bms.
 

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I'm having a similar issue, but less extreme so far...I'm wondering if I have a bad cell?

One cell (in a 16s bank) started both 1) reading higher up to 0.06v more than average on charge, and 2) dipping up to 0.06v lower on discharge. I top balanced all cells a few months ago and everything was very even and stable (all within 0.03v) until this week... Bad cell? Or just need to rebalance?
 
So like if most are around 3.320-3.330v, this one cell will be at 3.380v while charging or 3.270v under load.
 
And it's more likely a problem with the connection to the cell than the cell itself. Clean the terminal and bus bar / lug and try again.
 
penandpike said:
So how is your system still working. Didn't the BMS shut it down?
Click to expand...
Huh? No, my BMS (Overkill) would only shut it off when a cell reaches an extreme minimum or maximum, 2.7 and 3.6 in my case.

I'll probably just do a whole battery top balance again sometime soon - if it's still wonky after that, then I'll know it's just that one cell. I just wonder if the place I got my cells from will offer to trade for a new one 6 months later...
 
0 or 0.7% seems pretty extreme to me If the BMS works properly It should have stop the system when it sees no voltage at that cell
 
I have a single cell misbehaving in a 15S pack,
I have noticed that cell-1 voltage is 0.06v lower than other cells when discharged, and voltage on cell 1 starts dropping down feom 95% charged to 100% charged.

Any ideas or recommendations ?

Below are some records of the cell voltages:

When charging:
Cell 35%70%95%99%
13.2613.3303.3573.354
23.3243.3593.3623.367
43.3273.3623.3653.375
Diff0.0660.0320.0080.011

When discharging:
Cell38%76%95%
13.2053.2763.313
23.2623.3113.315
43.2653.3123.315
Diff0.0700.0360.002
 
Riderice said:
Huh? No, my BMS (Overkill) would only shut it off when a cell reaches an extreme minimum or maximum, 2.7 and 3.6 in my case.

I'll probably just do a whole battery top balance again sometime soon - if it's still wonky after that, then I'll know it's just that one cell. I just wonder if the place I got my cells from will offer to trade for a new one 6 months later...
Click to expand...
Did you ever figure this out. I have almost the exact same problem. Just got 16 105ah cells from18650 battery store so should be grade A cells. One is high during charging just like you described, and then that same one is low during discharge. Seems to settle to about the same voltage if no load. I am going to check the connection on that cell today, but hard to believe a bad connection would cause voltage to be high on that cell during charging. Anyone care to explain that phenomeno?
 
You could swap the suspect cell to a different position and see if the problem moves with the cell, this also gives you a chance to check your connections. The slight differences in this particular thread do show a cell that isn't quite a match to the others but generally of no great concern. If you start getting into tenths of a volt difference then I would be more concerned.
 
Chris30559 said:
Did you ever figure this out. I have almost the exact same problem. Just got 16 105ah cells from18650 battery store so should be grade A cells. One is high during charging just like you described, and then that same one is low during discharge. Seems to settle to about the same voltage if no load. I am going to check the connection on that cell today, but hard to believe a bad connection would cause voltage to be high on that cell during charging. Anyone care to explain that phenomeno?
Click to expand...
Did you perform a top balance with the cells in parallel?

A connection that has resistance will lead to a lower voltage reading under load.
 
Quattrohead said:
You could swap the suspect cell to a different position and see if the problem moves with the cell, this also gives you a chance to check your connections. The slight differences in this particular thread do show a cell that isn't quite a match to the others but generally of no great concern. If you start getting into tenths of a volt difference then I would be more concerned.
Click to expand...
Good idea. Ill move it while I redo the connection. It was approx 0.03 higher during charge. All the others were within 0.002 or so. And during a 20amp discharge for just a few minutes last night, with pack at 90%, it was almost 0.08 lower.
 
Zwy said:
Did you perform a top balance with the cells in parallel?

A connection that has resistance will lead to a lower voltage reading under load.
Click to expand...
Yes I did. But I stopped at 3.55v. Current wasn't quite zero but fairly close. Then I let them sit paralleled overnight. I completely agree about a high resistance connection under load. Just can't wrap my head around why that same cell was the highest during charge.
 
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