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BQ40Z50-R1: Can't enable cell balancing

Luis Pinillos
Prodigy 220 points
Part Number: BQ40Z50-R1
Other Parts Discussed in Thread: BQSTUDIO

Hello, 

I am new to using the BQ40z50-R1 evaluation board and I wish to activate the cell balancing. I have been researching and reading the Technical Reference Manual and followed its instructions, but I can't seem to get it to work. The balancing configuration register has bit CB enabled and also CBR enabled, the Balancing time/mAh for all of my cells is configured to be 360 s/mAh, the minimum start balance delta is 3mV, and the Relax Balance Interval is 18000s. When the BMS enters rest mode the BAL_EN bit turns on as well as when it is charging. Although this bit is enabled, the cell balancing seems to be disabled since my battery cells remain with the same voltage they had before activating. Is there something I'm doing wrong? I would appreciate any help I can get.

Have a nice day

  • 0
    TI__Mastermind 34775 points

    Hi Luis,

    Welcome and thanks for choosing our bq40z50-R1. Can you provide a snapshot of your cell voltage readings and well as exported gg.csv file? I would also recommend that you turn on logging in bqStudio with 1s intervals and provide us with the logs for the conditions you mentioned above.

    What are your cell voltages and what's the worse case voltage difference between the cells? How long are you waiting? Based on you setting it take 1hr to change the capacity by 10mAh. This will not give any significant change in voltage.

  • 0 in reply to Damian Lewis
    Prodigy 220 points

    Hi Damian,

    I will attach the gg.csv file and will give you a log as soon as I finish recording. In some cases the voltage difference between the cells is 200mV, specially when the state of charge is close to cero. I used to have the balancing time/mAh in 90 s/mAh  but didn't get any results and read that the maximum current that balancing system could handle was 10 mA, so I raised it up to 360s/mAh. To which value do you think I should set it to?

    BQ40z50-R1.gg.xlsx

  • 0 in reply to Luis Pinillos
    TI__Mastermind 34775 points

    Hi Luis,

    Your setting is ok for 10mAh CB in an hour. CB can only be done at relax or charging. What might be happening is the cell resistance mismatch is greater at low voltage (close to 0% SOC). If this is the case then CB will only occur during charging at low cell voltage. Do you see the cell imbalance (cell voltage difference) during charging of the cells from 0% SOC?

  • 0 in reply to Luis Pinillos
    TI__Mastermind 34775 points

    Hi Luis,

    log your charging and discharging and send us the log to review as well.

  • 0 in reply to Damian Lewis
    Prodigy 220 points

    Hi Damian,

    I left the battery pack in rest mode during the weekend and apparently the cell balancing works, I will still attach a log later today after I discharge it and charge it. An additional question that I have is, the state of health of the cells should be close to 100% since they are new, but the BMS shows it has a 53% state of health. Is there something wrong in the configuration I sent you that is causing this? The cells I am using are Lishen LR1865SK with a 2600mAh capacity.

    Thank you for your help.

  • 0 in reply to Luis Pinillos
    TI__Mastermind 34775 points

    Hi Luis,

    That great you were able to observe the cell balancing at rest (CBR) working. What is your pack configuration? X series and Y parallel cells? Looking at your gg file your design capacity is set to 7800mAh, it's most likely should be 2600mAh if you don't have parallel cells. capacity is increased by the number of parallel cells, not series cells.  Voltage is increased by the number of series cells.

    Design Capacity = Y parallel cells * Cell Capacity

    Design Voltage = X series cells * Cell Voltage

  • 0 in reply to Damian Lewis
    Prodigy 220 points
    Thank you for the quick response,

    Our configuration is 3S3P, so we have the the design capacity = 3*2600mAh = 7800 mAh.

    Best regards,

    Luis
  • 0 in reply to Luis Pinillos
    TI__Mastermind 34775 points

    Hi Luis,

    What capacity is the gauge reporting when you reach charge termination? It maybe your application FCC is much lower than the cells design capacity. In that case you may need to adjust for design capacity to match your application.

  • 0 in reply to Damian Lewis
    Prodigy 220 points

    Battery_test1.log

    Sorry for the late reply,

    Attached find the log of one cycle I think that this will answer the previous question you asked me. 

    Best regards,

    Luis