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BQ77915: Even cell discharging in 3-cell setup

Jomon
Intellectual 660 points
Part Number: BQ77915

Tool/software:

We are using 3-cell setup.

Charging, discharging, protections and everything is working well.

Except, the middle cell in the 3 series cell is discharging when kept idle for few days. The middle cell has only this connection to it for balancing.

We have replaced that 43R with 0R. Is that the problem ? I haven't added 1Kohm resistance between the cells and VCx pin. Will replacing the 43R / 0R with 1Kohm fix this issue ?

When balancing, odd cells are balanced together and even cells are balanced together. What reference voltage will the middle cell be balanced to ? Will they be balanced to the voltage of the odd cells, but at a different time ?

  • 0
    TI__Mastermind 42060 points

    Hello Jomon,

    The cells would not balance at all unless a charging current is detected. So I would not think that balancing is happening in this case. You could try replacing the 43-Ohm as a quick test.

    Is there any leakages that you can measure around the pack, I think the cell could be leaking from some other external component.

    You could also potentially replace the cell, in case it is a faulty cell.

    Best Regards,

    Luis Hernandez Salomon

  • 0 in reply to Luis H. S.
    Intellectual 660 points

    Hi Luis,

    I have figured it out.

    I should not have used the 0R in place of 43R. The internal resistance of the balancing circuit is around 12-20R. And it is recommended for about 50mA of balancing current only. When I use 0R, the balancing current is above this 50mA limit.

    The balancing current would be 4.2V / (12R + 0R + 0R) in this case, which is above the limit.

    The observation after each cycle was that the cell with the least voltage on charge completion, will keep discharging through the internal circuitry and will drop voltage eventually. While testing, the voltage dropped from 3.99 to 3.56 in 24-36 hours. The other cells had stable voltage of 4.12 for these hours.

    The fix is to use 43R and keep the balancing current below the threshold of 50mA. Tested it for few days and that seems to have worked.

    Thanks a lot for the support.