How to Fix Battery Cell Imbalance (BSL Battery)

How to Fix Battery Cell Imbalance (BSL Battery)

A battery cell imbalance may present through unusual SOC (State of Charge) behavior, red LED indicators, or system instability. If one battery in the stack is out of balance, it can cause inconsistent charging and affect overall performance. The goal of this guide is to isolate each battery, verify its SOC individually, and rebalance any unit showing abnormal charge behavior.

 

Check Monitoring for SOC Irregularities

  • Open your system's monitoring platform or inverter interface.
  • Look at the State of Charge (SOC) history and current readings.


     


  • If you notice that the SOC value drops significantly in a short period or instantly drop its value this usually indicates a problem with battery balancing.
  • This kind of sudden change suggests that one or more batteries are either not charging correctly or are internally unbalanced.

 

2. Inspect LED Indicators on Each Battery

  • Even if LED lights show normal on all batteries, there may still be imbalance.
  • In some cases, one or more batteries may show a red light error, but not always.

 

3. Isolate the Current Master Battery

  • Locate the Master battery (typically the top of the stack or battery with DIP switch set to Master and Master CAN cable).
  • Disconnect the RS485 communication cable going from this Master to the next Slave battery.
  • This allows you to view and tes the Master battery independently.






  • Check the SOC level of this battery on the inverter screen at the battery icon in the bottom left.
    Note: That the monitoring status from the Live Monitoring App will update every after 10 minutes so better check the screen of the inverter for real time update.

 

4. Reassign a New Master Battery

  • While the RS485 cable is still disconnected, unplug the Master communication cable from the current Master battery. Move this CAN cable to the next battery in the stack that you want to test individually.
  • Set the DIP switch on this 2nd battery to Master mode - UP / DOWN / DOWN / DOWN / DOWN / DOWN
  • Once the DIP switch is set correctly, the system will recognize this battery as the new Master.
  • Check the SOC level on the inverter screen
  • Note: Your monitoring system may show an error during this process  this is normal and expected until the DIP switch has been updated.

 

5. Check SOC Level of the New Master

  • Once the system successfully recognizes the new Master battery, review its State of Charge (SOC) reading in the monitoring interface.
  • You will likely observe a significant difference in SOC compared to the previous Master Battery.

 

6. Charge the Reassigned Master to Match SOC - Link to guide

  • Begin charging this new Master battery until its SOC matches or closely aligns with the previous battery.
  • This step is critical to synchronize charge levels across the stack before reconnecting everything.

 

7. Repeat for Remaining Batteries

  • Repeat the same process for the rest of the stack:
    • Move the Master communication cable (CAN Cable).
    • Set the DIP switch to Master.
    • Check SOC level and charge if needed.
  • Always test and charge one battery at a time, with all other batteries disconnected to each other. (RS485 cable disconnected)

 

8. Power Down and Rebuild the Stack

  • Turn OFF the DC breaker of the battery bank before touching any wiring.
  • Reconnect all communication (RS485) and Master cables to the original configuration.
  • Reset DIP switches according to the correct Master-Slave setup based on your battery model.



Final Checks

  • Turn the system back by switching the DC Breaker ON.
  • All LED lights should be normal (no red lights).
  • The monitoring system should now show consistent SOC levels across all batteries.
  • The inverter should operate without fault.