One cell is lower than others

purdum

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Jun 10, 2020
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Screenshot_20211108-110906_xiaoxiang.jpgI built a 12v 280ah eve cell battery in a milk crate for my 5th wheel. I top balanced before building but did not bottom balance. Works perfectly in the 5th wheel so far. When storing it I disconnected it from solar so it would not be sitting at full charge. Well went to check it today and one cell is lower than the others. Above 3.1 volts I have not seen this difference in voltage
So, do I worry, do I fix it. (if so then how) has this bms

RadioB Tech Smart BMS 4S 12V 120A Lifepo4​

 

smoothJoey

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Typically one of the cells is going to "fall off the cliff" first.
In your case it appears to be cell #2.
Charge the pack and see what the cell delta is at full charge.
If the bms doesn't trip and the cells converge and you get satisfactory capacity then its all good.
Let us know how it turns out.
Why is the "charge" toggle off?
 
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fafrd

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View attachment 71739I built a 12v 280ah eve cell battery in a milk crate for my 5th wheel. I top balanced before building but did not bottom balance. Works perfectly in the 5th wheel so far. When storing it I disconnected it from solar so it would not be sitting at full charge. Well went to check it today and one cell is lower than the others. Above 3.1 volts I have not seen this difference in voltage
So, do I worry, do I fix it. (if so then how) has this bms

RadioB Tech Smart BMS 4S 12V 120A Lifepo4​

Show the same picture after charging up to HVD (one cell reaching 3.65V or all cells exceeding 3.55V).

If cell #2 is also the highest, that is the cell with lowest capacity which is limiting the capacity of your entire battery.

If all cells including cell #2 are reasonably well-balanced when the battery is full (ideally all cells above 3.55V or at least 3.50V), your doing as well as you can.

If one or more cells reach 3.65V while cell #2 remains well below that (under 3.55V), you can gain a bit of capacity by improving your top-balance (use an Active Balancer or inject charge into cell #2 until its voltage is close to the others).
 
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time2roll

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Not much energy left at those low voltages. I would be more concerned about dropping below 2.5 volts as the cell is getting close. Better to leave the solar on. Maybe set the controller to a lower absorption voltage.

There is no real fix for the low cell if the voltage matches at the top.
 
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Dzl

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I top balanced before building but did not bottom balance.
That's good and right, you can't do both. If you top balance you are actually making it so your cells will be less aligned at low state of charge, and if you bottom balance the reverse is true (cells will be less aligned at high state of charge). If you have well matched cells they should be pretty balanced through most of the voltage range (but still probably diverge at the extremes). But one trade-off of buying budget cells from grey market sellers is unless you are lucky they will not be particularly well matched near 'the knees (steep parts of the voltage curve). So your choice is optimize for balance at the top or the bottom. We choose the top for two reasons with solar (1) that is where BMS balancing happens (2) that is where batteries spend most of their time in energy storage applications.
capacity_balance-png.22329


So in short, its expected for the cells we tend to buy to diverge at the bottom of the voltage range with top balanced cells. Everything we and the BMS do is optimizing for the top. Some cell will inevitably drop faster or slower than the others when you get to the lower knee. If it happens before the knee, or equates to a big difference in SOC, it is an issue worth worrying about, but there isn't a ton of energy left when you get to the knees, so moderate differences in voltage, don't necessarily equate to large differences in SOC.

So, do I worry, do I fix it. (if so then how) has this bms
I don't think there is anything to worry about or fix based on the data you have right now, apart from maybe storing your cells at a moderately higher state of charge. ~30% maybe, and/or making sure there are not loads draining the cells while they are being stored.
 

mikefitz

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So, do I worry, do I fix it.
The battery is discharged.
You have a load on the battery that discharged down to low voltage cut off by the BMS, cell 2 reached the low volts cut off and opened the discharge path. You need to discover what is discharging the battery.

With cell balancing you either balance at the top or balance at the bottom. If you top balance the cells will be unbalanced at the bottom and vice versa. Thus the variations at a low state of charge you are seeing is normal.

I advise charging the battery as soon as possible. For storage, discharge to say 70% and disconnect the battery from all circuits.
Voltage chart B.jpg
Mike
 
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purdum

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Jun 10, 2020
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Thanks for all the info
answers to your questions
Why is the "charge" toggle off? I think it's just that I cut the input from solar so it would slowly drop voltage
Why so low I found an intermittent fan was coming on which drew it down faster than I thought (took a month to get where it is today
I will report back when charged
 
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