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BQ77915: What happens when CBI is driven low, but no balancing cell terminals are connected ?

Jomon
Intellectual 660 points
Part Number: BQ77915

CBI pin low enables cell balancing

But, what happens when cell balancing is enabled, but we do not connect the balancing terminals of the battery pack ? Will it charge, discharge and work normally, except the balancing part ?

Or should balancing be disabled whenever this happens for the IC to work ?

Thanks a lot in advance

  • 0
    TI__Mastermind 42060 points

    Hello Jomon,

    I do not understand exactly what you are trying to accomplish here. The balancing terminals of the pack are the same inputs as the cell inputs. Can you clarify exactly what you mean? Or write it out? 

    If CBI is pulled to Vss, then cell balancing would be enabled. For cell balancing to occur a charging current would also have to be detected an the cell would need to be above a set threshold value.

    If balancing is not desired, CBI should be left floating.

    Best Regards,

    Luis Hernandez Salomon

  • 0 in reply to Luis H. S.
    Intellectual 660 points

    We are designing a system where the user will plug in the battery pack of 3S. Battery pack comes with power terminals (2 pins) and a balancing plug, which will have the connections to the cells of the battery pack.

    So, since the users are supposed to plug in both the power terminals and the balancing terminals, what if they do not plug in the balancing terminals ? So BQ77915 will have just 2 terminals connected.

    We have a 3S pack. Let the battery pack cells be C1, C2, C3 with C3 being the most positive cell. When balancing is connected, C1-, C1+, C2+, C3+ will be connected to the BQ77915. When power terminals are connected, C1- and C3+ only will be connected to the BQ77915.

    In the above image, the yellow connector is just for the power (- + of the pack). The white connector JST-XH connects individual cells.

    My question would be, what happens if the user connects only the yellow terminal, without connecting the white terminals, with balancing turned on in BQ77915 (CBI pulled low) Will BQ77915 work in this case ? Or should the CBI be left floating if individual cells are not connected, for the IC to work ?

    In short, we designed the system to have balancing, and pulled the CBI pin to VSS, but the user didn't plug in the balancing terminals. If user does so, will the system work ?

  • 0 in reply to Jomon
    TI__Mastermind 42060 points

    Hello Jomon,

    The "cell balancing inputs" are the same as the "cell inputs". The part may not properly function if these pins are not populated, regardless of balancing. This IC is typically placed inside the battery pack, so the cells would always be attached to the IC.

    Connecting BAT+/BAT- will not really work for this device, because this part has OV/UV protections that would likely trigger if the cells are not attached.

    Best Regards,

    Luis Hernandez Salomon