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BQ24800: Possibility to disconnect batteries from system: to avoid emptying them or battery boot up

William Courbat
Prodigy 20 points
Part Number: BQ24800


Tool/software:

Hello,

I am wondering if there is a way to disconnect the batteries from the system. With the design recommended in the datasheet, the body diode of both upper transistors will connect the batteries to the system voltage, no matter what is the status of the transistor.

In my case, two batteries of 3.3V are plugged in series, total output of 6.6V.

This means that even after removing Battery Only Boost Mode to 12V (detecting Vbat Depletion Threshold in 0x3B 14:15), the batteries will stay connected to the system and will empty themselves slowly in the system and pass the minimum voltage to maintain at their terminals that is recommended by the manufacturer to prevent damages (2V in my case, in series : 4V). 

This also mean that as soon as the batteries are inserted, the system and BQ24800 will boot up, making it impossible to shut it down while batteries are not completely empty.

This seems like a common issue and that possibly solutions were already found in the past. Do you know of any? 

Thank you very much!

  • 0
    TI__Mastermind 35856 points

    Hello William,

    I'm looking into this question and will provide an update after I gather more information.

    Best Regards,

    Christian.

  • 0 in reply to Christian Moyer
    TI__Mastermind 35856 points

    Hello William,

    Do you have Low Power Mode enable?

    This will help reduce the quiescent current in battery only mode.

    Best Regards,

    Christian

  • 0 in reply to Christian Moyer
    Prodigy 20 points

    Hello Christian. 
    I can't have Lower Power Mode enabled during backup as it is not compatible with Battery Only Boost Mode. When Battery Boost Mode disables as Battery Threshold is reached, I could. But the transistors still connect the batteries to the system bus, allowing them to discharge in the whole board. 

    I just wonder why there is a Battery Threshold to detect when battery voltage gets too low, but that is used only to stop boosting and do not prevent damages to the batteries due to under voltage that would happen later. Would there be a way that when Battery Threshold is reached and Boost Mode is disabled, this also disconnects the batteries from the system bus and therefore VCC of the BQ24800? Perhaps by adding a second transistor to BATDRV or HIDRV?

    Thanks!

  • 0 in reply to William Courbat
    TI__Mastermind 35856 points

    Hello William,

    We don't have a way to completely remove the quiescent battery current. You will neeed to implement an external circuit such as a battery monitor or guage to stop battery discharge when voltage gets too low.

    Best Regards,

    Christian.

  • +1 in reply to Christian Moyer
    Prodigy 20 points

    Hello Christian,

    Ok we will implement our own circuit to disconnect the battery from the system.

    Thanks.