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BQ34Z100-G1: Golden Image with loads below C/20

Graham Cameron
Prodigy 30 points
Part Number: BQ34Z100-G1
Other Parts Discussed in Thread: BQ34110

We are using a bq34z100-G1 part to fuel gauge three battery packs where the load is a constant current of less than C/20. The application typically takes 3-5 hours with a maximum of 16 hours of operating time before recharging. I picked a battery chemistry of 0100 for my LiCoO2/graphitized, carbon, 18650 cell from Panasonic (or Samsung). Is this image good enough to serve for our application? Say for a few adjustments, e.g. Design Capacity of 4.8 Ah. I expect 3-5% error on Remaining Capacity as opposed to 1% error -- this should make little difference to the user. Besides, it is unclear to me what learning cycle currents would apply from such a small current load that is below C/20.

  • 0
    TI__Mastermind 42535 points

    Hi Graham,

    At a load of C/20 you will not update resistance values. You need a rate > C/10 for Ra updates. For such low loads, you may want to look up the bq34110. It's a simpler gauge and is slightly less accurate but can work with low currents as it's based on a different algorithm.

  • 0 in reply to Batt
    Intellectual 270 points

    I can understand that the Learning Cycle wouldn't be very successful with a C/20 load, but I don't understand why an Impedance Track fuel gauge wouldn't perform well with a lightly loaded battery.  Referring to Figure 3 in SLUA450 (for example), it seems to me that as battery capacity is consumed, there would be plenty of dV to be measured and thus Ra calculations could be made.  Am I missing something fundamental?  Thanks... 

    Chris

  • 0 in reply to Chris Kimble
    TI__Mastermind 42535 points

    Hi Chris,

    We keep the battery in specific modes, chg, dsg and relax based on current thresholds. At low rates of dsg, you are typically depending on OCV to update capacity rather than a combination of coulomb counting and DOD updates. This happens when you have C/20 or so rates of dsg. We will update resistance only when the rate of dsg exceeds C/10. That's why we advise you to use a C/10 rate at a minimum.

  • 0 in reply to Batt
    Intellectual 270 points

    Hi again.  I have a lingering worry that I didn't state my question clearly enough, because it doesn't seem right that an impedance-track fuel gauge would work well only if battery lifetime between charges is less than 10 hours. 

    To be specific, for a 2.4-Ah battery, if we use a >240-mA dsg rate during the Learning Cycle, would the bq34z100-g1 perform well in a system where the actual load range is 50-120 mA? 

    Thank you.  I appreciate TI's excellent customer service in this forum!

  • 0 in reply to Chris Kimble
    TI__Mastermind 42535 points

    Hi Chris,

    If you have programmed and learned your battery correctly, yes, with a 120mA dsg, you will learn resistance and qmax if you exceed the chg and dsg thresholds. If your load is going to be 50mA at a minimum I suggest setting your chg and dsg current thresholds to 45mA.

    Thank you for your kind words. We wish you success in your application.