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BQ40Z50: Remaining Capacity and Run Time to Empty

Juliet DeNapoli
Prodigy 30 points
Part Number: BQ40Z50
Other Parts Discussed in Thread: BQ24616

Hello,

I was using bq40z50 and when I inspect the logs, I am having trouble understanding the Remaining Capacity and Full Charge Capacity, and how they affect Run Time to Empty. When the battery is fully charged (RSOC=100), I expect both the Remaining Capacity and the Full Charge Capacity to equal ~63000 mWh. However, on some batteries I am seeing ~2000mWh, ~4000mWh, or even ~900mWh, and I am not sure why.

For one specific battery, it is fully charged (RSOC=100), and the Remaining Capacity and Full Charge Capacity are both equal to 924mWh. As I begin to discharge it, the remaining capacity jumps up to 65510 mWh and decreases from there, while the Full Charge Capacity remains constant. After the battery is fully drained (RSOC=0), I charge it and the Full Charge Capacity jumps up to 1124mWh and remains constant, and the Remaining Capacity gradually increases to 65535mWh. When the RSOC=99, the Remaining Capacity drops down to 4mWh and increases a little more from there. 

At the beginning of the discharge:

At the end of the charging:

I assumed that Run Time to Empty was Remaining Capacity (mAh) / Current (mA). However, when the Remaining Capacity changes so drastically, the Run Time to Empty is not affected. I know bq Studio is recording the Remaining Capacity as mWh and not mAh, but I still thought there should be a change in the Run Time to Empty.

Why is the Remaining Capacity value changing so drastically, and why isn't the Run Time to Empty affected by it?

  • 0
    TI__Mastermind 27321 points

    Hello Juliet,

    RunTimetiEmpty is only updated when discharging. The gauge will report 65535 when charging or relax. 

    Did you obtain a ChemID and completed the learning cycle? 

    Did you follow section 2 in the Achieving The Successful Learning Cycle App Note? You might have not configured the dataflash parameters correctly. 

    For questions about Impedance Track Algorithm, please reference to this FAQ page where the theory of the algorithm is outlined. 

    Regards,
    Jose Couso

  • 0 in reply to Jose Couso
    Prodigy 30 points
    Jose Couso said:
    RunTimetiEmpty is only updated when discharging. The gauge will report 65535 when charging or relax. 

    Yes, that makes sense. Why doesn't the RunTimetoEmpty change when the RemainingCapacity goes from 14 to 65510 (seen in the first picture)? Doesn't the RunTimetoEmpty use the RemainingCapacity?

    I completed the learning cycle by doing the following: I fully discharged the battery, let it sit for 5 hours, and fully charged it, let it sit for 2 hours, and I repeated this twice. It didn't seem to straighten out any values.

    I compared all the values of importance in the learning cycle (Design Capacity, Design Voltage, Charge Term Taper Current, Discharge (Dsg) Current Threshold, Charge (Chg) Current Threshold, Quit Current, Term Voltage) of the battery behaving weirdly to the one that behaved correctly, and they were the same for all. 

    When the batteries with the lower remaining capacities and full charge capacities (~2000mWh or 4000mWh) start discharging, the remaining capacity will drop to zero on the discharge, and then reset at 65530 mWh and decrease from there (I assume this is the max). When I charge it, the remaining capacity increases all the way to 65530, and then starts increasing again from zero, again I am assuming because it has reached its max. Why is the full charge capacity constant (and for the most part lower) than the remaining capacity?

    Would it be more helpful for me to attach logs? 

  • 0 in reply to Juliet DeNapoli
    TI__Mastermind 27321 points

    Hi Juliet,

    Yes, please attach the log and configuration (gg) files. 

    Regards,
    Jose Couso

  • 0 in reply to Jose Couso
    Prodigy 30 points

    TIE2E_logs.csv

    Here are the logs. I am not sure how to find the ChemID or the .gg file. Our battery uses a standalone charger, BQ24616.

    Other than the remaining capacity and fully charge capacity being too low, the battery seems to have the same run time as the battery that has the correct remaining capacity and full charge capacity. We are just trying to understand how these numbers are calculated, and why the full charge capacity seems to vary so much. Again, I checked all the parameters of the correct battery to the one with the low capacities, and they are the same.

  • 0 in reply to Juliet DeNapoli
    TI__Mastermind 27321 points

    Hello Juliet,

    I think this is a reading problem in the reported remaining capacity values. I checked the Filtered and true remaining capacity columns and they seem fine. Please see below.



    It looks like the reported value is being scaled or read improperly. Can you try disabling the smoothing feature to see if the reported value changes behavior? 

    To export the gg file, please navigate to Data Memory and click on Export.


    Regards,
    Jose Couso