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After 3 years live the secondary comparator triggers at the wrong levels

Jana Ray
Prodigy 90 points
Part Number: BQ76PL536

I am using 4 Stacked BQ76PL536 in combination with a TM4C123G controller on a 14s battery system in a 4,4,3,3 configuration. The system is online since late 2015, with some longer offtimes during winter.

As i tested one battery, in total with 140 cells in series so 10 BMS boards, i recognised that the system is tripping overvoltage with the secondary protection but the cell voltages a actually lower.

The AFE is set 4.2V and 1000ms delay. This was verified in multiple tests and worked for 3 years. Just now the OV bits and the secondary protection are tripped before the voltage of 4.2V is reached.

We are talking about voltages between 4.12V to 4.18V depending on the boards, most of the time if i am rebooting (reinitialize and readdressing the AFEs) the cells which are indicating a OV are changing. They are not even the highest cells. CellVoltages are acccurate as always within +-2mV. For now i have lifted the OV Limit to 4.4V, but this is not acceptable since all the hardware safety circuits are now inactive and the safety relies on the software.

So what is happening here? I have no idea what to do! Let me know what data you need to know so we can figure this out.

  • 0
    TI__Genius 9401 points

    Hi Jana,

    Welcome to our forum. Can you please provide more details on the application/system where these devices are being used? I'll review the information you provided and let you know what extra information we need if any. 

    Best regards,

    Leslie

  • 0
    TI__Genius 10785 points

    HI Ray,

    Sorry to hear that.

    Are you charging or discharging? Or you see the issue in idle mode?

    200mV is big change. our protector accuracy is much better than this.

    Have you restart and rewrite the register?

    Or, is it the OTP programming?

    roger

     

  • 0 in reply to Leslie Marquez
    Prodigy 90 points

    Leslie Marquez Arroyo43 said:

    Hi Jana,

    Welcome to our forum. Can you please provide more details on the application/system where these devices are being used? I'll review the information you provided and let you know what extra information we need if any. 

    Best regards,

    Leslie

    The Devices are used in an electric vehicle, temperatures are never below 10°C and limit to 70°C.

  • 0 in reply to Roger Hwang
    Prodigy 90 points

    Roger Hwang said:

    HI Ray,

    Sorry to hear that.

    Are you charging or discharging? Or you see the issue in idle mode?

    200mV is big change. our protector accuracy is much better than this.

    Have you restart and rewrite the register?

    Or, is it the OTP programming?

    roger

    The variance is between 20mV to 80mV like i described in the first post. The error first occurred while charging, but is still present in IDLE. First i thought it was caused by some charging problems, but the problem is also preset with boards which are not live at the time.

    Yes restarting and reprogramming, we are not using OTP

  • 0 in reply to Jana Ray
    TI__Genius 10785 points

    Hi Jana,

    OV is simple comparator.

     you cell config is 4,4,3,3 for 14S.

    Can you tell me what you do with unused cells?

    Also let me know which cell is having problem.

    What about adjacent cells? 

    Can you also provide cell voltages and measurements as well?

    Roger