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BQ76952: External cell balancing resistor and capacitor design

Francis Jiang
Expert 3190 points
Part Number: BQ76952

Hi Team,

Customers worry about the big range of internal cell balancing resistor, and they want to minimize cell balancing current.

So they choose the biggest external cell input resistor 100ohm, and based on calculation, the capacitor should be designed as 50nF, but according to DS, the min value is 100nF.

Could you please provide your professional advice whether 100ohm+50nF is a right design? By the way, why RCB is tested under the condition of Vvcn - Vvcn-1 = 1.5V, not >3V?

BRs,

Francis

  • +1
    TI__Mastermind 27321 points

    Hello Francis,

    I am not sure how they calculated the Capacitor value. I do not think the capacitor value would have an effect on the balancing current. 
     -> Using a smaller cap value can reduce the filtering, but if they do not have high transients in their system, they can probably live with the 50nF cap.
     -> Typically we recommend 0.22-uF when using 100-Ohm

    The Vvc(n-1) = 1.5V represents the voltage across the pins of the IC (the voltage across the internal cell balancing resistance). For this test, a 20 Ohm external cell input resistance was used.

    Regards,
    Jose Couso

  • 0 in reply to Jose Couso
    TI__Expert 3190 points

    Hi Jose,

    Thanks for your reply.

    Sorry for delay, why Rcb so big? Is it because it's an analog switch? I remember this device can only choose odd cells or even cells to enable cell balancing.

    BRs,

    Francis

  • +1 in reply to Francis Jiang
    TI__Mastermind 27321 points

    Hello Francis,

    Rcb technically acts like a switch, and it is an expensive solution to implement a lower Rcb. And, the BQ76952 cannot do adjacent cells when operating in autonomous cell balancing, this is to avoid excessive power dissipation within the device. 

    Regards,
    Jose Couso 

  • 0 in reply to Jose Couso
    TI__Expert 3190 points

    Hi Jose,

    Thanks for your great help. No problem here.