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BQ40Z80: Explanation of State of Charge and Capacity Readings

Jamie Milliken8
Prodigy 230 points
Part Number: BQ40Z80
Other Parts Discussed in Thread: BQSTUDIO

Hello,

We are testing a 6S1P pack with nominal 3450mAh capacity. I am trying to understand how the capacity and state of charge readings are generated. See screenshot below:

This was taken during a 2A discharge test after fully charging the pack. First, I should mention that the Full Capacity reading will get up around 2650mAh after charging, but never reaches the 3450mAh programmed into the data registers. So my first question is why can't we get to the full capacity? Second, during this discharge test, the full capacity reading jumped abruptly from ~2600 to ~800mAh, even though cells are still at ~3.7V. What caused this?

Finally, what are the differences between absolute and relative SoC? What are the differences between Full charge and remaining capacity?

Thank you,

Jamie

  • 0
    Prodigy 230 points

    Follow-up - As I was writing the above post, the test was still running in the background. When I finished the post (a couple minutes of writing), the values have now jumped back up. No change in the test setup at all - it was just running continuously at 2A the whole time for this screenshot and the one in the previous post:

  • 0 in reply to Jamie Milliken8
    TI__Mastermind 27321 points

    Hi Jamie,

    Full Charge Capacity (FCC) is the usable capacity that the gauge predicts from full to empty. FCC is a function of (load and current), if any of these two change, it will affect FCC. The gauge establish usable capacity before the start of discharge. 

    Please see Section 2.6 in the Theory and Implementation of Impedance Track Battery Fuel-Gauging Algorithm. Here it is explained in details how FCC and RM are updated. 

    For battery capacity, you should be referencing to RSOC, not ASOC. RSOC will continue to transition from 0% to 100% as the pack learns. FCC continues to decrease as the pack ages. However, ASOC will eventually not be able to reach 100%, because the Design Capacity never changes. RSOC should be used for StateofCharge()

    RSOC  = RM / FCC

    ASOC = Remaining Capacity / Design Capacity

    If you still have questions, please attach the following files and we can take a look. The minimum files needed for debugging an Impedance Track question are configuration (gg) and log file which describes the behavior. 

    To learn how to log or extract a gg file from BQStudio, please follow the EVM user guide. I am assuming you have already obtained a matching ID and you are in the process of completing the learning cycle. 

    Regards,
    Jose Couso