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BQ40Z50: What is the internal FCC calculation formula

Scott Wang
Prodigy 60 points
Part Number: BQ40Z50


Dear team,

Need your help.

I found a FCC calculation issue when check a customer's log file. In my understanding about IT algorithm, FCC should equal to InitialQ plus PassedQ plus RemainQ. But I find it is not in this case. Look at the picture, the yellow part is the original data from log file and the green part is the calculation result of "TrueRemQ + InitialQ + QMaxPassedQ". The FCC calculated by theory is not equal to the FCC reported by the fuel gauge. Why? What is the internal FCC calculation formula?

Plus: The log data were obtained at low temperature charging.

Thanks,

Scott

  • 0
    TI__Guru** 105870 points

    Hello Scott,

    The QmaxPassedQ may not be the direct register the gauge uses. You will not be able to back calculate the FCC using the register readouts since the registers used to calculate FCC are not all visible.

    FCC also takes into account expected load with the load select setting, you do not have enough information to calculate this.

    Sincerely,

    Wyatt Keller

  • 0 in reply to Wyatt Keller
    TI__Prodigy 60 points

    Hi Wyatt,

    Thanks for your reply. I think it is RM that takes into account expected load with the load select setting. FCC is just a sum of Qstart, passedcharge and RM (like the picture below). Is that correct? 

    I don't understand why QmaxPassedQ may not use in FCC calculate. Isn't it equal to passedcharge?

    Thanks,

    Scott

  • +1 in reply to Scott Wang
    TI__Guru** 105870 points

    Hello Scott,

    You will not be able to accurately back calculate these parameters, the way each gauge estimates these is slightly different and there are a lot of variables.

    The FCC is also affected by the load and temperature along with the RemCap. You do not have the same FCC value under high load because you will hit terminate voltage faster, this leads to less usable capacity from full charge. RemCap is mostly coulomb counting down with adjustments made to it so when FCC moves RemCap moves as well to prevent jumps (when smoothing is enabled).

    Sincerely,

    Wyatt Keller