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BQ28Z610: BQ28Z610 external balance resister select

Star Xu
Genius 9645 points
Part Number: BQ28Z610
Other Parts Discussed in Thread: BQ40Z50

Hi 

I have a query need your help.

If external balancing is used, the voltage sampling resistor of the first cell is 100 ohms and the voltage sampling resistor of the second cell is 5 ohms.

Do I need to replace it with a larger value?  Like the BQ40Z50, if external balance is used, the sampling resistor should be changed from 100 ohms to 1kohms?

Please recommend the suitable resister value for external balance.

Waiting for your reply.

Thanks

Star

  • 0
    TI__Mastermind 23576 points

    Hi Star,

    I am a little bit confused on what you mean by voltage sampling resistor.  Are you referring to the input series resistor of the VCx pins? If so, these should be identical for each pin to avoid bigger voltage drops in other pins when the ADC samples a voltage. But, I think for this gauge, it has 5 ohm resistor in VC2 to prevent a big IR drop on this pin. VC2 is not only the cell voltage pin, but also acts as BAT pin in this hardware design. 

    In regards to the external balancing resistor, the designer should make sure that the maximum allowed current through the VCx pins is not exceeded. I believe it is in the 10mA range. 

    Please reference to this thread for more insight on cell balancing. 

    Mostly all the current would be dissipated in the external cell balancing resistor. I suggest customer see this Section 2.2 External Cell-Balancing Circuit Design Using N-Channel FETs in this app note for external cell balancing circuitry and design. Different device, but same principal. 

    Regards,
    Jose Couso

  • 0 in reply to Jose Couso
    Prodigy 61 points

    Thank you for your reply! It's a series of VCx pins,The two references should be the same, should be 100 euros, but VC2 is 5 euros，The external equalization reference circuit should be changed from 100 ohm to 1K，5 euros or how much should I pay? Is there a recommended value?

  • 0 in reply to chaochao zhang
    TI__Mastermind 23576 points

    Hello,

    Adding 100 Ohm in VC2 adds bigger voltage drop. BAT pin will read a lower voltage. If you are ok with this voltage drop, it should be ok.

    Regards,
    Jose Couso

  • 0 in reply to Jose Couso
    Prodigy 61 points

    With external equalization, VC2 uses resistance increased from 5 Ohm to 100 Ohm, and the VC2 display voltage on the software interface will be low, which is not desirable，At present, to ensure that the voltage collection display is normal, external balance must also be achieved, I do not know how to adjust the VC2 collection resistance 5 ohm to ensure that these two functions are normal?

  • 0 in reply to chaochao zhang
    TI__Mastermind 23576 points

    Hello,

    You can use external cell balancing configuration and keep 5 Ohm at the VC2 pin. Let me explain. 

    As seen below, most of the cell balancing current will be dissipated at the external Rbal resistor. From the design point of view, you will only need to ensure I_5ohm is less than the max allowed current. 

    Typical I_max is 20mA, assuming the cell voltage is 4 and the internal Rds_on of the FET is 200-Ohm. I think having I_5ohm equal to 15mA or 10mA is safe enough. The majority dissipated power will occur at Rbal.



    Please see section 2.2 External Cell-Balancing Circuit Design Using N-Channel FETs from the Cell Balancing With BQ769x2 Battery Monitors

    Regards,
    Jose Couso