Battery discharge question

[Conductor570]

Hello. I am a beginner in the solar community and this is my first forum post. Please forgive any mistakes here.
I recently built a basic 24 volt emergency back-up system using two 100ah lifepo4 batteries in series with a 2000 watt pure sine inverter. I balanced the batteries as recommended prior to connecting them in series. Both batteries read 13.37 volts when connected. I have since done a test run connecting the system to my home chest freezer. My batteries have internal bms and each battery has it's own status display integrated on the battery from the factory so I did not connect a shunt to this system. Here is my issue.
I checked the status of each battery periodically while it was running my freezer and noted that one battery was discharging faster than the other. After several hours, I unplugged the freezer and checked the status. One battery was at a 77% and the other shows 65%. Is it normal for batteries connected in series to discharge at different rates?. And if not, will this be an issue with the batteries when charging them?
Thank you in advance for any replies to this question.

 

[timselectric]

BMS controlled batteries in series is not recommended. Because they will drift apart from each other. But it shouldn't happen while discharging.
The drifting happens during charging, while the BMS's are balancing.

 

[Conductor570]

Thank you for the response. So your saying they should not be discharging at different rates? What could my issue be and if I don't correct this will I still be able to use my system as is?

 

[chrisski]

Although I don’t have experience with them, there are battery balancers to keep series packs in balance. I do believe if kept in series, the batteries will need to be balanced somehow. The battery balancer is the only way I know how to do this.

IME, BMSs may not read SOC correctly. I have a Overkill BMS, and I can’t trust the SOC readings.

 

[Conductor570]

Thank you Chrisski. I have looked into the battery balancer but wanted to hold off until I knew whether my issue was normal or not. like you stated, I was wondering how much to trust the built in battery monitor. I'm ok with the difference in discharge as long as I'm not destroying the system. Thanks again for your input.

 

[timselectric]

Thank you for the response. So your saying they should not be discharging at different rates? What could my issue be and if I don't correct this will I still be able to use my system as is?

The BMS balancing is the reason for most of the drifting. It's possible that the BMS in those batteries run continuously. But I wouldn't think so.
I would disconnect them and place them in parallel. And charge them fully together. Make sure that you let them float for a while to completely balance.
Then try again and see how they do.

 

[Conductor570]

Thank you. The bms display does have an on/off button and I only cut it on to check the status. I'm not sure if that cuts the actual bms off or not. I will definitely try your suggestion of charging in parallel and hope for the best. I appreciate all the information.