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Original question:

BQ76952: accumulated charge measurement calculation

BQ76952: Accumulated charge measurement calculation

René Lalla
Prodigy 20 points
Part Number: BQ76952

Hi there,

I use the BQ76952 for battery management and want to use the coulomb counter to calculate the SOC.

In response to this old question: https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1006473/bq76952-accumulated-charge-measurement-calculation/3722551
I would like to know how to interpret the fractional portion of the accumulated charge, if the integer portion is 0? 
For example the fractional portion evaluates to 0.5 mAh. Do I need to check the current, if it es negative or positive to distinguish it?

Regards, René

  • 0
    TI__Mastermind 42060 points

    Hello René,

    Yes, you should read the whole register, since it is in 2's complement, reading the integer portion will let you know if it is a negative or positive value. All F's for the integer portion indicates a -1.

    If the integer portion reads 0's, then it is 0. For example, 0.5-mAh would be returned as:

       00 00 00 00 = Integer portion returned as 0

       80 00 00 00 = Fractional portion returns 0.5

    Together, this is 0.5-mAh.

    Best Regards,

    Luis Hernandez Salomon

  • 0 in reply to Luis H. S.
    Prodigy 20 points

    Hello Luis, 

    thank you for your response. But how would a -1.5mAh would look like? 
    FF FF FF FF = -1

    80 00 00 00 = 0.5

    Will this result in -1.5 or -0.5? Do you see my point?

    Everything beyond |1A| is clear, as it is either negative or positive.

  • 0 in reply to René Lalla
    TI__Mastermind 42060 points

    Hello René,

    That is a good question! The fractional value is always a positive value, while the integer portion is signed and can be positive/negative.

    So I think that it is possible that it would not be able to interpret a value between -1 and 0, since the integer portion's largest negative number seems to be -1.

    I can look into this internally to confirm.

    Best Regards,

    Luis Hernandez Salomon

  • +1 in reply to Luis H. S.
    TI__Mastermind 42060 points

    Hello René,

    I am back. Now I understand how it works exactly. Sorry for the wait! I was actually wrong initially, after discussing it internally, I now understand how this is calculated. Yes, you can get negative fractional values above -1.

    The overall accumulated charge is actually the sum between the integer portion and the fractional portion.

    Accumulated charge (userAhr) = integer + fractional.

    So to get a -0.5, you need the integer to be - 1 and the fractional to give you 0.5:

            Accumulated charge = -1 + 0.5 = -0.5.

    Similarly, to get a value such as -1.5: 

            Accumulated charge = -2 + 0.5 = -1.5.

    Best Regards,

    Luis Hernandez Salomon

  • 0 in reply to Luis H. S.
    Prodigy 20 points

    Hello Luis,

    thank you very much for your time and effort! This helps me a lot :)