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BQ20Z90-V150: Capacity monitoring at less than full charge

George Mortimer
Expert 1205 points
Part Number: BQ20Z90-V150
Other Parts Discussed in Thread: BQ40Z50-R2

TI team,

I have discovered an anomaly in the calculation of state-of-charge that I could use some assistance with.

The International Air Transport Association (IATA) requires that lithium-ion batteries be shipped at a maximum of 30% state-of-charge for air freight transport.  As a result, we have developed for one of our customers a custom battery charger that monitors relative state of charge and cuts off charging when RSOC reaches 30%, as reported by the SMBus.  The problem is that, while the charger performs that function correctly, and even reports the RSOC at 30% after the pack is removed from the charger, as soon as any other action is done to the battery, such as applying any load to the pack, the gas gauge recalculates both FCC and RC, and as a result the RSOC changes, sometimes a significant amount.

Is this normal for the Impedance Track gas gauging on these chips?

If so, is there any way to either prevent the pack from recalculating so that the gas gauge continues to read the 30% until the pack is put to use (note that our instructions to our end user require them to fully charge the battery pack before using), or find some other method to fulfill the IATA requirement?

George

  • 0
    TI__Genius 14985 points

    It is true that actual capacity will change with load and temperature, which is why the register is called RSOC (relative state of charge) in many of our gauges.

    In Impedance Track gauges there will be occasional re-simulations of the estimated RM and FCC and SOC and the registers will be updated.  In older gauges like the bq20z family the updates will directly be reflected in the SBS registers.  In newer gauges we have an option to try to "smooth" or hide jumps from the user, but even then they can change over time.  The latest multi-cell gauge, bq40z50-R2, actually has a feature to assist with IATA regulations by storing and displaying the shipped SOC.

    For your case you could consider charging to a lower level to give yourself some headroom, or charge at a higher temperature with the assumption that if they are inspected it will be at a lower temperature and SOC will be lower.  You could also increase the values of Avg I Last Run and Avg P Last Run in dataflash since that is the load which will be used in simulations during rest, charge, or start of discharge for most settings of Load Select.

    Hopefully this helps!