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BQ34Z100-G1: SOC increasing dramatically during storage

Chen Jia
Intellectual 850 points
Part Number: BQ34Z100-G1

To whom it may concern:

BQ34Z100-G1 is used on a 9S1P battery. During storage test, the IC is set to deep sleep mode, and several battery got increased SOC. The FCC looks OK. The worst case (18 weeks storage) SOC increased to 93% from 30%, and another (14 weeks storage) was 55% from 30%. Here is the log data after the storage.

Current(A) Voltage(V) StateOfCharge(%)
0 31.96809 93
0 31.96793 93
-0.00085 31.36862 93
-0.05 31.94114 93
-0.05 31.94022 93
-0.05 31.9398 93
-0.05 31.94005 93
-0.05 31.93896 93
-0.05 31.93913 93
-0.05 31.93821 93
-0.05 31.93813 93

Obviously the voltage and SOC don't match. The 9S1P battery is set to charge to 37.8V, so 93% SOC should be over 36V.

This battery could be discharged, and at the end 27V (3V/cell), the SOC is still 70%. Then the battery was fully charged, and SOC reached 100% while before the CV mode. In the next discharging cycle, SOC was back to normal.

Here is the Pack Config C register: 0x33, so RSOC_HOLD is not enabled, and RELAX_SMOOTH_OK and SMOOTH are enabled.

Could you help to check the root cause of this SOC rising issue, and how should we set the configuration?

Thanks!

Chen

  • 0
    TI__Guru 57697 points

    Hello Chen,

    Was the battery matched to a Chemistry ID and was the correct chemistry ID programmed on the bq34z100-G1?

  • 0 in reply to Shirish
    Intellectual 850 points

    Yes, the ChemID is correct and programmed in. The ChemID is 5271, as MOLI P28A cell.

  • 0 in reply to Chen Jia
    TI__Guru 57697 points

    Hello Chen,

    You can check if this is an effect of smoothing by reading TrueRC() and comparing to RemainingCapacity().

  • 0 in reply to Shirish
    Intellectual 850 points

    Hello Shirish:

    Thanks for your reply. Unfortunately, the True RC and True FCC were not recorded. However, this change was dramatic from 30% to 93%  and the True RC need to change over 200%. We can try to remove the smooth function, but not sure if it would help. Also how about other settings in Pack Config C register?

    Thank you!

    Chen

  • 0 in reply to Chen Jia
    TI__Guru 57697 points

    Hello Chen,

    I don't think removing the smooth function will help, but you can try it.

    For the discharge: setting FF_NEAR_EDV may help in getting more accurate SOC reporting in the discharge phase, but it only works when voltage drops close to EDV.

  • 0 in reply to Shirish
    Intellectual 850 points

    Hello Shirish:

    The FF_NEAR_EDV is already enabled. The Pack Configuration C is set as 0x33. But it didn't update the SOC to correct value when the battery is fully discharged. The end voltage is 27V (3V/cell), but the SOC remains 70%.

    Another related question, if the battery only work from 10% to 80%, how to set the calibration during use? Could the fuel gauge get self calibration from 10% to 80% SOC?

    Thank you!

    Chen

  • 0 in reply to Chen Jia
    TI__Guru 57697 points

    Hello Chen,

    If the battery is only used from 10% to 80%, then the charge taper and termination will need to be set appropriately. EDV algorithms cannot handle a battery that never gets below 10%, so that explains why the EDV settings did not help. You should use Reserve capacity to prevent the battery from going below 10% but the host will need to act in order to remove the load.

  • 0 in reply to Shirish
    Intellectual 850 points

    Hello Shirish:

    Thanks for your reply. For this test (not the real application), the battery was fully discharged to 27V(3V/cell), but EDV algorithms didn't help.

    Test_Time(s) Current(A) Voltage(V) StateOfCharge(%) MaxError(%) RemainingCapacity(mAh) FullChargeCapacity(mAh)
    3690.971 -0.60001 27.09222 70 1 1835 2639
    3697.767 -0.59999 27.07222 70 1 1834 2639
    3704.024 -0.59998 27.05213 70 1 1833 2639
    3710.677 -0.59999 27.03213 70 1 1832 2639
    3717.078 -0.60002 27.01212 70 1 1831 2639
    3720.962 -0.59999 26.99998 70 1 1830 2639
    3781.137 0 27.47599 70 1 1830 2639
    3841.137 0 27.5853 70 1 1830 2639

    So in order to have a more accurate SOC, for 10% to 80% application:

    For low capacity range 10%, reserve 10% capacity and set the FD at 0%?

    For high capacity range 80%, what is your suggestion? set the charging current to a lower value? Or could we set FC at 80%?

    Thank you!

    Chen

  • 0 in reply to Chen Jia
    TI__Guru 57697 points

    Hello Chen,

    For low capacity range 10%, reserve 10% capacity and set the FD at 0%? Yes

    For 80% max capacity use, set the design capacity to 80% of full capacity and set parameters to make sure that charge termination is detected correctly. See section 3.7 Charging and Charge Termination Indication in TRM for details.