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BQ34Z100: Max. Lifetime Current

Andreas Teske
Prodigy 30 points
Part Number: BQ34Z100
Other Parts Discussed in Thread: BQ40Z80

Hello,

i have a battery-managment-system (BMS) with based on the BQ34Z100 chip here on a 7-cell Li-Ion battery pack.

Everything seems working like expected, only the value of the maximum lifetime current is too low.

Our application normally (about 85% of the time) draws a (nearly) constant current of about 1A.

Only sometimes, it needs a current of about 12A for a time  period of about 200us. When i read out the max. lifetime discharge current,

it says something about 6-7A, which seems to be the average value (probably).

So i want to know, if the max. lifetime discharge current is integrated over some time, or why else, the value is too low.

Even it would be helpfull, what settings i need to change, to readout the correct value.

Greetings, Andreas

  • 0
    TI__Guru** 105590 points

    Hello Andreas,

    I believe the sampling rate of the coulomb counter is much more than 200us, so the gauge is most likely missing this pulse or only captured the taper of the pulse.

    The conversion time for the coulomb counter should be in the datasheet: https://www.ti.com/lit/ds/symlink/bq34z100-g1.pdf?ts=1594132200601&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FBQ34Z100-G1

    Sincerely,

    Wyatt Keller

  • 0 in reply to Wyatt Keller
    Prodigy 30 points

    Hello Wyatt,

    thanks for your replay. I also thought that maybe the sampling rate is too low. In the datasheet i've found the following:

    Coulomb Counter

    tCONV - ConversionTime - Single conversion - 250ms

    Does this mean, that the current is converted over 250ms? Could be, i understand it like that. But i see no sense in averaging a maximum value.

    But one important thing, i have to correct: Now on the BMS there is a BQ40Z80 used, not like i wrote before. Sorry for that! Before we had a BMS with a BQ34Z100 on it, and this old chip delivered correct values for the max lifetime discharge current. So i think the new one must also, mustn't it?

  • 0 in reply to Andreas Teske
    TI__Guru** 105590 points

    Hello Andreas,

    Yes that is the sample rate. The previous gauge may have sampled right at the time of the max load, and your new test the current sample may have been taken when the current is tapering. I am not sure of the specifics of your test but this would be my first conclusion.

    The only way the gauge will be able to catch that pulse is if the pulse coincides with the current sample of the gauge.

    Sincerely,

    Wyatt Keller

  • 0 in reply to Wyatt Keller
    Prodigy 30 points

    Hello Wyatt,

    just to be clear, only the chip on the BMS changed, our application is the same as before.

    If this is the sample rate that means for me that a current value is sampled every 250ms.

    Then you're right, that maybe the sample point is not at the maximum current value.

    But this is only a matter of time. If we use the battery pack long enough, sometime the sample point will hit the maximum.

    So the only case, when i have no chance to get i suitable value is, when the maximum current is averaged over 250ms.

    But why should someone average a maximum value?

    Greetings,

    Andreas

  • 0 in reply to Andreas Teske
    TI__Guru** 105590 points

    Hello Andreas,

    The value could be some average of the current in the pulse because of the ADC method the gauge uses, the gauge is not designed to measure fast high current pulses.

    Sincerely,

    Wyatt Keller