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BQ27Z561EVM-011: charging and fuel-gauging two different LiPo batteries in parallel

Carl Olen
Expert 1160 points
Part Number: BQ27Z561EVM-011
Other Parts Discussed in Thread: BQ27Z561

I have a small battery-operated portable device for which I want to extend its use-time with an additional external battery.  The two batteries are 3.7V LiPo of different size: The small battery is about 70mAh and the larger external one is about 280mAh (but could go as high as 1800mAh).

However, when unplugging or swapping the larger external battery, I want the device to stay "on" with the small internal battery still connected.  I would then reconnect a freshly-charged larger external battery, or charge both in parallel when overall charge is low.

What would be my challenges for fuel-gauging and charging this 1S2P configuration with the BQ27Z561EVM-011?

Any recommendations for power path management or other issues?

Please let me know.

Thanks,

Carl Olen

  • 0
    TI__Genius 10370 points

    Carl,

    The bq27z561 gauge is a pack side gauge. This means it expected to gauge the same battery the entire time. This act of swapping the large cell to a different size is not something this gauge can support. The possibly drastic change is overall capacity would destroy our gauging algorithm. Even the act of swapping the same cell type and size would be an issue. The impedance profile could be different or one is aged more than the other. 

    Over all i dont think this is the right solution. i would recommend looking into a "System side" gauge like the bq27531 for example. These normally involve system interaction but are more suited for battery swapping. 

    Thanks,

    Eric Vos

  • 0 in reply to Vos
    Expert 1160 points

    Thanks Eric.  I will be using just one battery.  But I need more information on the difference between "Pack-side" and "System side" gauge configurations.

    Is there a white paper or other reference/training material that explains the differences more fully?

    Also is there an evaluation kit for the bq27531?  Please let me know.

    Thanks,

    Carl Olen