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BQ34Z100-G1: Charge termination - Accumulated change in capacity calculation

Part Number: BQ34Z100-G1

BQ34Z100-G1 charge termination indication is explained in SLUSBZ5B as revised July 2016, section 7.3.11 but 1(b) gives a puzzling description of how the threshold for accumulated change in capacity can be calculated. The reference to 0.25 mAh presumably refers to whatever value is stored in flash starting at subclass ID 36 and offset 2 which is labelled “Min Taper Capacity” and said to have a default of 25 mAh (not 0.25 mAh). The reference to Taper Current Window must refer to the flash location of the same name (subclass ID 36 and offset 6) for which the unit seems to be seconds (not hours). A worked example of a calculation for any accumulated change in capacity threshold would therefore be helpful, especially to people not using the evaluation hardware or software. Could anyone spare the time to provide one, please?

  • Hi Tim,
    e2e.ti.com/.../how-accurate-is-your-battery-fuel-gauge-part-2-2

    This blog explains how to calculate accumulated charge. There is an excel sheet there showing the step by step procedure.
    I believe the snippet you posteds has an error. 1(b) should be : During the same periods, the accumulated change in capacity > min taper capacity. The default for this parameter is 25mAh.

    thanks
    Onyx
  • Thanks, Onyx. Your wording for 1(b) does make much more sense than the TI literature. So, since there are 3600 seconds in a hour, if Taper Current Window is set (in seconds) to 40 and Min Taper Capacity is set (in mAh) to 25 then average charge current of 2.25A would just reach the set threshold for accumulated change in capacity during the set window period.

    2.25 A * 40 seconds / 3600 = 0.025 Ah

    But, that assumes charge efficiency is really 100% and is believed by the chip to be 100%. In reality, actual charge efficiency in lead acid batteries might be much less. Presumably the chip does not know the real accumulated change in capacity but calculates it by measuring charge current and modifying that by static PbA charge efficiency as set at flash subclass 34 and offset starting at 4 (but, at present, not by using other efficiency settings). So, if flash subclass 34 and offset starting at 4 were set, for example, to 50% then what real current would the chip regard as reaching the accumulated change in capacity set in the previous paragraph?

    Thanks for your help.
    Tim.

  • Hi Tim,

    I responded

    thanks
    Onyx