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BQ27520-G4: accuracy and delay of current values

Nicholas Carley
Expert 8495 points
Part Number: BQ27520-G4
  • What is the expected current measurement accuracy with a 0.01Ω resistor?
  • For the average current calculated in the fuel gauge, what is the averaging time?
  • What is the lag time in calculating the instantaneous current?
  • 0
    TI__Guru 57747 points

    Hello Nicolas,

    The accuracy will improve by a factor of 5 if a resistor of 5 times the value is used. However make sure that the current will always be low enough that the ADC sense input voltage is not exceeded.

    The average current is a filtered value of the last 5 samples. IMO this is hard to explain because it depends on the sampling interval in different modes.

    The instantaneous current is calculated from the sample that is taken. The sampling interval depends on the mode - in sleep mode it can be 20 seconds and in normal mode between 1s and 2s. The coulomb counter integrates the current over this time period.

  • 0 in reply to Shirish
    TI__Expert 8495 points

    Just summited an edit right as you responded. Its actually a 10mOhm resistor rather than 50mOhm.

    To your first point, do we have a data point of accuracy with a given resistor? Not SOC accuracy just measurement accuracy.

  • +1 in reply to Nicholas Carley
    TI__Guru 57747 points

    10mOhm is the default. The tolerance of the resistor plays a big role obviously. The datasheet has the characteristics of ADC and coulomb counter. The actual accuracy is much higher because the firmware compensates for temperature and applies filters. After calibration the accuracy improves again. Typical observed accuracy is 1mA, 1mV for a calibrated gauge. Ageing and other factors could cause some loss of accuracy over time. Since the PCB design can impact this accuracy, the accuracy of the "system" is not published.