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BQ34Z100 learning cycle issue and wrong passed charge value

Matteo Rigamonti
Expert 1170 points
Other Parts Discussed in Thread: BQ34Z100

Good morning,

I'm trying to perform a learning cycle on a 10S4P Li-ion battery pack with a total capacity of 11400mAh. I've followed exactly the procedure reported into the SLUA597 with also some further advices received some months ago (when I did the same on a different pack), but after the proper execution of the preliminary discharging phase and first charging phase, I've got a problem at the end of the first useful discharging phase:

  • Qmax value hasn't been updated
  • Update Status remains at 05 value (instead of 06)

I've analyzed all the configuration and I have some doubts about a couple of parameters, but I don't know if they can influence the wrong result I've obtained:

  • Cell Terminate Voltage: has such a parameter to correspond to the protection circuit cut-off voltage, i.e. to the minimum allowable value for the cell voltage?
  • Qmax Max Delta %: is any update into the Qmax value accepted only if it is less than this percentage of the design capacity?

Concerning the second question, I've noted the Qmax value registered at its first update (i.e. after the first charging phase) is completely different from any reasonable value: it is around 16000mAh instead of about 11400mAh (i.e. the nominal value). With this wrong value of Qmax estimation (why is that been accepted if it was completely different from the starting value, set equal to the design capacity?), maybe a further update of Qmax, after the discharging phase, has been rejected because too distant from it, but closer to the correct real value...

At this point, I've tried to understand the reason for a wrong Qmax estimation. In a document received from TI I've found the following formula: Qmax = PassedCharge / (SOC1 - SOC2), where SOC1 and SOC2 are calculated at full charge and full discharge conditions (and they should be always something like 100% and 0% respectively). Looking at Passed Charge, I've found it is completely wrong and correspond to 16000mAh at full charge and at full discharge. So this seems to be a wrong behavior. How does the passed charge is calculated?

The current value is measured by the BQ34Z100 with a small error (below 0.5%, but the calibration procedure does not compensate such an error). Anyway such an error cannot justify the 40% error on passed charge (16000 instead of 11400). Maybe an error on timing could be?

Please let me know if you have any idea for understanding this wrong behavior or any suggestion for reaching a successful learning cycle.

Thank you in advance and best regards

Matteo

  • 0
    TI__Guru 73830 points
    Matteo,
    Can you provide the log file for analysis?

    The Cell Termination Voltage should be set to the voltage where you wanr SOC = 0%.

    The Omax Max Delta% will only limit Qmax updates after Update Status = 06. It has no effect during the optimization cycle.

    Qmax = Passed Charge / (DOD0-DOD1), where the DOD points are set during the relaxation period.

    Tom
  • 0 in reply to ThomasCosby
    Expert 1170 points

    Hi Tom,

    thank you for your reply: very fast, as usual!

    In attachment you can find the log file.

    Regards

    Matteo

    LOGGING_28.01.log

  • 0 in reply to Matteo Rigamonti
    TI__Guru 73830 points
    Matteo
    I reviewed the data and the problem is not with the DOD point, but with the accumulated Passed Charge. This is a curious situation that I have not seen before. You get about 11500mAh of passed charge in both the charge and discharge directions, if you calculate the charge based on the reported current and time. But, the device is reported about 16000mAh of Passed Charge. This would imply that the current measurement is close, but the device uses this data to accumulate the Passed Charge. We will check to see how this can happen, but you could try calibrating the V, T and I and try again. Error is accumulating and causing your Qmax over estimate. Qmax should be about 11600mAh based on the current and change in DOD. Of course, this will be off if the current calibration is not accurate.

    Tom