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2-cell Battery Reverse Protection

MR Flashlight
Intellectual 420 points
Other Parts Discussed in Thread: BQ28Z610, BQ34Z100-G1

I am hoping to use the bq28z610 as a protection device and fuel gauge for a product with two user-replaceable batteries. How do I protect the device and my circuit from both batteries being inserted in reverse? Since the minimum voltage on the VC1 and VC2 pins is -0.3 volts, I have to imagine that it would cause damage. However, the appnotes I have been able to find do not account for a system with balancing taps. I presumably need to gate the center tap as well as the positive tap.

  • 0
    TI__Guru 73830 points

    The device was not designed to be used in host side applications where the cells are replaceable. If you do use the device in this manner, then you will need to use some type of mechanical protection features to insure that the cells are not inserted incorrectly. In addition, the gauge will need to learn the new cells and this could take several charge and discharge cycles, because the amount that Qmax and the Ra tables can change with each update is limited by the firmware. The bq34z100-G1 can be used as a host side gauge and Qmax and the Ra table can learn more quickly.

  • 0 in reply to ThomasCosby
    Intellectual 420 points
    The bq34z100-G1 leaves me without any battery protection, though. And the only protection devices that I seem to be able to find for a 2-cell configuration do nothing but overvoltage protection, while I'm looking for a full slew of protection features.

    For my first prototype I'm planning on using two reverse-protection mosfets, a PMOS on the top side, and an NMOS on the bottom, to prevent current from flowing in either loop. I am not able to include mechanical protection features, since I am using loose cylindrical batteries in a battery holder.