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BQ78350-R2-DEVICE-FW: Coulomb counter deadband and Sleep mode

Pietro Vignali
Intellectual 895 points

Part Number: BQ78350-R2-DEVICE-FW

Hello,

we have a battery pack for a special power tool where the bq78350 is set with an IPSCALE of 0x1 (the current is scaled by 10mA).

The Current deadband is set to 1, so we can read the SBS.Current with a granularity of 10mA.
The Sleep Current is set to 2, so If I understand well any current below 20mA cause the evice to go to SLEEP MODE.

If the device to which thw battery is connected drains something like ~5mA, I expect the battery do not detect it and coneinue to sleep, isn't it?

What happens to the Coulomb Counter in such case ?
If we have a Culomb Counter deadband that does not allow detecting the 5mA, I presume the Coulomb counter does not count any energy flowing out of the pack, and due to the sleep mode only the Electronics Load is taken into consideration (plus the Self Discharge %): is that understanding correct ?

If instead we lower the Coulomb Counter deadband below 5mA, does the gauge count correctly the real coulomb being drained from the battery even if in Sleep Mode ?

Thanks a lot.

Pietro

  • 0
    TI__Guru* 77283 points

    Hi Pietro,

    There is a separate parameter for the Coulomb Counter Deadband. This is different from the Current Deadband - the Coulomb Counter integrates charge over a large number of samples.

    The Coulomb Counter ADC on the AFE has a LSB of 8.44 uV, so the sense resistor value determines how much voltage the ADC will see when you are discharging 5 mA. A very small sense resistor allows you to measure large currents and to stay within the optimal range for the coulomb counter ADC. But a small sense resistor makes it very difficult to measure very small currents accurately because you will be below the resolution of the ADC. If your 5 mA is creating less than 8.44 uV across the sense resistor, then you will not be able to gauge the small discharge.

    The Coulomb Counter Deadband should be set to '9' according to our validation team for optimal performance. Any setting lower than this will result in the measurement being strongly impacted by noise. 

    Best regards,

    Matt

  • 0 in reply to Matt Sunna
    Intellectual 895 points

    Thanks Matt,

    on that pack we have an equivalent sense resistor of 0.6mOhm, so having a minimum detectable threshold of 8.44uV the minimum current that we can detect and account is 0.014A, is that correct ?

    So, in order to be able to account for smaller loads we should increase the sense resistor... but since the Power tool can generate loads up to 60A that we need to detect corectly, what Sense Resistor do you suggest to use ?

    Also, considering the Coulomb Counter and the Sleep Mode: during sleep, if there are currents highter than the Coulomb Counter Deadband, but lower than the Sleep Current threshold, does the gauge counts for that detected energy being drained ?

    Thanks

    Pietro

  • +1 in reply to Pietro Vignali
    TI__Guru* 77283 points

    Hi Pietro,

    You may be able to increase the sense resistor size, but you also need to consider your short-circuit protection threshold and over-current thresholds. The maximum short-circuit detection threshold is 200mV. There is a Detailed Design Procedure section in the BQ769x0 datasheet that goes through a sense resistor value selection example. 

    The Coulomb Counter will still accumulate in Sleep mode, but the sampling rate is slower. This can be adjusted with the Sleep: Current Time parameter.

    Regards,

    Matt

  • 0 in reply to Matt Sunna
    Intellectual 895 points

    Matt, a further clarification:
    having a 0.6mOhm sense resistor the minimum detectable current with a LSB of 8.44uV is 0.014A.

    Does this means that the system can read and account coulombs with a granularity of 14mA, so 28mA, 42mA and so on ?

    Thanks
    Pietro

  • 0 in reply to Pietro Vignali
    TI__Guru* 77283 points

    Hi Pietro,

    I think the granularity will be better since the LSB will toggle and the devices will integrate/average the reading.

    Matt