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BQ78350: Charge & discharge BQ78350 behavior

Albert Galofre
Intellectual 600 points
Part Number: BQ78350
Other Parts Discussed in Thread: BQ76940EVM, EV2400,

Hi there,

I'm working with the BQ76940EVM which has the gauge BQ78350DBT. I'm making some charge and discharge cycles and I don't understand the main behavior. I'm working with a 14S topologie.

Attach a picture where you can see a charge cycle (1A); and just after a discharge (-0.872A).

I have two questions:

- The "absolute state of charge" (green) and the "relative state of charge" (red) start to increase because the battery is charging at 1A, even you can see 4 cell voltage increasing as well. At 1950 seconds these signals stops to increase even with a 1A maintained!!!!  Between 1950 to 2200 seconds the current charging with 1A and the signals are fixed. WHY?

- At 2200 seconds I start the discharge cycle with -0.872mA and both state of charge falls to 0 really quickly. WHY?  I excepted a slowly decreasing values. Why falls to 0% so suddenly?

Probably I'm setting wrongly the registers.

Thanks

  • 0
    TI__Guru* 81034 points
    Hi Albert,

    If you go through the steps in the Gas Gauging section of this app note, it should help you to set up the gauge for your battery: www.ti.com/.../slua924.pdf
    The fast fall from 100% to 0% on the SOC is most likely because the lowest cell voltage crossed the EDV thresholds you have programmed to the gauge.

    Best regards,
    Matt
  • 0 in reply to Matt Sunna
    Intellectual 600 points

    Hi Matt,

    You was right, of course. Adjusting the EDV seems that the % acquire more logical values.

    Let me ask you more things regarding this absolute and relative SOC.

    1.- What is the diference between them? they are not the same.

    2.- In the picture attach you can see a charging phase; and at the end the relative achieve the 100% and the absolute the 85%. You can see the balance cells over the 3 cell voltages. Almost at the end  I made a reset, I mean I unplugged the battery and the EV2400. After restart this connections the relative and absolute signals doesn't restore his previous value. I expect the same value but the relative goes from 100% to 75% and the absolute the 85% to 60%. Why?

    Thanks a lot

    thanks a lot.

  • 0 in reply to Albert Galofre
    TI__Guru* 81034 points
    Hi Albert,

    RSOC and ASOC should be the same value when you set up the BQ78350-R1 with a new battery. Here is the difference between the two:
    RSOC = Remaining Capacity / Full Charge Capacity (When you first set up the gauge, you should set your 'Learned Full Charge Capacity' in data flash the same as your 'Design Capacity'. Full Charge Capacity is updated by the gauge over time as the battery ages and the capacity is reduced.
    ASOC = Remaining Capacity / Design Capacity Unlike Full Charge Capacity, the Design Capacity never changes over time

    When you cycle power or execute a Reset command, the gauge loses data that is not stored in flash. The Remaining Capacity value is lost, so the gauge must estimate the Remaining Capacity and SOC from the battery voltage alone. If you have selected the correct ChemID for your battery and it the battery voltage has settled (it can take a long time for a battery voltage to settle after charging/discharging), it should give a pretty close estimate. However, the Remaining Capacity will not be as accurate as before the Reset. Accuracy comes from coulomb counting the charge entering and leaving the battery.

    Best regards,
    Matt
  • 0 in reply to Matt Sunna
    Intellectual 600 points
    Wow, really cool!!!

    So, the RSOC goes always from 0 to 100% although the 100% doesn't means the original battery capacity. The 100% RSOC comes from the "learning process"; and I assume that the learning process comes from the Coulomb counter.

    Then my question is, what are the settings configurations which take in account the coulomb counter, in order to assume the 0% and the 100% RSOC?
    Thanks.
  • 0 in reply to Albert Galofre
    TI__Guru* 81034 points
    Hi Albert,

    The algorithm is a little complex, but it is outlined in Chapter 9 of the BQ78350-R1 Technical Reference Manual: www.ti.com/.../sluubd3

    There are some good presentations from previous conferences that cover the CEDV gauging algorithm - here is one example: training.ti.com/.../Download Presentation Part 2.pptx

    Best regards,
    Matt
  • 0 in reply to Matt Sunna
    Intellectual 600 points

    Hi Matt,

    I will read carefully.

    Thanks