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BQ77915: Stacked Pack with System Ground at Center of Stack

E H
Prodigy 10 points
Part Number: BQ77915

Tool/software:

Hi, E2E-

I'm developing a replacement for an old 20S Ni-Cd pack found in a piece of test equipment, and I am considering using a 6S Li-Ion stack with a pair of BQ77915s. The "catch" is that this pack needs to be ground referenced at the center of the stack such that split positive/negative voltage rails are available on the output. Similarly, it will charge using a set of +13 / 0 / -13 V supplies. Any issues using the BQ77915 in such an application? Please let me know if this question isn't clear, and I can attach a rough schematic of what I have in mind.

Thanks,

-Erik

  • 0
    TI__Mastermind 42060 points

    Hello Erik,

    A schematic would be great! Not entirely sure how your idea is being set-up.

    Best Regards,

    Luis Hernandez Salomon

  • 0 in reply to Luis H. S.
    Prodigy 10 points

    Hi, Luis-

    Thanks for the prompt response. Ideally I would like to be able to do something like this modified figure 9-14 from the datasheet, but I'm assuming connecting to system ground at the indicated point in the stack defeats the upper cells' protection by providing a non-FET-controlled return current path.

    bq77915mod.pdf

    Instead, I'm thinking that two independent stacks of three will need to be used, connecting the bottom of the top stack and the top of the bottom stack at system ground as shown in the schematic below. Besides the fact that a fault condition in one stack will not trigger a fault condition in the other stack, and also that the cells in the top stack could become unbalanced from those in the bottom stack (but not within a given stack), are the any other "gotchas" to consider with this topology?

    HP419C_BattPack.pdf

    Thanks again!

  • 0 in reply to E H
    TI__Mastermind 42060 points

    Hello Erik,

    I am concerned about the protectors cutting of in the middle of the stack. Not entirely sure of the effects this could have, but it may not be good. Vss of the top device with respect to the bottom device would be essentially floating if the FETs are OFF.

    I will give it some more thought about it.

    Best Regards,

    Luis Hernandez Salomon