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BQ77915: Schematic Design Review

eso18
Prodigy 150 points
Part Number: BQ77915
Other Parts Discussed in Thread: BQ25792

Hi everyone,

For the 4S2P Li-Ion battery, do you see any issues with the BQ77915 schematic design?

I will be using the system with the BQ25792 battery charger.

Thanks for your supports

  • +1
    TI__Mastermind 36780 points

    Hello Eso,

    Your schematic looks good! The only comment I have is that with LEDs on the cell inputs, there will always be current flowing through the cells, so this may contribute to the cells to discharge faster.

    Keep in mind that if the CHG pin can be exposed to voltages below -30-V, it may be good to add the components described in Section 10.1.1.2 Protecting CHG and LD of the datasheet. 

    Best Regards,

    Luis Hernandez Salomon

  • 0 in reply to Luis H. S.
    Prodigy 150 points

    Hi Luis, 

    Thank you for your answer. 

    I have a question about cell balancing. I use internal balancing. Is cell balancing active during charging?  If the cell balances during charging, do we need to reduce the charging current?

    For 50 mA cell balance current,  4,1V / (2*Rin) + Rbal ,  where Rbal is typ 12 ohm. Rin = 33 ohm.  

    Assuming the charging current is 2.5 A, high current will have passed through the resistors.

    Does this cause problems?

     

  • +1 in reply to eso18
    TI__Mastermind 36780 points

    Hello Eso,

    No, cell balancing is solely passive cell balancing. There is no need to reduce the charging current during balancing. The high charge current would flow solely through the cells, as this is the path of low-impedance for the charger. There is no risk of it flowing through the resistors.

    I would recommend you watch our video on cell balancing using the BQ77915 in order to understand how it works:

    https://training.ti.com/how-cell-balancing-feature-works-bq77915

    Hope this helped Slight smile!

    Best Regards,

    Luis Hernandez Salomon

  • 0 in reply to Luis H. S.
    Prodigy 150 points

    Hello Luis,

    Thank you for your supports