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NORMAL - cell voltage deviation at 13.8 volts while charging at 10 amps

JCALD

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Oct 5, 2022
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I have an SOK 206 AH battery with one cell that causes a BMS charge shutdown at approximately 13.7 volts. The other three cells have very little deviation in voltage but never reach max capacity since the single cell terminates the charge. The battery has been cycled a minimum of 5 times with time in between for BMS cell balancing.

Do I have a bad cell or is this normal? Can the BMS ever get this cell to take a charge like the others?

I have two other SOK 206 batteries that do not have this issue.

I am trying to determine how hard I push SOK for a replacement of cell or complete battery.

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You have one cell at higher SoC than the others (imbalanced). It could be that it's a bad cell, but you can't say that for certain.

The cells at 3.35 aren't "balanced." Cells rarely deviate much in the working range. I suspect that if you load the battery, the high cell will fall in line with the other 3 very rapidly.

They shouldn't come new like this, but it's not horribly uncommon.

You can't charge to the recommended voltage, so it doesn't meet spec. I would report this to SOK immediately. You need to document it.

The next sensible step would be to test the battery capacity. If the same cell turns out to be the low cell on the discharge, it's also the lowest capacity cell.

If the discharge test doesn't yield the stated capacity, this is strike 2 - report it again.

HOLDING the battery at 13.6V on a charger might allow the BMS to balance the cells all to 3.40V. If it does so, hold again at 13.8V until it balances all cells to 3.45V. Then on to 14.4V until all are at 3.60V.

If SOK offers to replace it expeditiously, I would take them up on it.
 
The battery has been cycled a minimum of 5 times with time in between for BMS cell balancing.
Either the time in between was in fact not long enough to properly balance the cell or the BMS is not working correctly. Many BMS only balance near the top end of the charge voltage range. Typical Balance Start Voltage would be 3.4V. At 13.7V charging that is only 3.425V per cell so the high cell is probably reaching the cut off value shortly after the BMS just starts to try and balance. Reduce charging voltage 0.1V at a time until the BMS does not cut off and let the battery float at that voltage for several hours and see if the high cell begins to respond. Most BMS are only capable of balancing at low currents, it takes time.
 
I am in the process of dischargeing the battery. It is at 19% SOC and all cells are within 0.003 volts. Discharge rate is 15 amps, so it takes a while. I will reach out when I get through the next charge cycle and float if it reaches the BMS balance threshold. Thanks
 
It has been 35 hours since my last charge was completed. I had to reduce the charge voltage to 13.7 to prevent the BMS from entering charge protection mode. I have been floating the battery at 13.7 for 30 hours with no appreciable balancing taking place. The problem cell is still at a 0.319 deviation from the other 3. The BMS was replaced about 2 weeks ago under SOK warranty due to firmware updates.
 
It has been 35 hours since my last charge was completed. I had to reduce the charge voltage to 13.7 to prevent the BMS from entering charge protection mode. I have been floating the battery at 13.7 for 30 hours with no appreciable balancing taking place. The problem cell is still at a 0.319 deviation from the other 3. The BMS was replaced about 2 weeks ago under SOK warranty due to firmware updates.

Bad BMS or bad BMS balance settings.
 
Bad BMS or bad BMS balance settings.
The problem I am having trouble getting my mind around is that cell 3 in this battery has always bumped charge protection mode causing my whole bank to have trouble. I will reach out to Min and see the response. Thanks for your input. Much appreciated.
 
The problem I am having trouble getting my mind around is that cell 3 in this battery has always bumped charge protection mode causing my whole bank to have trouble. I will reach out to Min and see the response. Thanks for your input. Much appreciated.

A high cell only means it is at the highest state of charge.

What capacity did your discharge test yield, and was #3 also the one that dropped out first?
 
The same cell dropped lower than the others at full discharge. I only have the capacity calculated by the SOK app and it is below specifications by only 4 AH. I didn’t time the discharge.
 
The same cell dropped lower than the others at full discharge. I only have the capacity calculated by the SOK app and it is below specifications by only 4 AH. I didn’t time the discharge.

That is a pretty good sign that cell is lower capacity than the others

I use a hobby charger that can charge single cells. I use it to top off other cells when this kind of imbalance occurs. You might try something like this. Or try to get SOK to replace the battery
 
The same cell dropped lower than the others at full discharge. I only have the capacity calculated by the SOK app and it is below specifications by only 4 AH. I didn’t time the discharge.

High cell on charge and low cell on discharge means it's the weakest cell. Missing cap by 4Ah means the battery doesn't meet spec.

1) battery can't charge to manufacturer specified level
2) battery does not meet rated capacity

Replace.
 
While cycling my problem battery I used an inverter with a 15 amp load to discharge the battery and then charged it back up using a 12 volt power supply so I could closely control the voltage. SOK now says that the battery cannot be discharged with an inverter for their app to give correct readings And it must only be discharged with a 12v load device. This sounds like BS to me. What say you.
 
First thing you need to discharge until bms shut down. However the inverter will not do that it will shut down with it own parameters.

Before charging, disconnect battery completely. Check to see if zero, if not you need to use a low wattage load until bms shut off. If initially you read zero voltage wait for battery to rest. If zero proceed.
 
Apparently that a method of recalibration according to sok and it will show your battery capacity estimate more accurate. Figures will be more accurate using the Bluetooth app.

Lot of equipment that do measuring are calibrated and over time usually need recalibration. Even victron bms require resetting.

If you plan to use the app to determine the health of battery or quality then to be fair do the process before claiming defective.
 
As a follow up to my original post:

As requested my Min I am using and cycling the batteries to see if my one problem battery will eventually balance. It is getting much closer to a balanced pack so I am not pursuing replacement, although it is not as well balanced as the other two batterie. Deviation readings today were .056, .003, and .004. The questionable battery had a deviation of .250, so we are getting closer.

SOK app is very inconsistent and all I use it for is to check individual cell voltage and deviation. I rely on my Victron smart shunt to monitor charge and discharge of the pack.
 
An inverter will shut down at its own setting. But, I also have lights and other loads that that are directly connected to the batteries and do in fact pull all of my batteries down to BMS shutdown.
 
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