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BQ25306: Reverse Battery Connection

digilog
Prodigy 115 points
Part Number: BQ25306

Hi I am designing a portable battery powered device using 2S 18650 batteries and have chosen the BQ25306 as the charger. I have looked through the forum and read a few interesting problems of which I have included in my design but I have a major problem when it comes to the batteries being installed incorrectly.

If the batteries are installed reversed what will the charger do?

I have read your article SLVA139 showing how to add reverse battery protection. Will this work correctly for the charger even when the batteries are installed correctly but are discharged to 0 volts?

I am also assuming that for this to work correctly the FET source must be on the charger side and not the battery side as there would be a current path through the FET diode when the batteries were reversed.

I have also included a low battery shut off circuit but this will only disconnect  the load and not the battery charger circuit.

  • 0
    TI__Mastermind 24075 points

    Hi,

      The chargers cannot provide reverse polarity protection but adding a protector or following the app note above will help protect against reverse polarity. Li-ion batteries discharged to 0V is not really a use case as it is extremely dangerous to have them discharged this low.

  • +1 in reply to Kedar Manishankar
    Prodigy 115 points

    Hi Kedar,

    Yes I do realise that 0 volt batteries are not good but since the batteries are user replaceable there is the possibility that one of the batteries could be reversed leading to a zero volts across the battery charger.

    What I am more interested in is knowing the behaviour of the battery charger under various battery configurations and making sure that the charger is not destroyed.

    What happens when the charger see's -7.4V?

    What happens when the charger see's 0V?

  • 0 in reply to digilog
    TI__Mastermind 24075 points

    Hi,

       If you apply a reverse polarity battery with negative voltage on the terminal, there is a low impedance conduction path through the body diode of the LSFET which cannot be mitigated by turning off the LSFET. You would also violate the abs max specification.