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Bq20z655-R1 parasitic current

Tony Brenke15
Intellectual 480 points
Other Parts Discussed in Thread: BQ20Z655-R1

We have the BQ20Z655 with a 0.02-ohm sense resistor.

We can measure current to 10A within the 0.2V adc input.

The MIN current we can measure with this setup is 1 ADC count of .305mA.

This is not low enough to see the parasitic current when the BMS is asleep.

With the battery on a shelf in sleep mode, the battery can discharge in a year but the fuel gauge still reads 4 bars.

Is there a register to set parasitic current in the BQ20Z655-R1?  if so I have not found it.

  • 0
    TI__Guru 57697 points

    Hello Tony,

    The bq20z655 should automatically update SOC based on voltage drop if the correct chemistry was programmed.

  • 0 in reply to Shirish
    Intellectual 480 points

    The sense resistor is 0.02 ohms. this allows reading of 10A with the -0.2 / 0.2V limit. but the reverse of that, we can only measure 6.103E-06 or 0.305mA across the sense resistor. the sleep current on the BQ20Z655 is 0.250mA.  Too small for the BMS to see. this results in the gauge being full and the battery dead.  The battery chemistry does not change that.

  • 0
    Intellectual 480 points

    We are using the BQ20Z655 in a high capacity/high discharge battery pack design where our peak current is 15A, and the battery is on the shelf often for 1-3 years before being used.  The self-discharge of this pack is ~214uA.   Because of the sporadic use, the self-discharge over a 1 year period creates a large difference between the reported SOC from the battery gauge vs the actual SOC of the battery pack (the battery is much lower).  We cannot choose a large enough Rsense to measure the self-discharge and at the same time measure the peak current, so we cannot solve the SOC difference by changing Rsense alone.   How do we set the battery aging to account for the 200uA self-discharge so that the battery gauge can account for this in the SOC estimate?

  • 0 in reply to Tony Brenke15
    TI__Guru 57697 points

    Hello Tony,

    bq20z655 has a shutdown mode which may be useful for your use case.

    In sleep mode, bq20z655 is supposed to update RSOC periodically based on the cell voltage. This is based off the chemistry data that is programmed. There is no other compensation mechanism.

  • 0
    Intellectual 480 points

    The current use during sleep mode is what the Gauge can not account for.

    during sleep the current is 215uA.  with a charged battery sitting on a shelf in sleep mode waking every 30 seconds, the battery will deplete but the gas gauge will still show full charge.

  • 0 in reply to Tony Brenke15
    TI__Guru** 105590 points

    Hello Tony,

    During these inactive periods it would be best to keep the gauge in shutdown if possible as Shirish mentioned, then wake it up when it is needed. Upon wakeup from shutdown the gauge uses the chemistry ID to do voltage correlation for the first SOC estimation. This will be the lowest power and probably the best performance for the gauge. If you leave the gauge in sleep mode it should update periodically based purely on the OCV voltage, with current draw this low it does not make sense from a power perspective to try to capture passed charge when the OCV can be used for direct SOC estimation since there is no voltage drop from internal resistance. 

    Sincerely,

    Wyatt Keller