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BQ27441EVM-G1A: BQ27441-G1A Design Capacity

Matt Leacock
Prodigy 170 points
Part Number: BQ27441EVM-G1A

Is the Design Capacity what is written on the cell or the useable capacity from the cell.  We are using the TPS65217C with a 1800mAh battery.  However the cut off for the battery is 3.0V.  The TPS65217C does not boost, so we are limited to a cut of voltage of 3.3-3.4V in our system.  Do we set the Design Capacity to 1800mAh or the available value what ever that may be.  I know it will learn so this value can change over time, but to get it as close as possible at that start which value should we be using.

Thanks,

  • 0
    Prodigy 170 points

    We did a discharge test with our battery.  Discharging to 3.4V we have a capacity of 1600mAh.  So do we use 1600 or 1800 for the design capacity.  This seems like a simple question that can be clarified very easily.

  • +1 in reply to Matt Leacock
    TI__Guru 53996 points

    Design Capacity should be set to the nominal capacity as specified by the manufacturer.

    The gauge uses this as a reference for parameters that are in Hr rate (C rate) and as a reference for chemical capacity (QMax). The latter is measured by the gauge when conditions allow so it will simply scale up or down, depending on measurements and how you specified Design Capacity. Note that Design Capacity itself, the parameter, is a constant. The gauge will learn QMax (which is expressed as QMax [mAh] = QMax[Num] * Design Capacity / 16384) but not Design Capacity. It will report Full Charge Capacity (and Remaining Capacity), which are results from capacity predictions based on discharge simulations that use QMax. FCC will change depending on conditions (temperature, load prediction) while QMax is directly measured using the coulomb count and OCV measurements.

    The bottom line is that Design Capacity is a reference and as long as it's in the ballpark, the gauge will measure what it needs to calculate gauging results.