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BQ76952: BQ76952: Effect of AFE sampling accuracy during balancing

Star Xu
Genius 9645 points
Part Number: BQ76952

Hi 

I have a query need your help.

By configuring the software to open odd or even cell balancing, it has an impact on the sampling error of other sections (see the figure below for detailed data), and not only on cell15. Therefore, during balancing, the sampling accuracy cannot be compensated only for cell15 (n * 35 uA* 100Ω/1000=3.5（mV） * n). What is the reason for the impact on the sampling accuracy of other cells? Is it inconsistent with the description in the specification? How to avoid the impact of sampling errors on other cells?

1，To open odd cell balancing，and then cell5、cell7、cell9、cell11、cell13、cell15 sampling accuracy is affected.

2，To open even cell balancing，and then cell6、cell8、cell10、cell12、cell14 sampling accuracy is affected.

Waiting for your reply.

Thanks

Star

  • 0
    TI__Mastermind 25556 points

    Hello Star,

    Can you please measure the voltage at the input of the cells and also measure the voltage at the pins of the AFE? It'd be good to log this voltage measurements and see if they differ while cell balancing. 

    The only thing I think might be affecting the cell measurements during cell balancing are the Zener diodes across the input pins. These Zeners are rated 7.5V, and the cell voltages could go as high as the 7.5V turning ON the diodes and causing voltage to drop. It'd be good to remove the Zeners and see if it fixes the issue. 

    Regards,
    Jose Couso

  • +1
    TI__Mastermind 25556 points

    Hello Star,

    I was discussing with my team, and turns out the BQ76952 has Measurement Disturbance During COV/CUV Checks. This is more detailed discussed in section 2.5 from the cell balancing app note.

     Also, to help with the voltage readings, smaller RC filter could be used in the input cell pins.



    A workaround on this would be to ignore 2 measurements out of three while balancing.

    Regards,
    Jose Couso 

  • 0 in reply to Jose Couso
    TI__Intellectual 920 points

    Hi Jose,

    • This is my customer's case. This sch. is the previous one. In the new design, they replace the Zener diode between inputs by capacitor to GND. Any affects by this capacitor?  
    • The RC  in customer's design, 100Omega/ 220nF is same to our demo. 

    Br,

    Hugh

  • 0 in reply to Hugh Yang
    TI__Mastermind 25556 points

    Hello Hugh,

    Please post the question in a new thread. This is simply to better keep track of threads and not discuss multiple topics.

    Please include the schematic in the new thread.

    Regards,
    Jose Couso