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BQ27542EVM: Inquiry About Ra Table Calculation and SOH Calculation on BQ27542

Tom Tsao
Expert 1850 points
Part Number: BQ27542EVM

Tool/software:

Dear TI Support Team,

I have some technical questions regarding the BQ27542 fuel gauge, specifically related to Ra table calculation, SOH abnormalities, and SOH calculation.

  1. When calculating the Ra table, could you confirm the sampling period of the BQ27542? Does the IC have any filtering mechanism to mitigate noise? What is the sampling method used? For example, does it take multiple samples (e.g., 5 readings) and average them?

  2. In actual usage, could noise interference on the I2C (SCL, SDA) signal lines cause abnormal SOH readings?

  3. According to the TRM, SOH is calculated as:  FCC(25°C, SOH Load I) / DesignCapacity()

    This formula appears to apply during discharge, but is a different calculation method used during charging?

    How is FCC(25°C, SOH Load I) calculated, and what is its relationship with the SOH Load I setting?

                          

I would appreciate any insights or guidance you can provide.

Best regards,

  • 0
    TI__Mastermind 25855 points

    Hello Tom,

    1. I've attached the documentation of Impedance theory below that can better explain how the Ra table is calculated. The first sampling period happens after 500 second of last discharge and changes dependent on the DOD. In terms of the sampling method it take the points of the cell's resistance and compares them to a database of cells (aka the chemID) to help it predicate the current SOC.
    - App report of Impedance tracking, page 4
    https://www.ti.com/lit/an/slua364b/slua364b.pdf?ts=1739549985142&ref_url=https%253A%252F%252Fwww.google.com%252F

    2.Typical if your board layout have close ground planes and a routed properly as a differential pair than you should see any interface with SOH reading. I2C is a robust protocol

    3.Yes, the SOH is heavily based on the discharge and not really much on the charge cycle (the algorithm can note if it take less time to charge a cell in the same setting than it's capacity is lowered meaning that the cell has a lower SOC). But typically SOH is taken during the discharge and compared to it's ChemID to see what it discharge curve should look like. I would read the mention document above to get a better understanding of how impedance tracking algorithm works.


    Regards,
    Alan