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BQ25708: 1S Cell application, BQ25708 stop charger Battery when set CV voltage exceed 4.288V , set lower than 4.288V, the charger is normal

Fandy
Expert 2285 points
Part Number: BQ25708

Hello

1S Cell application, BQ25708 stop charger Battery when set CV voltage exceed 4.288V , set lower than 4.288V, the charger is normal

so what probably happens ?

  • 0
    TI__Expert 5330 points
    Hello Fandy,

    When BQ25708 is configured for 1S Cell by biasing CELL_BATPRESZ to 25% of REGN, the MaxChargeVoltage register (0x15) is set to 4192 mV at POR startup.

    The MaxChargeVoltage register will cause the charger to stop charging the battery once the MaxChargeVoltage value is reached. This value may be modified via I2C by overwriting register 0x15. It is set to a conservative value at startup.

    Can you provide more detail on what you mean by the battery charger stopping when "set CV voltage exceeds 4.288V?" Are you saying that charging stops when the battery voltage (i.e. as measured at SRN pin 19) exceeds 4.288V? If so, then this is expected behavior as per the above.

    Or are you saying that when register 0x15 is changed from 4.288V to a value above 4.228V via I2C, the charger suddenly stops charging? If this is the case, then it would not be expected behavior, and I will need more detail to determine the cause. If so, please let me know:
    1. the value you are setting for register 0x15 when you see unexpected behavior
    2. if you are changing any other register values from their default POR value
    3. the voltage measurement at pin 19 as well as the measured current into the battery (i.e. is it 0 A or just less than expected?)
    4. the value of register 0x20 (charge status register) as well as register 0x21 (Prochot status register) during the unexpected behavior
    5. are you using the EVM or custom hardware?
    6. are you charging a real battery or simulating the battery with a CV power supply? If it is a real battery, does it have a protection IC?

    Regards,
    Steve
  • 0 in reply to Steve Preissig
    Expert 2285 points
    Hi Steve
    thanks for your reply, after CHRG_OK, I reset the 0x15 value, such as less than 4.288V ,it is charging ok when battery is low, but stopped when voltage at SRN rise approach to 4.288V ，This maybe caused by my protect action of protection IC on my Battery, so this problem can be suspend.

    I have other question: At POR,0x15 set to default 4192mV， After CHRG_OK, 0x15 could be changed, I want to know the range of MaxCharge Voltage Register(0x15) ? this doesn't descriped in datasheet.
  • 0 in reply to Fandy
    TI__Expert 5330 points
    Hi Fandy,

    The max value that can be set in register 0x15 is 19.2 V and the minimum value is 1.024V. The register has 11 bits with 16 mV LSB, which theoretically allows you to write a larger number than 19.2V, but any larger value will be ignored.

    Section 8.6.4 of the datasheet (SLUSCU2) has a few more details on the format and limit.

    Regards,
    Steve
  • 0 in reply to Steve Preissig
    Expert 2285 points

    Hi Steve

    For 1S application, 0x15 can be set in the range of ? V, for 1S, the OVP of SYS is 5V, so I can set 0x15 below 5V ? Actually I expect available value of 0x15 is about 4.35V

  • 0 in reply to Fandy
    TI__Expert 5330 points
    The max charging voltage set in register 0x15 indicates the voltage that the battery will be charged to. For 1S LI-ion, this is typically in a range of 4.1-4.35V, with 4.2 V being common, but really it is dependent on the specific battery you are using and you should contact the battery vendor to determine the proper max charging voltage to set.

    As you say, there is a relationship between battery charging voltage and system voltage. I agree that charging the battery above the maximum system voltage does not make sense at a system level. But it is difficult for me to give you a concrete range or set of values for these different parameters without understanding the system better. That said, the max charging voltage should be set directly from the characteristics of the battery and not from these other system requirements. If battery characteristics don't match system requirements, you need to move to different battery or multi-cell configuration.