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BQ27542EVM: SOC when charging and discharging at the same time.

user5768972
Intellectual 450 points
Part Number: BQ27542EVM

Hello.

When charging and discharging at the same time, the initial SOC of 92% at a cell voltage of 8.2V changed to SOC of 92% at a cell voltage of 8.02V.

We thought that the cell had simply deteriorated. However, when this battery was discharged to 6V, charged to 8.4V, and left overnight, the cell voltage returned to 92% SOC at 8.2V.

Question 1:
If the battery is not discharged to 6V and charged to 8.4V, but continues to be charged and discharged at the same time, will the relationship between cell voltage and SOC continue to deteriorate?
Or does it stay somewhere (e.g. SOC 92%, cell voltage 7.9V)?

Question 2:
Is there any other way to restore a battery that has changed to 92% SOC at 8.02V cell voltage other than discharging, charging, and leaving it alone? (For example, by sending some kind of command).

  • 0
    TI__Mastermind 28450 points

    Hello,

    If possible, can we please receive the .gg file and .log file of this instance occurring? This will allow us to look deeper into what is causing the SOC to act this way.

    Regards,

    Anthony Baldino

  • 0 in reply to Anthony
    Intellectual 450 points

    Hello, Anthony


    Unfortunately, there is no LOG for this phenomenon.
    The gg file is as attached.

    I would like to know if you know anything about this, even if it is a similar case in the past.

    regards.

    KazutoGGファイル.xlsm

  • 0 in reply to user5768972
    TI__Mastermind 28450 points

    Hi Kazuto,

    Without the log file, it is difficult to pinpoint the exact reason this could be occurring since we cannot see what is happening within the registers. However, I do not believe that charging and discharging should be possible with this device. Can you please tell me more about how this is being accomplished?

    Below is a document that contains more information on how the Impedance Track Algorithm calculates values such as SOC, SOH, etc:

    https://e2echina.ti.com/cfs-file/__key/telligent-evolution-components-attachments/00-24-00-00-00-00-02-91/Theory-and-Implementation-of-Impedance-Track-Battery-Fuel_2D00_Gauging-Algorithm.pdf 

    Regards,

    Anthony Baldino

  • 0 in reply to Anthony
    Intellectual 450 points

    Hi Anthony,

    You are correct, the battery is not being charged and discharged at the same time.
    Insert a 2 to 2.5 A discharge for 1 to 1.5 seconds in the middle of a 250 mA or 750 mA charge.
    ← This charge and discharge is repeated for several months at various ratios (I can't log this situation because of this).
    This is a situation where a device with a built-in battery is kept charged at all times, and the device is used when needed (with the battery still charged).

    Because of the above situation, the capacity of the battery is always at 80-92%. (several months)

    I think probably that this phenomenon may be due to the fact that the parameters for SOC calculation are not updated due to two reasons: the battery is not fully discharged and the battery does not get relaxation time.
    (I infer the above from the fact that it can be revived by "full discharge and full charge and further neglect".)

    I will ask again under the above conditions.

    Question 1:
    If I continue to use under these conditions (always charging and discharging within the range of 80-92%), will the voltage corresponding to 92% SOC continue to drop? Or will it stop falling?
    If it stops falling, what voltage will it be?

    Question 2:
    As mentioned earlier, with "full discharge, full charge, and leave alone," the voltage at 92% SOC will revive to 8.2V.
    Is there any other way to revive the voltage other than "full discharge, full charge, and leave the battery unattended"?
    (commands, etc.)

    best regards.

    Kazuto

  • 0 in reply to user5768972
    TI__Mastermind 28450 points

    Hi Kazuto,

    Thank you for explaining.

    I believe the main issue of this is that if you are constantly either charging or discharging the cell for long periods of time, there is not enough time of the cell relaxed for an accurate OCV reading to take place and gauge where the SOC at that point truly is. To constantly charge and discharge without a relax period will cause error to build up, due to the coulomb counting measurements not having the time to clear and readjust.

    Regarding the first question asked, I cannot estimate how the gauge will continue to act if this process will continue without more information or a log of this data. Below is how the SOC is calculated and where the updates of the RM and FCC that are used in that calculation occur.

    Regarding the second question, I believe including some sort of relax time between the constant changing from charge to discharge will allow for the SOC to adjust to its normal measurement.

    Regards,

    Anthony Baldino

  • 0 in reply to Anthony
    Intellectual 450 points

    Hello, Anthony.
    Thanks for your answer.

    It seems that Ra also had something to do with it and I needed to discharge as well as relax.

    I would like to address this by prohibiting this kind of usage.

    Kazuto