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BQ27426: SOH update timing

Koji Ikeda
Expert 6315 points
Part Number: BQ27426
Other Parts Discussed in Thread: GPCCHEM

Hello support team,

Customer are evaluating bq27426, however, the Remaining cap decreased soon. They found SOC went to 3% during couple of cycle, they assumed this cause the behavior. Could you support SOH inquiries?

* What is SOH definition? FCC = Design capacity x SOH?

*When or what condition bq27426 update SOH?

* Can SOH function be disabled?

Thanks,

Koji Ikeda

  • 0
    TI__Guru** 105760 points

    Hello Ikeda-san,

    You can check the TRM section 5.13 StateOfHealth(): 0x20 and 0x21

    This section explains how the SOH is calculated. You don't need to pull the SOH data, it's always calculated, I'm not sure why you would want to disable it?

    You can check the SOH status to see what stage the calculation is at.

    Did they run the relax-discharge-relax log to submit to the GPCCHEM tool to see if it's a good match for the default chem IDs on the ROM gauge?

    Sincerely,

    Wyatt Keller

  • 0 in reply to Wyatt Keller
    TI__Expert 6315 points

    Wyatt-san,

    Thank you for your support. They already confirmed the TRM section 5.13, however, there are no description, so, they ask.

    SOH seems to be the ratio of predicted FCC(25°C, SOH LoadI) / Design capacity. Could you explain more about predicted FCC(25°C, SOH LoadI) and SOH LoadI? What is difference between predicted FCC and FullChargeCapacity() command? And they can't find SOH LoadI detailed description.

    Could you show which description on TRM shows when SOH update?

    Thanks,

    Koji Ikeda

  • 0 in reply to Koji Ikeda
    TI__Guru** 105760 points

    Hello Ikeda-san,

    The Full Charge Capacity is calculated based on a load and temperature, so the gauge will run a simulation based on the SOH load (400ma) and at room temperature (25degC) and use that as FCC/DesignCap instead of taking the current FCC which would be calculated with the current load and temperature which can change significantly.

    The SOH will update when the Qmax or Ra tables update.

    Sincerely,

    Wyatt Keller

  • 0 in reply to Wyatt Keller
    TI__Expert 6315 points

    Hello Wyatt-san,

    Thank you for your support. Could you explain more about SOH load (400mA)? Is this impedance table at room temperature and 400mA load? I assume that gauge comperes actual impedance from measured voltage drop, current and temperature with the SOHload impedance table, then, gauge update new SOH at Qmax and Ra table update. Is my understanding correct?

    Thanks,

    Koji Ikeda 

  • 0 in reply to Koji Ikeda
    TI__Guru** 105760 points

    Hello Ikeda-san,

    Yes, the gauge will use the learned Ra and Qmax values and run a simulation with those values and the load set to a constant value (SOH load) and constant temperature of room temp. This prevents SOH from jumping as much during battery cycles.

    Sincerely,

    Wyatt Keller

  • 0 in reply to Wyatt Keller
    TI__Expert 6315 points

    Wyatt-san,

    Thank you very much for your support.

    Please let me confirm, what your last comment " set to a constant value (SOH load) and constant temperature of room temp" mean? I think that SOH is not fixed value.

    Thanks,

    Koji Ikeda

  • 0 in reply to Koji Ikeda
    TI__Guru** 105760 points

    Hello Ikeda-san,

    SOH is not a fixed value, It will be based on the Design Capacity and the simulated FCC value which is what I was describing, FCC can change depending on the learned Qmax and Ra values. SOH = FCC/Design Capacity. FCC can change without the SOH load or temperature changing due to Ra and Qmax.

    Sincerely,

    Wyatt Keller

  • 0 in reply to Wyatt Keller
    TI__Expert 6315 points

    Wyatt-san,

    Thank you for your support. I'm sorry to my many question, but, I need some another information.

    * Is predicted FCC which is used for SOH calculation separated value from  FullChargeCapacity() command result?

    * Is "SOH LoadI" a kind of impedance table at 400mA and 25degC? If learned Ra is higher than SOH load, does gauge set lower predicated FCC?

    Thanks,

    Koji Ikeda

  • 0 in reply to Koji Ikeda
    TI__Guru** 105760 points

    Hello Ikeda-san,

    Yes that's correct, the FCC predicted for SOH calculations is separate from the FCC reported on the command. Our gauges run simulations to find FCC and RemCap to calculate SOC, for the SOH we use the same functions we use to calculate real time FCC but we set constant current and temperature.

    This means the Qmax and Ra values are the only things that change during battery life to decrease SOH. SOH load is the constant current used in the FCC prediction to then calculate the SOH. SOH load isn't a resistance or impedance, it's a current value.

    Sincerely,

    Wyatt Keller