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BQ20Z655-R1: bqMTester - sense resistor

Part Number: BQ20Z655-R1
Other Parts Discussed in Thread: BQEVSW, BQMTESTER,

I am having an issue setting the sense resistor for current calibration while using the bqMTester for production. We are using three 0.01 Ohm sense resistors in parallel (for power) which is 0.003 Ohm. The bqMTester software does not want to accept such a low resistance. The calibration comes out completely way off. Is there a way to correct this? I can calibrate through the bqEVSW, but I cannot through bqMTester...

What is really driving this question is that I think I'm having an issue with my RSOC measurement. I believe it has to do with the CCdeadband called out in the solution here: 

I'm in urgent need for some guidance! I appreciate any and all help.

Thank you.

  • Hi JDrulia,

    1) bqMTester
    In production will you be performing per board calibration after programming the golden image or will you be using the average values of 10-30 boards (assuming variance is low) in the golden image so only the golden image is programmed (no per board calibration step)?

    If you won't be performing per board calibration, please use the bqEVSW to program and average the values of 10-30 boards and create a golden image with those values. If you will be performing per board calibration and bqEVSW is not an option, the calibration may need to be performed manually with commands via SMBus.

    2) RSOC measurement issue
    If you're having an issue with the RSOC drifting due to 'ghost currents' during relaxation, please increase the CC deadband to prevent 'ghost currents' from contributing to capacity accumulation. For the bq20z655-R1 the CC deadband is in units of 294 nV with a default value of 34 (9996 nV).

    For more information on the CC Deadband, please refer to C.13.4.3 CC Deadband (Offset 2) found in the TRM:

    Bryan Kahler
  • Thank you so much for the quick reply. Since I couldn't calibrate the current due to the small Rsense, we had already done the averaging of boards (as suggested) when creating the golden. Current measurement haven't been far off across the sample of production boards, but we have seen RSOC drift between full and empty while sitting on the shelf (when it should be ~60%).

    I will adjust the CC deadband as suggested. Thanks again!
  • Hi JDrulia,

    Glad to hear it! If you need anything else, please let us know.

    Bryan Kahler