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BQ76905: Cell balancing issue

Kailun Liu
Expert 3063 points
Part Number: BQ76905

Tool/software:

Hi Team, 

Please help to check below: 

1. The technical manual mentions that the minimum input filtering resistor is 20Ω, but the BQ76905_EVM design uses a 10Ω input filtering resistor, which is less than the minimum resistance of 20Ω mentioned in the technical manual. Which one is correct?

2. Currently, the adopted scheme uses an internal balancing method. The technical manual mentions a maximum balancing current Imax = 50mA; however, by using the minimum balancing resistance R(CB) = 53Ω and Rn = Rn-1 = 10Ω mentioned in the data manual, I calculated the maximum balancing current I = 4.2 / (2 * 10 + 53) = 57.5mA, which does not match the Imax = 50mA stated in the technical manual. Which one is correct?

3. When charging a 4.2V lithium battery with a current of 4A, the cut-off current for full charge is 100mA. When a certain battery exceeds 4.15V (while the other batteries have not yet reached 4.15V), I send a balancing command to discharge that battery. However, I found through testing that the voltage of the battery in a balancing state continues to rise, showing no balancing effect at all.

Preliminary analysis: The current charging current of 4A is much greater than the maximum balancing current of this battery (57.5mA), resulting in charging being greater than discharging.

Therefore, even if balancing is turned on, the battery cannot achieve a balancing effect. The specifications state that the full charge cut-off current is 100mA, which is greater than the maximum balancing current of 57.5mA, making this unable to meet the requirements for charging balancing.

Could you please provide any solutions and suggestions?

  • 0
    TI__Genius 17795 points

    Hello Kailun,

    Thank you for your patience.

    Kailun Liu said:
    Which one is correct?

    Thank you for catching that. 10 Ohms for the input filtering resistors should be correct.

    Kailun Liu said:
    which does not match the Imax = 50mA stated in the technical manual. Which one is correct?

    It is correct that the maximum balancing current for each cell is 50mA as stated in Section 6.1 Absolute Maximum Ratings of the datasheet. You may have a battery cell that has a voltage up to 4.2V, however, in order to make sure you do not go over the maximum balancing current, you may need to use larger input filtering resistors.

    Kailun Liu said:
    Could you please provide any solutions and suggestions?

    If the balancing current is too small, we recommend using external cell-balancing which will allow for a much larger balancing current. Please refer to our Cell Balancing with BQ7690x application note for how to best do so.

    Best Regards,
    Alexis