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BQ78350-R1: FCC value decrease very quickly for LG cell ID:0575

mehmet özel
Prodigy 120 points
Part Number: BQ78350-R1
Other Parts Discussed in Thread: BQ78350, BQSTUDIO

Hİ, 

I'm working on the e-bike BMS project that includes the Bq78350.
We apply long-term performance tests ( 8A discharge / 5A charge cycle test) to our batteries.
During these tests, we did not see any problems for 200 cycles but after 200 cycles, we noticed that the battery enters OC protection at the end of each charge. For this reason, the FCC value began to decrease very quickly because the battery was never fully charged. This is not about cells because we measured the actual capacity of the cells and capacity of cells is pretty good.
Below I share the values we found for you using the CEDV calculator.
What do you think might be the reason for this situation?

Chem ID: 0576, INR21799M50LT 

  • 0
    TI__Guru* 81034 points

    Hi Mehmet,

    It might be good to capture a log file as the battery approaches full charge to understand why the OC protection is triggering. Overcharge occurs when the internal capacity of the gauge measures a larger capacity than the FCC. Is valid charge termination detected before the OC protection triggers? Does the current reported by the gauge appear to be accurate?

    Best regards,

    Matt

  • 0
    Prodigy 120 points

    Hi Matt, 

    Thank you very much for the quick response.

    Our valid charge Term Taper Current was set to 300mA, but the charge current has never fallen below this value, the charge has generally ended at 1A levels (OC is triggering)

    There is no problem in our calibration, the actual values are compatible with the value read from the BMS.

    In addition, our OC threshold value was set to 750mAh, I think this value is quite a lot, so I never expected the OC protection to be triggered. These results make me think that the gauge is measuring the capacitance incorrectly.

    If you specify the logs you want me to get, I can get these logs for you.

    Finally, we do not see this problem in other cell chemistries, we used chem ID 2591 before and we had no problems.

  • 0 in reply to mehmet özel
    TI__Guru* 81034 points

    Hi Mehmet,

    I am not sure why this cell chemistry would behave differently. For learning FCC, the chemID is not used by the gauge. The gauge only uses the ChemID to estimate the SOC after the device has been reset or power cycled. So the issue is with learning the FCC - it might be that the OC protection is causing charging to terminate earlier than expected, or there may be a problem with the learning at the end of discharge. It would be good to see a log file (if you can connect to BQStudio) of the charging to full and discharging to empty.

    Regards,

    Matt