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BQ40Z50-R2: Measuring low currents

Emilien Chappatte
Intellectual 355 points
Part Number: BQ40Z50-R2
Other Parts Discussed in Thread: BQ40Z50

Hello all,

I would like to know if with the maximum recommended sense resistor of 3mR we are able to measure reliably discharge current in a few mA range ?

With 3mR, the reference is 0.1215V (Vref1/10) 1mA is 8 of 32767... or about 30uV.

Is this ok ? Or do we need to use a bigger sense resistor ? Nominal current is less than 1A (in both directions) and max short circuit 3-5 amps.
But in the datasheet it's mentioned using 1 to 3mR.

Thanks !

  • 0
    TI__Guru* 76230 points

    Hi Emilien

    3mohm is ok for your application.  You will want the gauge to properly measure your nominal values as those have more impact on the battery capacity than smaller current draws.

    thanks

    Onyx

  • 0 in reply to Onyx Ahiakwo
    Intellectual 355 points

    Hello Onyx,

    Thanks for your fast reply. Perhaps I should be more precise with my numbers :

    Average discharge current is 10-30mA.
    We have short pulsed currents in the 200-300mA range (max discharge current reported in lifetimes) and at other moments (MCU sleep) we use only 2-3mA.

    I did more testings with two pairs of same devices, one modified with 20mR sense resistor and the other with 3mR standard.
    I have better results with 20mR. With 3mR in most of the cases at the end I have a drop from 3%/8%  to 0% of RSoC. Some graphs below.

    It seems with the 3mR resistor the gauge miss some current at discharge, do you think we can use 20mR sense resistor for BQ40z50 ?

  • 0 in reply to Emilien Chappatte
    TI__Genius 12215 points

    Hi, Emilien

        Using a sense resistor with higher value always yields higher accuracy current reading, and this will also improve the SOC accuracy. The main concern of using a 20mOhm Resistor is for your system noise rejection, if your system is not sensitive to the noise caused by using higher value resistor, then a 20mOhm value should be OK from gauge side.