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bq40z60 - Current measured is over 65% too high

Charley A.
Intellectual 430 points
Other Parts Discussed in Thread: BQ40Z60, BQSTUDIO

Hi,

I have a board with a bq40z60 circuit copied from the evaluation module.  When I look at the discharge current value on bqStudio, I see values that are consistently about 65% higher than what I measure with an ammeter. However, it does return the correct pack voltage.

The board has the 0.005 ohm sense resistor (R43) between SRP and SRN.  And two 100 ohm filtering resistors (R38 and R44 in the EVM).

 What should I be checking (hardware and data flash) to isolate the problem?

Thanks,

Charley

  • 0
    TI__Guru 73830 points
    Charley,
    Are you trying to calibrate the current? The reported current should be very accurate when you are discharging. I have seen errors when charging, because the gauge is very sensitive to layout and an offset can be introduced causing measurement errors. If you cannot measure an accurate discharge current after calibrating the EVM, then it may be defective.
    Tom
  • 0 in reply to ThomasCosby
    Intellectual 430 points

    I was not trying calibration.  In your experience, is calibration usually necessary?

    With my custom board, the battery life was much longer than expected, so I guessed that the actual power draw of the system was lower than what was reported by bqStudio. That is when I checked the actual voltage and current with a multimeter.

    I tried the same test with the EVM board and the difference in measured discharge current between bqStudio and the multimeter was only 4%.

  • 0 in reply to Charley A.
    TI__Guru 73830 points
    I would try calibrating the current. If you still see the current difference, then it is probably a noise issue caused by the PCB layout.