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BQ40Z50-R2: Cell Balancing

JeremyG
Intellectual 631 points
Part Number: BQ40Z50-R2

Hi,

I've been cycling a two cell pack I am developing and have noticed that sometimes I get a value for Cell 1 Bal Time and sometimes it is for Cell 2 Bal Time.  Which timer gets a value doesn't seem to correspond with which cell has the greater QMax at the time, but perhaps that is not unexpected.  Are there some other parameters I can monitor to observe the balancing action take place, such as some of the many DOD related options?

I don't have access to the schematic for this BMS, so could it be possible that the default Balance Time per mAh values (367, 514) are causing overshoot?

Thank you,

Jeremy.

  • 0
    TI__Guru* 76230 points
    Hi Jeremy
    The cell balancing timer will be set based on which cell has a higher capacity over the other and not based off qmax. Although the gauge balances based on capacity and not voltage, it is not doing so based off qmax but rather remaining capacity.
    To see cell balancing working, You can connect a scope to the terminals of the battery connected to the vc pins of the chip. You will see the different puslsing of the balancing fets depending on which is active. Other than the cell balancing timer being non zero and the balancing flag in the register which indicates balancing is active, i don't believe there are any other indicators pointing that balancing is on going.

    thanks
    Onyx
  • 0 in reply to Onyx Ahiakwo
    Intellectual 631 points
    Hi Onyx,

    That makes sense, however, I'm interested in observing the cell balancing action converge.

    Is it possible to observe the remaining capacity for each cell, rather than the complete pack?

    Perhaps the Ra table?

    Regards,

    Jeremy.
  • 0 in reply to JeremyG
    TI__Guru* 76230 points
    When cells are unabalanced, it is the voltage that is really unbalanced. Cell balancing tends to be more effective during rest. If it is functioning properly, you will see the voltages converging to be equal. So you can monitor the voltages.
    thanks
    Onyx
  • 0 in reply to Onyx Ahiakwo
    Intellectual 631 points

    Hi Onyx.

    The voltages do seem to be converging with each cycle, so I am satisfied that it is working, however, I'd really prefer to compare the Remaining Capacity of each cell.

    Are the Cell 1 DOD0 and Cell 2 DOD0 fields suitable fort this?

    Regards,

    Jeremy.

  • 0 in reply to JeremyG
    TI__Mastermind 42535 points
    Yes, looking at the DOD0 finally after rest would suffice. However, please note that DOD0 is only updated after relaxation. So, it might not monitor it continuously until relaxation is complete.
  • 0 in reply to Batt
    Intellectual 631 points
    Thank you very much Batt.

    Regards,

    Jeremy.