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BQ25731: Inquiry Regarding BQ25731 for a 3-Cell Battery System

Yu-Heng Deng
Prodigy 10 points
Part Number: BQ25731
Other Parts Discussed in Thread: BQ40Z50

Tool/software:

Dear TI Team,

We are designing a system that utilizes the BQ25731 with a 3-cell battery, which includes a built-in battery gauge. The device is expected to operate primarily while connected to an adapter.

I have a few questions regarding the charger chip's behavior and best practices for maintaining battery safety and health:

  1. Battery Voltage and ChargeCurrent() Behavior:
    Is it safe to maintain the battery at 12.6V when the ChargeCurrent() is not set to 0? I understand that certain events may reset ChargeCurrent() to 256mA. Would you recommend leaving it at this value, or is it better to explicitly set it to 0?

  2. Long-Term Battery Health:
    Considering long-term battery safety and health, should I set ChargeCurrent() to 0 once the battery is fully charged and only restart charging (by setting ChargeCurrent() to a non-zero value) after the battery voltage drops below, for example, 11.55V? Alternatively, is it acceptable to maintain the battery at a fully charged state for good when the device is plugged in?

  3. ChargeCurrent() and ChargeVoltage() Behavior:
    Do ChargeCurrent() and ChargeVoltage() operate in a way that performs constant current and constant voltage charging modes respectively? Or do they simultaneously impose limits on both current and voltage? Does the charger automatically switch between constant current and constant voltage modes depending on the measured battery status with the values set with ChargeCurrent() and ChargeVoltage()?

  4. Observed Charging Behavior:
    We observed that with ChargeCurrent() set to 1500mA and the watchdog disabled, the actual charging current (as read from the ADCIBAT) sometimes drops to 256mA or even 0mA once the battery voltage reaches around 12.6V. Is the charging current being limited by the charger or by the battery's internal chip? Additionally, would it be safe to charge under these conditions with an unprotected battery?

  5. Undocumented Registers (0x10–0x1C):
    An engineer who previously worked on this initialized registers from 0x10 to 0x1C in the charger chip, but these registers are not documented in the datasheet. Could you provide information on their functionality?

Thank you for your assistance. Please let me know if you need further details about our design to clarify any of the questions.

Best regards,

Ryan

  • 0
    Prodigy 10 points

    FYI, the battery gauge built into the battery is TI bq40z50.

  • 0 in reply to Yu-Heng Deng
    TI__Mastermind 32715 points

    Hi, Ryan,

    When the battery is fully charged, set charge current to zero. 

    Yu-Heng Deng said:
    Do ChargeCurrent() and ChargeVoltage() operate in a way that performs constant current and constant voltage charging modes respectively? Or do they simultaneously impose limits on both current and voltage? Does the charger automatically switch between constant current and constant voltage modes depending on the measured battery status with the values set with ChargeCurrent() and ChargeVoltage()?

    Both loops are effective,it depends which one reaches the limit first. The transition is seamlessly, no need for external intervention. 

    Yu-Heng Deng said:

    • We observed that with ChargeCurrent() set to 1500mA and the watchdog disabled, the actual charging current (as read from the ADCIBAT) sometimes drops to 256mA or even 0mA once the battery voltage reaches around 12.6V. Is the charging current being limited by the charger or by the battery's internal chip? Additionally, would it be safe to charge under these conditions with an unprotected battery?

    When the battery reaches max charge voltage=12.6V, the charge current is automatically reduced by the charger. You can still safely charge it until the charge current reduces to termination current, say 256mA, then you can terminate the charge by set charge current to zero. 

    There is no undocumented register inside the chip. You need to clean your code 

    Regards,

    Tiger 

  • 0 in reply to Tiger Zhou
    Prodigy 10 points

    That gives us the information we need to move forward, thank you very much!