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How to set CC deadband

Brad Bushard14
Prodigy 210 points
Other Parts Discussed in Thread: BQ40Z60, BQSTUDIO

The design I'm working on uses a 2 mOhm Rsense with the bq40z60. What is the equation for adjusting CC deadband (coulomb counter)?

The default CC deadband is a value of 9. The TRM (August 2015) says nothing about how this register is used or what should be done when running a different Rsense from the EVM or default setting.

Is a CC deadband of 9 * 116 NV = 1 uV the proper setting for a 5 mOhm Rsense?

If so what do I set CC deadband too for a 2 mOhm Rsense?

My concern centers around the QMax update Offset Error and QMax time-out.

  • 0
    TI__Guru 73830 points
    Brad
    This device should not be used with a sense resistor smaller than 5mohm. You will run in into current measurement and gauging issues.

    Tom
  • 0 in reply to ThomasCosby
    Prodigy 210 points
    Tom,

    The bq40z60 datasheet (SLUSAW3C –DECEMBER 2014–REVISED JULY 2015) says the following:
    High-performance, integrated analog peripherals allow support for a sense resistor down to 3 mΩ, battery charge control, and simultaneous current/voltage data conversion for instant power calculations.

    Is the datasheet wrong?
  • 0 in reply to Brad Bushard14
    TI__Guru 73830 points
    Brad,
    We recommend using a 5 mohm sense resistor, but you can reduce the value to 3 mohm. You may see more error in the coulomb counting current with the smaller sense resistor.
    Tom
  • 0 in reply to ThomasCosby
    Prodigy 210 points

    Tom,

    The reason I ask about CC deadband is in reference to QMax update timeout. Given the design attributes:

    Cell capacity = 1.1 A/h

    Rsense = 5 mOhm

    CC deadband register =  9  or (9 * 116 nV = 1 uV)

    QMax update timeout = 21.07 hours

    21.7 hours is a long time and will exceed Qmax Max Time of 18000 seconds.

    How is CC deadband determined? Is it a setting I should adjust? Is it somehow determined by calibration or other parameter settings or by bqStudio?

    BR,

    Brad

  • 0 in reply to Brad Bushard14
    Prodigy 210 points

    Correction to previous post. Rsense in this design is 2 mOhm,

    Cell capacity = 1.1 A/h

    Rsense = 2 mOhm

    CC deadband register =  9   (9 * 116 nV = 1 uV)

    QMax update timeout = 21.07 hours

  • 0 in reply to Brad Bushard14
    TI__Guru 73830 points
    Brad
    We typically try to keep the Qmax update time at about 10 hours, so the CC Deadband is set a little low on the bq40z60 already. If you are using a 2 mohm sense resistor, then I would set the CC Deadband to 17. This will set the offset current to about 1mA and this is where we normally keep it.

    Tom
  • 0 in reply to ThomasCosby
    Prodigy 210 points
    Tom,
    Thanks for the advice and detail on targeted offset current of 1 mA for a 10 hour Qmax update timeout. I'll test this arrangement to see if QMax update success rate is improved.
    Brad
  • 0 in reply to ThomasCosby
    Prodigy 210 points
    I continue to be confused by the inconsistencies in TI documentation.

    The bq40z60 TRM (SLUUA04C–December 2014–Revised March 2016) says:
    “The bq40z60 measures charge and discharge activity by monitoring the voltage across a small-value
    series sense resistor (1 mΩ typical) between the negative terminal of the cell stack and the negative
    terminal of the battery pack.”

    So the bq40z60 datasheets mentions support down to 3 mΩ, but the TRM says a typical 1 mΩ, TI engineering says 5 mΩ is recommended.