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BQ35100: Weird values for scaled resistance and impedance

Hendrik van Essen
Prodigy 50 points
Part Number: BQ35100

Hello,

we are running an EOS draining experiment and we are gathering data for scaled resistance and impedance as described here in chapter 5 (Testing Procedure). The values we read out of the fuel gauge are odd. Most of the time the value for the resistance is around 32769 or even 64422. This is suspiciously close to 2^16  / 2 (or 2^16). Only three times we obtained some realistic values (1114, 1104, 1100). For the impedance we have a similar issue, most of the time it is 32767.

Any ideas what could be the cause for this? Any help is greatly appreciated.

Thanks,
Hendrik

  • 0
    TI__Guru** 105760 points

    Hello Hendrik,

    Depending on your test sequence the resistance estimations can be invalid. Can you share your test setup and some scope captures of the pulse you apply to the gauge?

    Sincerely,

    Wyatt Keller

  • 0 in reply to Wyatt Keller
    Prodigy 50 points

    Hello Wyatt,

    the major discharge that takes place between GAUGE_START and GAUGE_STOP has a duration of 73 minutes and has a constant discharge current of 140 mA at 3.3 - 3.7 V, which equals 1% of the battery.

    The test setup is based on one of our custom boards that contains the bq35100. It is powered by the tested battery and can enable and disable the discharge current on command. The test sequence is basically to do one hundred single 1% discharges (or until the battery is drained empty). All the obtained values are stored in an internal NOR flash.

    Best,
    Hendrik

  • +1 in reply to Hendrik van Essen
    TI__Guru** 105760 points

    Hello Hendrik,

    This is a very long pulse, the BQ35100 is designed for short pulses, on the scale of 100s of milliseconds to a few seconds. I think you may be saturating the filter for resistance estimation. For the accelerated testing we discharged by 1%, then relaxed, then did a short pulse of 500ms to learn resistance, then short relax before discharging another 1%.

    Sincerely,

    Wyatt Keller

  • 0 in reply to Wyatt Keller
    Prodigy 50 points

    Hello Wyatt,

    thank you very much for your suggestion. I see, so we are not supposed to take the measurements from the long discharge pulse, but from a smaller pulse beforehand. Do we still start and stop the gauge while discharging the 1%, though?

    Thanks,
    Hendrik

  • 0 in reply to Hendrik van Essen
    TI__Guru** 105760 points

    Hello Hendrik,

    No, the gauge start and stop should only be done around the small current pulse, similar to how it would be in normal application. The long 1% discharge pulse is just for characterization, the gauge can be completely off for this discharge.

    Sincerely,

    Wyatt Keller

  • 0 in reply to Wyatt Keller
    Prodigy 50 points

    Hello Wyatt,

    thank you very much, we will then re-run our draining experiment with the latest changes.

    Best,
    Hendrik