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State of Health System Side with BQ27421

Jonathan Valdez
Genius 16430 points
Other Parts Discussed in Thread: BQ27421-G1

Hello.

Background:

I am using the BQ27421-G1 for a system-side battery gas gauge which will connect to removable battery packs.

Question:

1) When I switch battery packs, how does the gauge accurately measure the state of health?

2) Does it take a full discharge cycle or some amount of time?

3) I can plug a 100 mOhm resistor in series with the battery pack to simulate aging and the state of health does not change more than just a few percentage points, is there a more appropriate value to use, or does it take a specified amount of time to update this value? We saw about a full discharge cycle to drop 2-3%

  • 0
    TI__Expert 4115 points

    Hello Jonathan,

    For the '421 (and other system side gauges) the SOH parameter also comes with an SOH Status value.  The SOH Status lets the user know how accurate the SOH value is.  The most accurate SOH value will be after the gauge has had a chance to update the Qmax value and update the resistance table.  SOH is calculated Full Charge Capacity (FCC) at a specified load (default is 400 mA for the '421) and at room temperature divided by Design Capacity.  So, to answer your questions:

    1&2: When the gauge first initializes SOH is a reflection of the initial FCC calculation which is essentially going to be based on the voltage, load and temperature the gauge sees at initialization.  SOH Status should be 0x01 at this point.  If you start a discharge, once a grid point is reached the gauge will do a resistance calculation and that should change the SOH Status to be 0x02.  Once the gauge gets a Qmax update the SOH status can go from 0x01 to 0x03 or 0x02 (if a resistance update has already occurred) to 0x03.  Please refer to SLUA450 for more information on the theory of Impedance Track.  Once SOH status has reached 0x03, the SOH value is at its most accurate point.

    3.  Depending on the type of cell being used, adding 100 mOhm in series could potentially impact the SOH reading, but most likely will not have a significant impact.  It would be better to double the resistance table stored within the gauge to get a better picture of an aged cell.

    I hope this helps.