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BQ34Z100-G1: Charge Termination Indication: Taper Current Setting

Fahad Akhtar
Prodigy 210 points
Part Number: BQ34Z100-G1


Can you please confirm that charge termination indication via tapper current, as described in section 3.7 of TRM is critical only for learning cycle?

I have 16 series cells with each cell 280Ah. My charging profile during normal operation will be CC/CV which essentially cuts off the current (almost step change from charging current to zero current) once desired pack voltage is reached. This means that under normal operation, end of charging profile will not meet requirement of taper current window. I need to make sure that it is ok for normal operation and battery guage performance will not be impacted. For learning cycle, I can force the profile required for charge termination indication since its just one time

Best regards

Fahad 

  • 0
    TI__Expert 8365 points

    Hi Fahad,

    It is important to meet the valid charge termination requirement. Without this, many vital parameters used for accurate gauging will not be updated.

    Can you tell me more about why you don't expect your CC/CV operation to work? We use CC/CV and expect the current to taper out under normal operation. Is it just that the slope of your CV current taper is so severe that it goes to 0A too quickly?

    Thanks,

    Jackson

  • 0 in reply to Jackson DiGiovanna
    Prodigy 210 points

    The way CV/CC typically works is that once pack reaches full battery voltage (58.4V in my case), it switches from CC to CV. This basically means that the charger stops increasing its terminal voltage to keep current constant. Once the switch happens from CC to CV, it takes almost no time (maybe a second or so) for the current to go to zero naturally since battery voltage quickly catches up reducing current to zero

    Also there are other charging methods whereby the charger cutsoff based on some external command, in which case it goes to zero right away

    So am I interepreting your response correctly that charge termination requirement are necessary even under normal operation and not just during initial optimizatiom/learning cycle? Note that in many applications, batteries are never fully charged hence charger termination stage may never occur anyway.

  • 0 in reply to Fahad Akhtar
    TI__Expert 8365 points

    Fahad,

    the DOD at EOC parameter updates upon a valid charge termination. It is not necessary to update this parameter on every charge cycle but over time the gauging accuracy will drop if this parameter is not updated as the gauge will not track how the max chemical capacity of the pack degrades over time.

    In terms of CC/CV, I am familiar with how this process works, I am just confused on how you are seeing current taper all the way to 0V within a few seconds. Typically this CV process takes minutes for the charge current to taper to the value of Taper Current where the charge cycle is complete. If you can lower the value of Taper Current (and the termination current from your charger) the CV curve should take longer to reach termination current and thus you would be able to meet the requirement of taper current window.

    Thanks and let me know of further questions,

    Jackson

  • 0 in reply to Jackson DiGiovanna
    Prodigy 210 points

    Jackson

    Is there a minimum value of taper window that you would recommend? I know the min you can enter is 1sec, but I wonder if a very low value of taper window negatively impacts guaging? Default value is 40sec

    Thank you

  • +1 in reply to Fahad Akhtar
    TI__Expert 8365 points

    Fahad,

    Lowering the value of taper window does not inherently negatively impact gauging performance. It can however, make it much harder to obtain a valid charge termination as certain conditions may not be satisfied within two consecutive 1s windows. For example, accumulating 0.25mAh in two consecutive 1s windows while also having AverageCurrent() be less than TaperCurrent() may not occur for your specific application.

    I personally would fine tune the values for both Taper Current and Taper Window such that these conditions will be satisfied.

    Is it also possible for you to change the settings on your charger such that the current tapers out to a much lower value before actually terminating charge? The lower this value gets, the longer the taper will be which may help avoid having to adjust Taper Current and Taper Window to satisfy the conditions necessary to achieve Valid Charge Termination.

    Thanks,

    Jackson