Cell 1 voltage issues

Dyotat100

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I have 2 8s 24v 280 ah eve batteries that I built. Both running chargery BMS. Built around a year apart. I have been running them paralleled together for around 4 months. They are charged with 6 405 w panels and victron controller.

I only charge them to 3.40v. Everything was good until about a month ago. That's when the new pack #1 cell voltage would be off by .3 volts at top. Also noticed when battery is at 70% or so #1 is the lowest cell. All other cells are with in 5 mili volts.

So I contacted vendor to purchase 1 cell and told her my problem. Out of stock until end of June. She told me to move the cell to a different location in the battery. So I swapped 1 and 3.

Next day everything looked good and I was like that was easy. Well it was only a matter of time before it started again. Yesterday it was out 150 mili volts so I shut down solar and let it discharge some. It balances back out as the cells hit 3.33-3.34v. I then did a little reconfigure of some of my wiring tying them together and moved the main plug to draw from + of one and - of other. Before I had a 2 plug setup where I could unplug one battery and run off other. This did nothing to change the problem.

What is wierd is the older battery doesn't have this issue.

Any ideas or help?

Thanks
 

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Samcat

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Have benchtop battery cell in question charging and discharging scenario with 2 hours waiting time between. Record all and post it here . If it did pass all and meet the Ah you probably have issue with terminals or not tight connection. Inspect bus bars so they are flat and make good surface connection when tighten. If all are pass you will need to Inspect BMS .There is many scenarios what could be wrong. But battery cell would be #1 step to Inspect. And remember to top balance all before putting back in series to eliminate AH energy differences.
 

Bud Martin

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Did you try swapping the suspect bad cell with another location to see if the problem follows?
 

Dyotat100

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Yes but it did it with cell 3 in cell 1 location also.

Cells were all top balanced and came out good
 

Dyotat100

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Maybe I will swap BMS from one to the other. I would lower voltage but 3.40 is lowest
 

Ellcon123

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I think your voltage is to low. At 3.4V you are barely in the knee so matching voltage is no indication of balance. You need to be well into the knee for voltage to mean anything. I start balancing at 27V and charge the pack to over 28V.
 

Dyotat100

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I think your voltage is to low. At 3.4V you are barely in the knee so matching voltage is no indication of balance. You need to be well into the knee for voltage to mean anything. I start balancing at 27V and charge the pack to over 28V.
I tried charging higher but things get out of balance more. I have have played with different voltages and I really saw no benefit going higher. Even at 28v it only takes 5 min or so to bring it down to 3.34 where it settles with 10 amp load.

I charge both batteries at the same time and discharge at same time. The one that has the issue is the negative terminal. Wonder if switch it up so it is positive from that battery instead.
 

Dyotat100

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So turns out that battery is the positive terminal draw only.

Here is pics of both BMS after running house all night. First pic is older battery and second is the troubled one..

Will post pics of them when the solar tops them off. Over cast today so not sure if it will happen all the way.
 

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Ellcon123

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I tried charging higher but things get out of balance more. I have have played with different voltages and I really saw no benefit going higher. Even at 28v it only takes 5 min or so to bring it down to 3.34 where it settles with 10 amp load.

I charge both batteries at the same time and discharge at same time. The one that has the issue is the negative terminal. Wonder if switch it up so it is positive from that battery instead.
Will only see a large imbalance at higher voltage. Look at the voltage curve. It's here where you need to get them back in balance and reset the capacity. The BMS or shunt needs the batteries to go full to reset the 100% level. Same reason Tesla says to charge full once a week.
 

Dyotat100

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Will only see a large imbalance at higher voltage. Look at the voltage curve. It's here where you need to get them back in balance and reset the capacity. The BMS or shunt needs the batteries to go full to reset the 100% level. Same reason Tesla says to charge full once a week.
The BMS resets everyday to 100%. I'm waiting for it to reach full today still. Had no sun until 11 am.
 

Samcat

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You will have to individually charge batteries in Series to preset voltage of your choice or arrange all in parallel and top balance. Then after the top balance is done try to keep eye on suspected cell you had problems with how they hold charge and using current clamp meter overtime to check if it is draining other cells because of suspected cell is not holding charge.
 

Dyotat100

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You will have to individually charge batteries in Series to preset voltage of your choice or arrange all in parallel and top balance. Then after the top balance is done try to keep eye on suspected cell you had problems with how they hold charge and using current clamp meter overtime to check if it is draining other cells because of suspected cell is not holding charge.
I have top balanced when I built it. I have been using this battery for 4 months or so.

Why would another cell do the same exact thing when put into #1 position on the battery. I moved cells 1 and 3 and now cell 3 is in #1 position and doing what cell 1 was doing in position #1
 

Bud Martin

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So the issue did not follow the cell, it sounds like BMS issue as other has suggested.
 

Samcat

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I have top balanced when I built it. I have been using this battery for 4 months or so.

Why would another cell do the same exact thing when put into #1 position on the battery. I moved cells 1 and 3 and now cell 3 is in #1 position and doing what cell 1 was doing in position #1
You will need to do some detective work on your side. It could be power draw to keep BMS running making your #1 cell getting out of SOC over time . I have experienced this problem with some BMS platforms. It could be issue with balancing terminals on the bms that can bleed energy constantly even when not balancing. Definitely inspect terminal bars and make sure they are flat when tight. Do you have any low voltage users that only feed from one battery cell 3.5V or less?
 

Dyotat100

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You will need to do some detective work on your side. It could be power draw to keep BMS running making your #1 cell getting out of SOC over time . I have experienced this problem with some BMS platforms. It could be issue with balancing terminals on the bms that can bleed energy constantly even when not balancing. Definitely inspect terminal bars and make sure they are flat when tight. Do you have any low voltage users that only feed from one battery cell 3.5V or less?

Nothing drawing from one cell. I have 2 batteries setup identically with chargery BMS. Will check all connections and leads.

Overcast again today so not sure I will get enough sun to top off to see what happens.

Thanks
 

Samcat

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Nothing drawing from one cell. I have 2 batteries setup identically with chargery BMS. Will check all connections and leads.

Overcast again today so not sure I will get enough sun to top off to see what happens.

Thanks
I would try to use different BMS for experimental purposes and try to see if this still happening. Definitely interested in your problem and what was the issue. Keep us posted so we all learn together.
 
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