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BQ40Z50-R3-DEVICE-FW: flat region and OCVFR - chem-ID 0x2158

Bernhard Gross
Prodigy 20 points

Part Number: BQ40Z50-R3-DEVICE-FW

Hi! 

Our battery pack is based on the bq40z50-r3 (2 cells in series, Chem-ID 0x2158). After a few partial charge and discharge cycles (between 5.90 and 7.66V) we noticed that the RSOC value is drifting. We added a rest phase (at 6V, 4 hours) later on because we hoped that an OCV measurement would limit the effect oft he drift. But unfortunately the OCVFR bit is set throughout and no OCV measurements are taken.

If I understood correctly, the values Vqdis_min and Vqdis_max in chemdat.txt define the location of the flat region. Since the values are identical (Vqdis_min = 3699, Vqdis_max = 3699), I suspect that no flat region exists for this chem id. However, in our tests, the OCVFR flag is set anyway, which is apparently why there are no OCV measurements during the rest phase.

In the Technical Reference I found the following about OCVFR:

  1.  The GaugingStatus()[OCVFR] flag indicates if the cell voltage is inside this flat region.
  2. Impedance Track Configuration Options: An OCV reading is taken when a dV/dt condition is met. This is not the case if charging stops within the flat voltage region. By default, this flag is set. The bq40z50-R3 device will take a 48-hour wait before taking an OCV reading if charge stops below the FlatVoltMax. A discharge will not cancel this 48-hour wait. The 48-hour wait will only be cleared if charging stops above the FlatVoltMax level. Setting this flag to 0 removes the 48-hour wait requirement, and OCV is taken when the dV/dt condition is met. Removing the 48-hour requirement can be useful sometimes to reduce test time during evaluation.

The wording in 1) seems to me to be somewhat misleading. The behavior obviously corresponds to 2).

Under what conditions is the OCVFR bit in IT Status cleared? Interrupting the charging process for a few seconds at a voltage above Vqdis_max did not cause this.

Can OCVFR be deactivated in the Impedance Track Configuration Options at Chem-ID 0x2158 OCVFR without negative side effects? Or is this not recommended?

Best regards,
Bernhard

  • 0
    TI__Guru** 105760 points

    Hello Bernard,

    If your batteries do not have a very flat voltage discharge curve (like LFP chemistries ) The chances of disabling the OCVFR function causing an issue is lower, it is strange you able to set the OCVFR bit unless one of the discharges was stopped exactly on that voltage.

    It looks like your time frame is very long, so the gauge will update SOC as the voltage settles even if you disable OCVFR it should equalize fairly quickly with your time frame.

    Sincerely,

    Wyatt Keller

  • 0 in reply to Wyatt Keller
    Prodigy 20 points

    Hello Wyatt!

    Thank you very much for the feedback!

    Are you sure about the implementation of the OCVFR condition? Is it actually checking if the voltage is between Vqdis_min and Vqdis_max? Or merely below Vqdis_max?

    Every time (3 out of 3) when we started the test after an OCV measurement, the OCVFR bit was set after the next load. That would be quite a coincidence if we had hit the voltage here exactly every time. See also plot below. Here we had 7564 mV when charging and then jumped to 7305 mV when discharging (next sample). The values between logged samples are unfortunately unknown.

    With OCVFR functionality enabled, I had the module at my desk for 8.5h with RSOC 99% at 6.45V. A waiting time of 48h with OCVFR set until the next OCV measurement (as mentioned in the Technical Reference) would be too long in our application.

    What are the potential problems or negative effects we have to expect if we disable the OCVFR functionality?

    Sincerely,
    Bernhard Gross

  • 0 in reply to Bernhard Gross
    TI__Guru** 105760 points

    Hello Bernhard,

    It should only disable an OCV taken in the flat region, you can disable the OCVFR functionality in 16.2.1.15 IT Gauging Configuration.

    Looking at your chem ID it doesn't really have much of a flat region so it should be fine disabling this feature so you can take an OCV quicker. Usually this feature if helpful for battery chemistries that have a flatter OCV region that the one you are using.

    Sincerely,

    Wyatt Keller