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BQ2970: Is it required reverse battery protection?

Part Number: BQ2970
Other Parts Discussed in Thread: BQ25100,

Hi everyone,

I have included the BQ29707 battery protection into my design and I am considering if I have to protect the IC against reverse battery connection. For that reason I connected a P-Channel MOSFET on BAT pin as shown below. Is this required or should I omit the MOSFET?

  • Hi Nikos,

    It seems like that could work. You may want to consider placing a high-value (5.1M) resistor between gate and source to discharge the gate, similar to what you did with the charge and discharge FETs. It'll dissipate a little more power during normal operation though.

  • Thank you for your reply Albert!!

    My initial question was if the MOSFET is required. I mean, do I have to include reverse battery protection OR the BQ29707 provides reverse battery protection itself? 

    I will combine the BQ29707 battery protection with the BQ25100 battery charger and I have connected both ICs with the Vbat power signal. Is the battery going to be charged normally? or somehow the MOSFET will affect/disrupt the charging process?

  • Hi Nikos,

    The BQ2970 does not provide reverse battery protection.  BAT can not go negative., similarly V- can not go above BAT.  Your extra FET seems it should work for a reversed cell.

    When used with the charger the cell is protected by the BQ2970 from over charge or over discharge.  Normally the BQ2970 will be "on" in normal mode and the charger can provide current and the load use current from the battery as needed.  If the charger does not behave properly and overcharges the cell the BQ2970 will protect.  Similarly if the load does not stop when the cell is discharged to its normal limit the BQ2970 will protect to disable further discharge.  Connecting the charger will cause a recovery from the overdischarge. 

    Your diagrams seem generally correct, the 5M at the charge FET should generally be from gate to source of the FET

  • Thank you for your assistance WM5295,

    Your diagrams seem generally correct,

    Thank you to verify that!! My concern was if the MOSFET will effect the wireless charging as it is connected

    the 5M at the charge FET should generally be from gate to source of the FET

    You mean like this?

  • Hi Nikos,

    The reverse charge FET should always be on when the cell is connected properly.  If the cell degrades to be less than VGSTH of the FET it will become a diode and you won't be able to charge.

    Yes, the new placement looks more common.