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BQ76952: accumulated charge measurement calculation

Zhonghui Xu
Expert 5445 points
Part Number: BQ76952

Hi team,

I use the BQ76952 for a battery management.I should use the coulomb counter to calculated the SOC and I don't understand exactly how it's work.

In the technical reference, it's indicated: subcommand 0x0076:
- Accumulated charge (integer portion) signed int32 user AH.
- Accumulated charge (fractional portion), fractionnal portion 32 Bit

In the TI's web site: it's completed, the second 32-bit (unsigned) data reports the fractional charge in units of userAh/2^32.

The Bq is configured in mAH for the units of userAh.

I have battery connected, with a discharge current of 280 mA. I read the coulomb counter all the 10s. and I have the following frame:
- FF FF FF FF (integer portion)
- E0 C2 C1 D3 (fractionnal portion)

My questions are:
- How I decode these data (what is the calculation) to obtain a number in mAh ?
- How is indicated the current direction (charge or discharge) in the fractionnal portion?

Thank you very much for your help.

Best regards,

  • 0
    TI__Guru* 80964 points

    Hi Zhonghui,

    This register reports the charge accumulated over time. The Accumulated Time is also reported in 0x0076. So I recommend sending the RESET_PASSQ() command 0x0082 and then reading the register after a certain time period like 60 seconds. At this point the Accumulated Time should show 60 seconds and the integer portion should read ~4-5 mAh. 

    Can you try this and let me know the results you see?

    Best regards,

    Matt

  • 0 in reply to Matt Sunna
    TI__Expert 5445 points

    Hi Matt,

    thank you for your reply.

    Thank you for your answer.

    I have done the procedure and I have the following result:

    Current is 260 mA.
    The following data are the result of the 0x0076 subcommand with the format:
    Accumulated charge (integer portion) // Accumulated charge (fractional portion)
    B3 B0 B7 B4
    00 00 00 00 00 00 00 00


    After the reset t = 0:
    00 00 00 00 00 00 00 00

    1ere measurement t= 1 seconde:
    00 00 00 00 6E FC 72 C3

    2nd measurement t = 2 secondes:
    00 00 00 00 61 29 8F FF

    3th measurement t = 3:
    00 00 00 00 53 56 AD 3F

    8 measurement t = 8:
    FF FF FF FF F2 92 79 FF

    60 measurement t = 60s:
    FF FF FF FD 73 7D 10 D3


    For my application, I need the calculation to convert these data to value mAh.

    Thanks in advance.

    Best regards,

  • +1 in reply to Zhonghui Xu
    TI__Guru* 80964 points

    Hi Zhonghui,

    This online converter is helpful: https://www.rapidtables.com/convert/number/hex-to-decimal.html

    FF FF FF FD is the integer portion of the measurement. It is in 2-s complement format, so this represents -3 in decimal.

    73 7D 10 D3 is the fractional portion. The decimal number is 1937576147 / 2^32 = 0.451

    So this is measuring -3.451 mAh of accumulated charge. 

    Have you calibrated the coulomb counter to make sure the readings are accurate for your board?

    Best regards,

    Matt