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BQ27541: BQ27541

Christopher Paul22
Intellectual 260 points

Part Number: BQ27541

It is difficult to find an app note on how to calibrate a non-host side (pack side) gauge. For my custom pack-side bq27541-M200 gauge (derviced from -V200?), I suspect the following to be true:

If a Board Offset is to be calibrated on every single gauge, the correct sequence for calibration is CC Offset Calibration, Board Offset Calibration, Voltage Calibration, Temperature Calibration, and Pack Current .Calibration.

Our pack Assembler proposes NOT to run Board Offset on a sample of 20 - 30 boards, average that, and use the average for all PCBs. Instead, they propose the following for when no current flows: "We read the gauge's reported current. If the current is zero , we will not calibrate CC offset and board offset; otherwise, we will."

This concerns me. Please comment. Thanks in advance for your reply.

  • 0
    TI__Guru* 76230 points
    The board offset calibration ensures the gauge measures 0 A when there is no current. The gauge routinely performs auto CC offset calibration. I agree that if they do not see measured current when there is no current flow, then there is no need for board offset calibration. if they do, then both board and offset calibrations need to be done. Doing an average of 30 boards for board offset and running with that is more riskier than doing board offset on each board that sees a non-zero current.
    thanks
    Onyx
  • 0 in reply to Onyx Ahiakwo
    Intellectual 260 points

    Thanks for your reply, Onyx.

    Can you please explain what Board Offset is doing that takes 30 seconds? I thought it was using the time to average a large number of readings to arrive at a very small, sub-1mA offset value. If so, a report from the gauge of 0mA, which could reflect an actual value between +/- 0.5mA, would not eliminate the desirability of making a Board Offset measurement. Again, thanks in advance for your reply.

  • 0 in reply to Christopher Paul22
    TI__Guru* 76230 points
    hi Chris,

    Board offset first requires a CC offset to be run. So when you run just board it initiates both.

    In both cases it takes 16 samples of current to average together for the offset result. That is why it takes > 30s
    thanks
    Onyx
  • 0 in reply to Onyx Ahiakwo
    Intellectual 260 points

    HI Onyx,

         I'm sorry to beat a dead horse here, but I'd like to understand better than I do.

         If you do only CC Offset, it takes nowhere near as long as 16 seconds to run. I am assuming that this CC Offset correction makes one measurement and that that has a resolution of 1mA. In such a case, if the gauge already reports 0mA when no current flows, there is no reason to run a CC Offset alone.

         However, if sets of 16 samples are being taken, I assume they are to be averaged. Unless this average results in a resolution of much lower than 1mA, I can't see any benefit in doing this. I assume that this much lower resolution measurement is capable of improving the zeroing out of of the CC register. But there is no way of knowing whether a battery with a particular sub-1mA offset would benefit from such, because the gauge reports with only a 1mA resolution.

        With this line of reasoning, a reading from the gauge of 0mA provides no guidance as to whether or not a Board Offset would be beneficial.

        Thanks again in advance for your response.

         

  • 0 in reply to Christopher Paul22
    TI__Guru* 76230 points
    Hi Chris,
    if your board measures non zero current when there is actually no current flow, performing the board offset corrects that. Other than that there is no reason to perform a board offset calibration. The gauge on its own performs auto calibration of the coulomb counter. I am not sure what you are driving at with your question.
    thanks
    Onyx
  • 0 in reply to Onyx Ahiakwo
    Intellectual 260 points

    Hi Onyx,

        I am trying to be clear. Please recall that my pack assembler reads the current register when no current is flowing. If the value is zero, they do nothing. If non-zero, they run a Board Offset on each unit in production. I question this.

    1. What measurement result directs me to perform the much longer Board Offset correction in preference to the shorter CC Offset calculation?
    2. Is the resolution of a CC Offset measurement 1mA? Or does it have a smaller resolution?
    3. In the presence of noise, an averaged series of measurements can produce a result which has a smaller (better) resolution than a single measurement. Example: the average of .001, .001, .001 and .002 is .00125. If such a value is applied, it can result in a more accurate correction. Is this what Board Offset is doing with multiple measurements? If not, what is the result of a Board Offset measurement - what is being done with it?

  • 0 in reply to Christopher Paul22
    TI__Guru* 76230 points
    Hi Chris,
    I agree with their procedure.
    1. My experience has been, the CC offset calibration by it self typically does not remove the error when a non-zero current is measured when in actuality there is no current. That tends to be caused by offsets from the boards traces and poor layout. C
    2. CC offset has a smaller resolution. The gauge resolution of reported current is 1mA, but CC offset is smaller.
    3. Yes it takes the average, 16 samples.