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BQ76940EVM: BQ76940EVM problems with the discharge

Albert Galofre
Intellectual 600 points
Part Number: BQ76940EVM
Other Parts Discussed in Thread: BQSTUDIO, BQ78350, BQ78350-R1, BQ40Z50

Hi there,

I'm starting to work with the BQ76940EVM with his built in BQ78350DBT and BQ7694000DBT.

I'm learning the knowhow regarding the gauge. My application is a BMS for a medical equipment. It's a 16S1P with 100A peak of current and link via SBS 1.1

I have the qbStudio working: I can communicate with the PCB and read the cell voltages values. Calibrate the V I T measurements without problems.

The Vbat is 47V and the Vpack is 39V measured with a multimeter.

Why the Vpack is 39V without load? Even worst, If I load the Vpack with 200ohms the it fall to 0V!!!!

Could be, that I have the registers set wrongly? I mean the PCB configuration with the bqStudio.

Maybe I have to activate the discharge mosfet....

Thanks

  • 0
    TI__Guru* 81035 points

    Hi Albert,

    There is a FET Enable command (look for the FET_EN button on the right side of the bqStudio window in the list of commands). Can you try to enable to FETs to see if this resolves the issue? Also, on your register screen, you should see bits labeled CHG and DSG - these show whether the FETs are active. A red color indicates a '1' and green indicates a '0'.

    If the FETs are not active even after executing this command, check for any fault conditions that might be present on the Register screen. Many of the faults respond by opening one or both FETs, so it is important to understand these and adjust the fault detection settings if needed.

    Best regards,

    Matt

  • 0 in reply to Matt Sunna
    Intellectual 600 points
    Hi Matt,
    You are right. Sorry.... I'm a beginner.

    I have looked the "bq78350 technical reference.pdf" but a bit ..... hard.
    Could you give me some link to start understand the main performance of the bq78350. I know that in the future I must read technical reference device even in detail; but at this moment It will be useful some pdf a bit.... more easy.

    TI provide a really good devices, starter kits and PC eclipse IDEs; but unfortunately the literature really technical.

    In any case thanks a lot.
  • 0 in reply to Albert Galofre
    TI__Guru* 81035 points
    Hi Albert,

    I understand - battery gauging devices do have many features, but they tend to be difficult to learn quickly because of the complexity. There is an app note called Using the BQ78350-R1 that might be helpful (www.ti.com/.../slua924.pdf). I revised this a few months ago to cover many of the important steps that are easy to overlook. I also recommend the EVM User Guide for important examples on the setup.

    I you have some time, this video might help to give you an idea of the whole development flow for a battery gauge. It is for a different device (the BQ40Z50), but it will help you get a feel for BQStudio. You can ignore the portion of the video on 'Learning Cycle' since this does not apply to the BQ78350. training.ti.com/bq40z50-setup-and-going-production

    I hope this helps. I may work on a video in the future for the BQ78350. You should likely be able to use the default settings for most features, but the Using the BQ78350-R1 app note will guide you through the critical parameters.

    Best regards,
    Matt