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BQ27520-G4: Current is offset by over 500mA after reprogramming

Ross Johnson
Prodigy 230 points
Part Number: BQ27520-G4

We have been using the bq27520-G4 (3.29) for a while now. I recently needed to reprogram the gold image due to a change in battery pack composition. I unsealed it, reprogrammed the original firmware (senc file using EVSW) then programmed the new golden image. The current reading is now offset by over 500mA on two units (535 and 543 to be precise). Otherwise the readings appear generally correct whether under load or charging.

I tried restoring the stock firmware without our golden image and Gauge Studio still reports the same erroneous current. Other parameters such as voltage and temperature appear to be correct.

Does anyone have any idea why this might have happened and what can be done to correct it? Or have the chips failed somehow?

Cheers, Ross

  • 0
    TI__Guru* 78563 points

    Hi Ross,

    It seems like there must be some voltage across the SRP/SRN pins. Can you check this on your board?

    Have you tried to calibrate the CC offset to see if it helps?

    Best regards,

    Matt

  • 0 in reply to Matt Sunna
    Prodigy 230 points

    Hi Matt,

    Thanks for responding. I did try calibrating CC offset (and others) with no effect. I'll check on the SRP/SRN voltage when I get an opportunity, but probably not for a few days (due to our current lock-down).

    However, I'm not sure it's actually possible for this to be a voltage on the sense resistor. With the battery being the only power source in the system, the sense resistor would have to be detecting 500mA charging the battery! Would be nice to have a product with a self charging battery, but sadly I doubt it. Also, the magnitude of current variation suggests that the sense resistor does still have the correct 10mohm resistance. So this is an offset (in the positive direction), and not a scale issue.

    Is it possible there is some register/flag in the fuel gauge which hasn't been properly cleared by unsealing and reprogramming the stock image? Are there any additional steps I should take to restore factory state?

    Cheers, Ross

  • 0 in reply to Ross Johnson
    Prodigy 230 points

    Just tested it and zero volts on the sense resistor.

  • 0 in reply to Ross Johnson
    TI__Mastermind 24870 points

    Hello Ross,

    Can you do a board calibration and cc calibration?

    That should resolve it.

  • 0
    Prodigy 230 points

    Hello Kang,

    As mentioned previously, I have already tried the CC and board calibrations which completed, but didn't affect the erroneous current reading. However, I was comparing the calibration values to those in my golden image (derived from averaging 15 units), and there are some notable anomalies.

                    GOLDEN  PROBLEM

    CC Gain         20.792  24.327  mOhm
    CC Delta        20.751  24.28   mOhm
    CC Offset       -1442   -1503   Counts
    Board Offset    0       59      Counts
    Pack V Offset   0       -2      mV

    Most of these values are probably okay, given the variability observed during golden image preparation. However, the board offset may be of concern. All 15 units in the golden image had board offset of zero.

    Regards, Ross

  • 0 in reply to Ross Johnson
    TI__Mastermind 24870 points

    Hello Ross,

    The board offset is the one that has impact on your offset issue.

    Please calibrate the board offset with no current flowing, but don't short SRN and SRP.

    Thanks!