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BQ76942: cov Protection issues

sheng liu
Prodigy 40 points
Part Number: BQ76942
Other Parts Discussed in Thread: BQ76952, BQSTUDIO

Hi,TI`s Team;

When the single cell voltage is greater than the COV threshold, the COV Safety Alert registere goes high,but after COV delay time，the COV safety status A register doesn't go high，When I adjust the single cell voltage to a higher level, the COV status register will go high; 

Why the actual protection value in test is greater than the set value in the configuration; 

The following is my COV parameter configuration, when I increase the voltage to COV threshold and higher,we can find the voltage when the warning register go high , and the voltage when the status register go high;

 parameter configuration

when the single cell voltage is4.196V,the COV safety status does not go high;

when the cell voltage is 4.237V,the cov status goes high


  • 0
    TI__Guru* 81035 points

    Hi Sheng,

    The threshold should not be different for the Safety Alert and the Safety Status. I suspect the voltage is right at the threshold and the noise is not holding it high for the full COV:Delay duration in the first case.

    I notice you are measuring the cell voltage by reading from the BQ76952. Did you calibrate the cell voltage readings. The over-voltage protection is a hardware comparator protection, so it is not using the ADC reading to trigger. Try measuring the voltage with an accurate meter directly at the cell when testing COV. 

    The cell voltage measurements can be calibrated for better accuracy and the COV threshold can also be calibrated to improve the accuracy further.

    Best regards,

    Matt

  • 0
    Prodigy 40 points

    Hi Matt;

    Now I have the sample test results, when the voltage reaches the COV threshold, the Safety Alert  will be red, but the COV Safety status flag will not be set; and all my actual protection voltages on each cell are around 4.18V instead of 4.15V

    Best regards;

    Sheng

  • 0 in reply to sheng liu
    TI__Guru* 81035 points

    Hi Sheng,

    How are you controlling the cell voltages. Do you have a very stable voltage source and have you looked at the voltages with a scope? As you hit the threshold, there may be noise. Battery cells are much more stable than most power supplies, so you need an accurate stable supply (I think our lab uses a Kronhite supply for testing). If there is noise, the voltage will go above and below the threshold and the COV:Delay time will not be met.

    Regards,

    Matt

  • 0 in reply to Matt Sunna
    Prodigy 40 points

    We are using Angilent's power supply, and there will be no voltage fluctuations, but in our configuration, there is balance during charging. Whether the balance causes the detected battery cell voltage to jump, which affects the battery's COV protection

  • 0 in reply to sheng liu
    TI__Guru* 81035 points

    Hi Sheng,

    Balancing will cause the voltages at the pins to move, but the device stops balancing to measure OV/UV periodically. Can you tell me more about your board and configuration? What value are you using for the cell input resistors? From you BQStudio picture, it looks like you have connected for 6 cells? Did you configure the VCell Mode register for 6 cells?

    Best regards,

    Matt

  • 0 in reply to Matt Sunna
    Prodigy 40 points

    Hi，Matt

    The following is the circuit schemtic and  configuration,please check it.

    2211.BQ76942-7S8P-2021-05-08.gg.csv

  • 0 in reply to sheng liu
    TI__Guru* 81035 points

    Hi Sheng,

    I cannot see the schematic image very well, but it looks like the cell input resistors are 1k. This is outside of the datasheet maximum value - the maximum recommended is 100 ohms. I recommend changings these resistors to 100 ohms. They can even change the resistor for VC10 to 20 ohms (minimum recommended value) to reduce the IR voltage drop on the top cell measurement. 

    It also looks like there is a large capacitor on REGIN? The REGIN capacitor should only be 22nF. 

    They should analyze the cell balancing circuit with the new resistor values to make sure no other component values need to be changed. 

    Best regards,

    Matt