Unequal SOC?

[rfguy]

I installed in a motor home in 2020 a 4S4P pack built from 16 ea Fortune 100AH LiFePO4 batteries I purchased from Electric Car Parts Co.. I also bought their BMS that is rated for max charge/discharge 200 amps. I was using it with a Xantrex inverter/charger rated at 2Kw/100amp charge. All was fine until I sold the motor home early this spring. I removed the battery pack and BMS but left the Xantrex and new FLA batteries for the new owner.
In May I installed a new Xantrex inverter/Charger-3Kw/150A charger along with my battery pack. While the pack was out of service and I was doing the Inverter install I strapped all 16 of the cells in parallel and let them sit for a week or so since the voltages were identical. Installation went well and I charged the pack and took the new RV out to dry camp. We have left it here at our lot for about month. We come every few days charge batteries (never below 50% SOC according to the Victron smart shunt) and stay for few days charging from genset through inverter charger at 150A verified with smart shunt. The charger parameters are set up as 14.5 V bulk/13.5V float. All started out well but I have noticed charger shutting down before it reached 14.5. The charge amps used to begin ramping down when voltage was at 14.2 and gradually going to zero when it hit 14.5V. I synced the SOC on smart shunt to 100% after the first charge cycle right after the installation and first charge. Now it is shutting down before pack reaches 14.0V and SOC on smart shunt is reading 94%. I suspect BMS is doing this because of cell inbalance. I checked the voltages shortly after last charge cycle and found them all running 3.35 +/- .01V. I realize that SOC may be very different even in cells that are reading same voltage. I thought about running RV on generator for a few hours while I take battery pack apart and move the cells into new positions as a way to equalize them but I wanted to check with the experts here to see if that is a good plan or not. Thank you for reading my lengthy post and welcome your input. In the meanwhile since I did not have any problems like this charging at 100A, I have adjusted my charger to charge at 90amps to see if that helps.

 

[DThames]

You might charge it to about 14.3v (keep the charger on) and see if one of the cell's voltage is starting to move ahead of the others. Very likely that is what is going on.

You will need a balance cable from your battery but with that plugged into a monitor like this, you can see what is going on. Also do some balance if needed.

 

[rfguy]

Thank you that is a good suggestion. Since I have 4 in P would it be safe to assume that each group of 4 is at the same SOC? They have been in this configuration for 3 months now. BTW after I did the install in the new RV it sat on the charger for at least a month with no discharge cycles as we were not using the RV.

 

[DThames]

Thank you that is a good suggestion. Since I have 4 in P would it be safe to assume that each group of 4 is at the same SOC? They have been in this configuration for 3 months now. BTW after I did the install in the new RV it sat on the charger for at least a month with no discharge cycles as we were not using the RV.

If the 4 cells are connected in parallel and have been somewhat discharged and then fully charged, they should have the same SOC. If you have 4 series strings of 4 in parallel (not likely as you would need 4 BMSs) no way to know without some cycle on each cell.

 

[rfguy]

Thank you again -I assumed correctly then as they are in a 4S4P not a 4 x 4S1P. I was thinking after I check your suggestion that I could take the pack apart and assemble with 4 new groups composed of one cell from each current group and charge as best I can. We are heading home tomorrow so the pack will be sitting for 4 days with no charge cycles and only a minimal draw. The RV when everything is off draws about 3 amps parasitic. So maybe things will equalize.

 

[DThames]

Thank you again -I assumed correctly then as they are in a 4S4P not a 4 x 4S1P. I was thinking after I check your suggestion that I could take the pack apart and assemble with 4 new groups composed of one cell from each current group and charge as best I can. We are heading home tomorrow so the pack will be sitting for 4 days with no charge cycles and only a minimal draw. The RV when everything is off draws about 3 amps parasitic. So maybe things will equalize.

Just curious, that 3 amp draw, is that something that you need to be on or just something difficult to disconnect?

Does your BMS have Bluetooth communications?

 

[rfguy]

I am guessing about half is the refrigerator. It is a large propane model but has circulating fans inside (12V). The rest of it is small electronics -smoke/co2 detectors, LCI wifi so you can control some of the RV functions from an phone app. I am thinking or breaking out the refer electronics to a separate fuse from the 12V fuse panel and then when we leave the RV for several days, I can turn off the 12V disconnect without turning off the refer electronics. BTW yesterday I took the battery pack apart labeled each parallel cell group and then reassembled the pack with one of each group into new parallel groups. I then lowered the charger to 80 amps and it is behaving and seems to be improving with each charge cycle. The paper work on the BMS states it is good for 200 amp of charging but when I accessed the pack to work on it, I saw a rating plaque on the BMS tht stated 100A of charge. I was charging at 150 A so I think I got it figured out. Thanks for your replies.

 

[DThames]

I am guessing about half is the refrigerator. It is a large propane model but has circulating fans inside (12V). The rest of it is small electronics -smoke/co2 detectors, LCI wifi so you can control some of the RV functions from an phone app. I am thinking or breaking out the refer electronics to a separate fuse from the 12V fuse panel and then when we leave the RV for several days, I can turn off the 12V disconnect without turning off the refer electronics. BTW yesterday I took the battery pack apart labeled each parallel cell group and then reassembled the pack with one of each group into new parallel groups. I then lowered the charger to 80 amps and it is behaving and seems to be improving with each charge cycle. The paper work on the BMS states it is good for 200 amp of charging but when I accessed the pack to work on it, I saw a rating plaque on the BMS tht stated 100A of charge. I was charging at 150 A so I think I got it figured out. Thanks for your replies.

Charging at that level of current, check the temp on the BMS. If it is not getting enough air and it is getting too warm, it might do weird things. Even at 100A, if that is a constant 100A for a while, the BMS may be warming up more than you want.