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BQ24073: Inquiry about charging efficiency

MJ Jeon
Genius 5240 points
Part Number: BQ24073

Hi,

-5V input stage current consumption: 0.137A at 5V

-BQ24073 IC's CC measured value: 0.12A at 3.7V,

  When the battery is in 3.7V condition, the output voltage of the charger IC is 4.2V.

Therefore, the overall charging efficiency of the board is calculated as (4.2V*0.12A)/(5V*0.137A)*100 = 73.57%.

(0.137A of input terminal is the value including charging current and system consumption current.)

If the above is not abnormal,

I want to know how much power bank battery capacity is needed when I want to charge the cradle with 300mA battery once using the power bank.

Assuming that the output efficiency of the 5V stage through the power bank boost ic is 90%.

[case 1]

Cradle battery power: 3.7V * 0.3A = 1.11W.

The efficiency to charge 1.11W is 73.57%, so 1.508W is required on the cradle side.

In order to output 1.508W from the power bank, 90% efficiency is generated,

so 1.675W must be supplied from the power bank battery and

1.675W / 3.7V = 452.7mA battery capacity is required.

In other words, 452.7mA of battery capacity is required in Power Bank to charge Cradle's 300mA battery once.

(Overall efficiency: 300mA/452.7mA=66.2%)

[case 2]

Cradle battery power: 3.7V * 0.3A = 1.11W.

However, 4.2V is output from the charger IC when the battery is output, so 4.2V *0.3A = 1.26W is required.

73.57% efficiency is generated to charge 1.26W, so 1.712W is required on the cradle side.

In order to output 1.7121W from the power bank, 90% efficiency is generated,

so 1.902W must be supplied from the power bank battery, and 1.902W / 3.7V = 514mA battery capacity is required.

In other words, 514mA of battery capacity is required in Power Bank to charge Cradle's 300mA battery once.

(Total efficiency: 300mA/514Ma=58.3%)

Which of the above two efficiency calculation methods(case 1, 2) is calculated correctly?

If the calculation method is not correct, please tell me how to calculate the required power bank capacity to charge a 300mA battery once.

Regards,

MJ

  • 0
    TI__Mastermind 23410 points

    Hi MJ,

    Can you clarify, are you asking about the BQ24073? Your power efficiency there is correct but are you using any other calculations later? Is the BQ24073 in the cradle or the power bank?

  • 0 in reply to Anthony P Pham
    Genius 5240 points

    Hi, Anthony

    My question is correct about BQ24073.

    The calculation formula I gave you is the overall calculation.

    BQ24073 is inside the credle.

    The question is about the charging efficiency when charging the battery inside the credle using an external power bank.

    Thanks.

    Regards,

    MJ

  • 0 in reply to MJ Jeon
    TI__Mastermind 23410 points

    Hi MJ,

    Our typical calculation for the requirements of charging are not dependent on efficiency. We generally cite current draw requirements as the deciding feature. Do you already have a schematic that is looking at this design?

    As the BQ24073 is a power path device, it is recommended that the input current be sufficient enough to provide the power draw of a combined output current and battery charge current (Input current >= Battery Charge Current + Output Load Current). 

    We do not look at batteries being used to charge batteries and so we cannot guarantee any of these calculations. If you are charging the battery with a 5V input to the BQ24073's input pin with a charge rate 120mA but total system load is 137mA, I can only say that the power requirement for this condition is the 5V * 137 mA = 0.685 W

  • 0 in reply to Anthony P Pham
    Genius 5240 points

    Hi, Anthony

    I have an additional question.

    When the charging speed is 120mA, how much input power is needed to actually charge the 300mA battery?

    If possible, please respond with a calculation formula.

    Thanks.

    Regards,

    MJ

  • 0 in reply to MJ Jeon
    TI__Mastermind 23410 points

    Hi MJ,

    To use the device without the effects of DPPM or VINDPM in effect, the input needs to be able to sufficiently provide current for both the battery charging as well as the system load that is at the OUT pin. 

    With typical applications seeing an input voltage of 5V, the power requirement would be 5V * 120mA = 0.6W.