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0°C normalization in Ra table?

Zhang Hui
Intellectual 625 points
Other Parts Discussed in Thread: BQ27421-G1, BQ27510-G3

Hello,

In bq27421-G1 Technical Reference (sluuac5) “6.4.3 Ra Table Class”, one read that the resistance values are normalized to 0°C. What is its rationale?

We know that battery’s resistance increases as temperature lowers, and heating battery can practically be used to extend the running time of a depleted battery. So why the gauge normalize resistance to 0°C? And what is the formula for converting between normal temperature resistance and the 0°C-normlized values read from the Ra tables?

 

Hui

  • 0
    Genius 3970 points

    I don't have knowledge about Impedance Track past what their documents SLUA450 and SLUA364B provide, but I think the resistance is normalized to 0 degrees Celsius so that the gauge has a "baseline" resistance that gets temperature-compensated during operation.

    From SLUA364B:

    Before storage to Data Flash, resistance values are normalized to 0°C as Ra[dod] = R[dod]/exp(Rb[dod] x T) where R is the measured resistance value at a given DOD, Rb[DOD] is the value of temperature coefficient of impedance change at a given DOD stored as a reserved data-flash table, and T is temperature in °C.

    Note that the aforementioned Rb table is in a hidden area of the gauge's memory (i.e. it is not user-accessible). The Ra data that's visible is intended for backup/restore when using data memory image files and not really meant to be interpreted directly by the user. However, I think the bq27510-G3 does normalize its resistance to 25°C, but in a typical use scenario the gauge is sealed so access to the gauge's memory is locked down anyway.