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BQ34Z100-G1: Vbat divider Calibration

Jeffrey Wright
Expert 2875 points
Part Number: BQ34Z100-G1
Other Parts Discussed in Thread: BQSTUDIO

Hello;

I'm having problems with the divider gain calibration - On our board, the value for Rseries computes to 1.12 Mohm for a max Vstack of 62 volts. However, bqStudio says its unable to calibrate because the calibration value is out of range.  We have 16 cells and calibrate at ~53 volts...but this fails.  But if I lower Rseries to 1.0Mhom, it appears to calibrate ok. But this value doesn't match the equation, and results in a max stack voltage of 55.4 Volts.

I even tried increasing the 16.5K to 17.6K, which also is correct by the math, and should result in a max Vstack of ~62 volts....but it too won't calibrate.

I'm concerned that, given the bug in the gauge's offset calibration, there may be some negative effect on the Vbat calibration as well.

What am I missing?

  • 0
    TI__Guru** 105590 points

    Hello Jeffrey,

    When you did the calibration, you set the voltage divider ratio according to the TRM and then tried to do a voltage calibration in bqStudio and that's where the error occurred?

    What value for the Voltage Divider are you inputting?

    Sincerely,

    Wyatt Keller

  • 0 in reply to Wyatt Keller
    Expert 2875 points

    Hello Wyatt;

    Yes, Studio says that the cal value exceeds the limits, despite the math being correct on the Rseries calc.

    We're targeting ~62 Vin max, and so the math says 1.12 Meg and 16.5K are correct. Nevertheless, Studio says it's out of range.

    We were calibrating when the battery stack volts was ~53.2 V.

    But as I said, if I reduce the 1.12 Meg to 1.0 Meg, it will calibrate, but that yields a max stack voltage of ~55.4V, which is too low for the battery.

  • 0 in reply to Jeffrey Wright
    TI__Guru** 105590 points

    Hello Jeffrey,

    Can you share your .gg file? I'd like to take a look at the other settings, if the voltage divider parameter in DF doesn't match the division of the real divider the calibration may fail due to the bounds in bqStudio.

    Sincerely,

    Wyatt Keller

  • 0 in reply to Wyatt Keller
    Expert 2875 points

    You'll note that I scale the cell count from 16 to 8 to get it to calibrate - as is discussed in slua760.  But I shouldn't have to do so. 

    hammerhead.gg.csv

  • 0 in reply to Jeffrey Wright
    TI__Guru** 105590 points

    Hello Jeffrey,

    If you are following SLUA760 you will need to apply the division to the voltage as well, it looks like the voltage divider is about half, and the cell count is half as well, so you will need to input your applied voltage divided by 2 so the sale factor is not too large during calibration.

    Sincerely,

    Wyatt Keller

  • 0 in reply to Wyatt Keller
    Expert 2875 points

    Yes, that's what I'm doing...I can only get the Vdiv calibration to take if I scale the stack voltage a la slua760, and so I set the cell count to 8 and enter my calibration voltage as ~53200/2 mV...this works....

    But my point is, I shouldn't have to scale in the first place...the R divider should place the max Stack volts below the maximum of 65535, and so I should be able to calibrate using 16 cell count.  As I said, the Rseries and 16.5K place the max Stack volts at ~62 volts, well below the 65535 max... 

    Can you dig and find out if this is another undocumented errata, just like the board offset calibration bug?

    Thanks

  • 0 in reply to Jeffrey Wright
    TI__Guru** 105590 points

    Hello Jeffrey,

    The Voltage Divider max is set to 65535 so it shouldn't be an issue to set it to this value. Can you send a screenshot of what you're modifying and the error you see?

    I may be able to resolve the board offset, I sent you a friend request on the forum.

    Sincerely,

    Wyatt Keller

  • 0 in reply to Wyatt Keller
    Prodigy 85 points

    Hello Wyatt,

    I am having the same issue for a pack with max voltage of 54.6V (13S at 4.2V design voltage).  Divider is 1M + 83.5k + 16.5K.

    bqstudio only allows entry of a max voltage of about 51V into the calibration tool. What is the procedure to resolve this?  Do you need a .gg file?  I am already applying Design Scale = 2 since the pack capacity is greater than 32Wh. 

    Please provide public resolution to this issue-hopefully TI will produce an errata sheet that details these issues documented on forums?

    Thank you,

    Nishant

  • 0 in reply to Nishant Pol24
    TI__Guru** 105590 points

    Hello Nishant,

    Were still looking into this, in the mean time I would recommend using the divider method to calibrate for the higher voltages.

    Sincerely,

    Wyatt Keller

  • 0 in reply to Wyatt Keller
    Prodigy 85 points

    Hi Wyatt,

    Thanks for the quick reply.  The pack we have selected is 13S4P so the only possible scaling factor would be 13.  Is this advisable?  Seems that the gauge scales this value anyway so there should be no impacts on resolution or accuracy of derived measurements?

    Nishant

  • 0 in reply to Nishant Pol24
    TI__Guru** 105590 points

    Hello Nishant,

    The scale factors are explained in the app note I've linked. I'm not sure which factor you're referring to but 13x would be too high for a 13S pack.

    www.ti.com/.../slua760.pdf

    Sincerely,

    Wyatt Keller

  • 0 in reply to Wyatt Keller
    Expert 2875 points

    Hello Wyatt;

    As I've said, I'm able to work around this issue using the scaling method, but am eagerly waiting for an explanation as to why I must do so. I'm frankly surprised that these issues have gone unnoticed these many years, and undocumented, but now that your team is on top of them, I'm hoping for a thorough errata sheet soon.

    Thanks!