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BQ40Z80: Balancing configuration

Daniel Librio
Intellectual 380 points
Part Number: BQ40Z80

Hi,

I have a couple of questions regarding balancing:

1.  The TRM states  "If external cell balancing is selected, [CBM] = 1, the gauge will perform a rotation cell balancing with only one cell to be balanced at a time..."

Is there any reason I couldn't leave CBM=0, even with external balance FETs fitted?  This way, balancing could be achieved faster through simultaneous balancing of multiple cells?

2.  I trialed an intentionally-unbalanced pack recently, which featured one cell approx 400mV lower than the other 5 (in a 6S pack).  Balancing stopped  (CB = 0) after only 5 hrs, leaving the pack still very unbalanced.  Changing any of the Bal time/mAh settings by a factor of 10 made no difference... I needed to reset the gauge to recommence balancing, and again it shut off after ~5hrs.  I had CB, CBM, CBR, and CBS all set to 1 under Balancing Configuration.  After leaving the pack for 3 days, balancing had still not recommenced.  I also did not recommence when placed on charge.   
Could the large imbalance have triggered an error state which stopped balancing?  A different test with a fairly well balanced pack yielded over 24hrs in balancing.

thanks in advance,

regards,

Daniel

  • 0
    Intellectual 380 points

    I should add, Rsoc was well above the threshold required to start balancing.

  • 0 in reply to Daniel Librio
    TI__Guru** 105760 points

    Hello Daniel,

    Were you able to measure any current flow between the cells? Can you share the schematic?

    The internal passive cell balancing is very slow, if you have large capacity cells it may take a lot of time to balance.

    Sincerely,

    Wyatt Keller

  • 0 in reply to Wyatt Keller
    Intellectual 380 points

    Hi Wyatt,

    Yes, I measured voltage across the balance resistors (and thus, current flow) as long as CB=1.  When the BMS cleared CB (after ~5hrs), the voltage across the resistors remained at zero.

    I understand that internal balancing is slow.  But I have found that with external FETs fitted, I can set the BQ40Z80 to internal balancing, and the external FETs still work.  The only difference appears to be that the FETs cycle through sequentially with it set to external... and with it set to internal, the external balance FETs switch on at roughly the same time.  Oscilloscope screenshots comparing both attached.  Schematic extract also attached.

    regards,
    Daniel

    CBM=0 (Internal)

    CBM=1 (External)

  • 0 in reply to Daniel Librio
    TI__Guru** 105760 points

    Hello Daniel,

    The gauge uses a capacity calculation to determine how long each cell needs to be balanced, if the chemID isn't a good match or there hasn't been a learning cycle it may lead to issues with cell balancing.

    The reason for alternating is because with external balancing it can happen quicker so it's safer to alternate to not balancing as quickly.

    Sincerely,

    Wyatt Keller