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BQ20Z95: FCC should only be a Portion of the maximum possible Chemical Capacity

Robert Andris
Intellectual 445 points

Part Number: BQ20Z95

Hi,

We currently are working on a "old" product that we have to maintain and support.
The current cells are not available anymore so our customer selected a new cell for us to assemble the pack. 
Since it is a new Cell I started with the cycles for the ChemID and got stuck at reconfiguring the bq20z95.

My Problem is that the old cell had way less capacity in comparison to the new one. Our Customer does not want a significant increase in capacity for reasons unknown to us and he will most likely not drop this Requirement.

The whole Gas Gauging Algorithm used by bq20z95 depends on the Chemical depth of discharge and the total chemical capacity Qmax. Therefore, as I understand we have to set the chemical Qmax for the new Cell with a Golden Cycle that will Result in the real chemical capacity the Cell has so that the ChemID and OCV-Tables as well as the Gauging in its entirety will still work.

Further i think we can increase the DF:Terminate Voltage and DF:Reserve Capacity to Reflect the FCC we actually want. As an Example
with a 3000mAh Standard Cell with 30% at 3.6V we would have a FCC of 2100 as long as we set the DF:TerminationVoltage to 3.6V
and the DF:ReserveCapacity to 900mAh. This way we could still use the "usual determined" ChemID and the OCV-Tables would also not 
disrupt anything as long as we stay above the 3.6V. Is this Assumption correct?

I was reading the "SLUA364 - Theory and Implementation of Impedance Track". It is not clear for me if the DOD1 and DOD2 used at 
the new "TerminationVoltage" would Result in a Qmax of 2100 when the previous settings in the example are used and if there would
be any weird behavior in the GasGauging itself.

Can u tell me if the simple parameterchange proposed and a Golden Cycle with these Settings as well as the ChemID i got from Cycling the 
Cell (min/max values specified in the Datsheet) would give us the Outcome our Customer wants or if we have to approach this from a different angle ?

Thanks in advance

    Robert Andris

  • 0
    TI__Mastermind 42535 points
    Actually your solution works. Change to the terminate voltage will restrict your DOD range. So, while your chem ID and OCV scales will go to the full range, your actually capacity will be restricted by the capacity from charge termination to terminate voltage. You can also scale reserve capacity up to reduce the cell's available capacity even further. Qmax is essentially delta passed charge by delta DOD. So that will effectively be set in stone by the chem IDs range. Now, if you really want to use the gauge this way, you may run a learning cycle with a higher terminate voltage which gets you close to the capacity you want and then fine tune it by changing reserve capacity upwards.
  • 0 in reply to Batt
    Intellectual 445 points

    Thanks for the answer. I will Test it during a learning cycle with the settings that gets me close to what i want. We now also have a List of commands/values the customer reads for his Application. The Qmax Values are not among them so it should not be a Problem that they read the higher values. FCC and Remaining Capacity will be with the lower Capacity also so no Problem there either. The only Thing left would be the Design Capacity. Can i use the lower actual usable Capacity as the DesignCapacity for correct ASOC/RSOC or do i have to Set this to the proper full chemical Cell Capacity for the overall Gaugin to work?

  • 0 in reply to Robert Andris
    TI__Mastermind 42535 points
    Please use the real, proper rated values and not the usable capacity.