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BQ25750: Battery Test Station

Achim Wack
Prodigy 230 points
Part Number: BQ25750

Tool/software:

Hi,

I´d like to build a battery test station (charger / electronic load) with the BQ25750. The electronic load I consider to connect to the SYSTEM connection of the BQ25750, but I don´t find any register to control the BATDRV pin... May plan was to disable charging and switch on the BATFET to connect the battery to the electronic load for discharging. Is this possible or I´m completely wrong?

Best regards,

Achim

  • 0
    TI__Mastermind 30915 points

    Hello Achim,

    The BQ25750 will automatically connect VBAT to VSYS through the BATFET when VAC is absent or out of range. Keep in mind though that the BQ25750 has a built-in overcurrent for the BATFET. If the current through the BATFET exceeds 8A, the BQ25750 will turn off the BATFET to prevent damage to the FET.

    If you need to discharge more than 8A, you can use an MCU to set auto reverse mode. Auto reverse mode will automatically provide power to the system through the converter when VAC is removed or out of range.

    Let me know if you have any questions about this.

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    Prodigy 230 points

    Hello Ethan,

    8A is more than enough for the actual project...

    The problem is, that I want automatically charge and discharge without connecting / de-connecting VAC. I think than it´s easier / better to connect the MOSGET of the discharging path directly to the battery and clean the CHG_EN (via I2C) bit if I want to dis-charge. Maybe then I can remove also the BAT-FET..

    Schematic BCM - shortened.pdf

    Best regards, Achim

  • 0 in reply to Achim Wack
    TI__Mastermind 30915 points

    Hello Achim,

    You could use HIZ mode to turn-off the ACFETs and turn-on the BATFETs. This will connect the battery to the system load without disconnecting VAC. HIZ mode is covered in section 8.3.3.5 Device HIZ Mode in the BQ25750 datasheet.

    Let me know if this works for you.

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    TI__Mastermind 30915 points

    Hello Achim,

    Also, I have a few recommendations for your schematic:

    • Make sure the BTST capacitors are 100nF
    • For the IAC and IBAT sense resistors, make sure to copy the filter network from the schematic checklist or the EVM.
    • Make sure to have HF filter caps on the input and output of the converter. Make sure these high frequency caps are close to the switching FETs.
    • Make sure the SYS pin has a 100nF cap close to it.
    • We list the max input current sense resistor as 10mΩ by the way. I would change the 15mΩ IAC sense resistor to a 10mΩ sense resistor.

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    Prodigy 230 points

    Hi Ethan,

    tooks some days till I´ve had time to test. Yes I can switch the BATFET on with HIZ bit. But sometimes it switches back to normal mode. Don´t know why...

    Best regards,

    Achim

  • 0 in reply to Achim Wack
    TI__Mastermind 30915 points

    Hello Achim,

    Make sure that the watchdog time is disabled. The watchdog timer will reset the EN_HIZ bit and adapter plug-in will also disable EN_HIZ.

    Are there any flags being set when the converter switches to normal mode?

    Best Regards,
    Ethan Galloway