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BQ24133: Does this device detect fully discharged Battery and isolate from Load?

Manjula Ellepola
Expert 1335 points
Part Number: BQ24133

Hi Community,

We are using the above device to charge two cell LIPO batteries. Everything works good.

I want to know the behavior of the IC when there is no external charging voltage present and the

battery is being discharged to load via via the BATFET controlled via the /BATDRV signal.

What happens when the LIPO is fully discharged to say 6.5V (8.2V fully charged) does the BATDRV

turn off and isolate the battery from the load so that the LIPO is protected?

This is very important to us as we don't want the battery to continue to discharge into the load

when it is fully discharged as it can damage the LIPO. I was reading about UVLO but this voltage

seems to be for the single cell configuration.

Your comments regarding the above is valued.

Regards

Manjula

  • 0
    TI__Guru**** 166626 points
    Hi Manjula,

    The charger is not intended to be used in place of an internal battery pack protector circuit that prevents over-discharge. Most packs have such a protector. I will need to confirm with my designers whether UVLO of AVCC, connected through a diode to a resistor divider from BAT, disables /BATDRV when tripped.
  • 0 in reply to Jeff F
    TI__Guru**** 166626 points
    Manjula,

    I checked on the bench and unfortunately, the /BATDRV signal does not go high when in battery only mode and battery voltage is less than UVLO. Therefore, to add protection, you would need to add a second PFET, controlled by a comparator powered off the battery, in series with the other PFET that only turns off when the comparator senses the voltage is too low.
  • 0 in reply to Jeff F
    Expert 1335 points

    Hi Jeff

    I appreciate your efforts very much to look into this. I did similar bench test to yours and can concur on your findings. Thank you for the comparator and pfet idea.

    What do you think about the idea of having an nfet in the gate drive circuit of BATFET to control it? The nfet could be turned off when the voltage gets too low thereby isolating the BATFET from the IC. This way we can eliminate the voltage drop of an additional series fet in the load path.

    Looking forward to your comments

    Regards

    Manjula

  • 0 in reply to Manjula Ellepola
    TI__Guru**** 166626 points
    Manjula,

    FETs have body diodes. To completely isolate the battery from system but also allow for supplement mode through one diode, I think you will ultimately need two PFETs with body diodes facing each other. In addition, NFET's need a voltage higher than their source to turn on. /BATDRV logic is for PFET not NFET.