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bq40z60 Current Calibration Routine

Cameron Tacklind
Intellectual 380 points
Other Parts Discussed in Thread: BQSTUDIO, BQ40Z60, BQPRODUCTION

Hello,

I'm trying to understand:

  • How to run Current Calibration routine with bqStudio correctly
  • What registers the current calibration actually changes
  • How do calibrate "Board Offset" option

First, the EVM guide only has this confusing and potentially ambiguous statement about how to apply a test current:

"Connect and measure a 2-A current source from 1N (–) and PGND (+) to calibrate [...]"

  1. Is "2-A" a typo? Why is there a dash ("-") in there? It makes me think someone was trying to say "Apply a reverse current of 2A". Add the fact that we're told to enter "-2000" into the calibration field and that almost confirms my suspicions.
  2. Should the battery be disconnected for this step? Does that affect anything? The bq40z60 stops responding if I remove the main battery connection in my system.
  3. It was not immediately clear which way to apply the test current as it was not obvious that the "(-)" and "(+)" were referencing the applied current. Is this correct?
  4. Does the current calibration work with other currents? Other directions?

It would be easier to understand if there was redundancy in the documentation. Along the lines of: "Apply the test current across the battery's current sense resistor", or "Apply the test current such that the voltage at SRN is above the voltage at SRP."

Second, what register(s) does this set? Does this calibrate the AC current sense resistor as well?

I noticed that after current calibration, a pair of values was set that matched the resistance used in my circuit. These were "CC Gain" and "Capacity Gain" in "Data Memory" > "Calibration" > "Current". However, I used different resistances for the charger and battery current sense resistors. I'm guessing that "CC Gain" is for sensing current in CC charge mode and so have adjusted that to match what my AC current sense resistor should be.

Is there a procedure for separately calibrating the AC current sense? Does the calibration routine do anything more than just read a voltage value and divide by a current to calculate resistance? I could imagine that the calibration routine could be a little more complicated to also account for the internal gains and offsets for voltage measurements.

Finally, is bqStudio only intended to be used with an EVM? I'd like to use the "Board Offset calibration option" in my circuit. I've tried bqProduction but it is even more confusing. Is there some other way to achieve this?

  • 0
    Intellectual 380 points
    I was just running the learning cycle and noticed that the current reported by my load was 3~4 times larger than the current reported by bqStudio/bq40z60. I'm guessing I messed up with my current calibrations. Any suggestions or pointers here would be greatly appreciated.
  • 0 in reply to Cameron Tacklind
    Intellectual 380 points
    I have not found any reference to these configuration variables in the TRM. I've tried many search terms, have read most of the manual, and scanned through the rest of it to no avail. I'm planning to do some tests tomorrow with these values to try and figure this out experimentally.
  • 0 in reply to Cameron Tacklind
    TI__Guru 73830 points
    Cameron,
    What configuration variables are you referring to? Current calibration? These would be the CC Gain and Capacity Gain and they will be set to the sense resistor resistance. The current calibration routine calibrates the coulomb counter interface and the charger current is not calibrated by the user. There are a couple of methods to calibrate the coulomb counter. One is to place a load from VSYS to PGND and enter the discharge current as a negative value in the calibration screen. Another method is to place a current source directly across the sense resistor and enter this value into the calibration screen.
    Tom
  • 0 in reply to ThomasCosby
    Intellectual 380 points

    Thank you for the response.

    I think that clarifies things.

    To make sure I have a good understanding, please confirm the following statement:

    Before running the calibration routine, correct values must be entered in the "CC Gain" and "Capacity Gain" fields for the sense resistors for measuring Charger and Pack currents respectively. The calibration routine does not adjust those values. It adjusts internal offsets that are only accessible in this way.

  • 0 in reply to Cameron Tacklind
    Intellectual 380 points
    I just tried the calibration routine again. It adjusted my "CC Gain" and "Capacity Gain" values. Clearly I'm missing something. Please advise.
  • 0 in reply to Cameron Tacklind
    TI__Guru 73830 points
    Cameron
    The current calibration should update the CC Gain and Capacity Gain. Are they set to value close to your sense resistor value? Is the reported current accurate? The CC Offset and Board Offset calibrations are used in the event that the gauge reports a current when no load is present.

    Tom
  • 0 in reply to ThomasCosby
    Intellectual 380 points
    I am observing that the calibration routine adjusts both "CC Gain" and "Capacity Gain" even though I am only attempting to calibrate the pack resistor. They get set to the same value even though I have different resistances for each. I don't understand why it sets both as the calibration current is clearly only being applied to the pack resistor.

    Some experimenting has indicated that "CC Gain" is "Cell Current Gain" and "Capacity Gain" is "Charger Gain"; which was not at all clear from the documentation. (In fact, my first guess was CC related to constant current charging mode indicating to me that the values were swapped from what they are.)

    My board is reporting -4mA pack current. I don't think this is an offset issue as my pack voltage is dropping. (This is a separate issue apparently. I've completely unloaded VSYS as far as I can tell and there is still a small load.