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BQ76952: 14S of 30Ah battery, 60A (2C) discharge, CUV setting is 2.83V, but no CUV protection occurs until 2.4V is discharged.

jmyang
Expert 1186 points
Part Number: BQ76952

TI experts,

               14S of 30Ah battery, 60A (2C) discharge, CUV setting is 2.83V, but no CUV protection occurs until 2.4V is discharged.

               The battery samples are about to be shipped to customers. Can you help me see what the problem is?

              

               14S-bq76952-0101#-60A-DIS-2021-10-31.xlsx

              14S30Ah-76952-0-36-10bei-0.2mR-R00-2021-10-24(CRC).gg.csv

  • 0
    Expert 1186 points

    This is when we used an oscilloscope to capture the P+/P- waveform, and found that there was a CUV time of about 8ms, and then the voltage quickly recovered.

     

  • 0
    TI__Mastermind 42060 points

    Hello Jmyang,

    I look into your schematic and wave and get you a response by tomorrow.

    Best Regards,

    Luis Hernandez Salomon

  • 0 in reply to Luis H. S.
    TI__Mastermind 42060 points

    Hello Jmyang,

    I apologize for the delayed response. I reviewed your .gg settings and log.

    You set your cell/stack shutdown voltage to be equal to your CUV threshold, so when you reach a cell voltage of 2.8-V your part starts going into SHUTDOWN. SHUTDOWN by cell/stack voltage has a delay of 60 seconds, however, because you are discharging (and therefore have a voltage in LD), the part is waiting for discharge to finish to turn-off. I believe this is what's causing the odd behavior in your part, as these two parameters were never meant to be set equal. 

    Something else I noticed, your OTD protection is being triggered as you are reaching a high FET temperature during the end of the discharge. I believe you should take this into account as it would also led to the CHG/DSG FETs turning off. I would either increase the OTD protection temperature or make sure your FETs don't reach this temperature during normal operation.

    Best Regards,

    Luis Hernandez Salomon

  • 0 in reply to Luis H. S.
    Expert 1186 points

    Hello Luis,

            My CUV is set to 2.83V/5 seconds, and the shutdown is set to 2.8V/60 seconds.

            In theory, CUV should always be triggered first. However, my problem is that BMS cannot trigger CUV protection.

            Until the cell voltage reaches 2.2V. Only manually cut off the load later.

            And my other battery pack uses the same BMS, but the capacity of the battery cell is increased to 60Ah.

           The same 60A discharge can trigger the CUV protection normally, and even the 120A discharge can trigger the CUV protection.


           This problem seems to be related to the capacity of the battery. If the capacity is low, there will be no CUV protection.

  • 0 in reply to jmyang
    TI__Guru* 81034 points

    Hi JM,

    I think it is important to lower the Shutdown voltage. It may also be good to decrease the CUV Delay time. We think the problem may be that Shutdown is being triggered before the CUV Delay time is completed. Once Shutdown is triggered, the Shutdown sequence starts so the device is just waiting on the Shutdown time at that point. Normally, users set the cell Shutdown voltage to something like 1.5V. Shutdown is not a protection feature (CUV and SUV protections are for this) - you want the monitor to stay awake to control the FETs and to recover if a charger is connected. Shutdown should only be initiated when the battery voltage becomes critically low.

    Best regards,

    Matt

  • 0 in reply to Matt Sunna
    Expert 1186 points

    Hi Matt,

     Thank you, the 33rd pin (RST_SHUT) of my bq76952 is a 0R resistor connected to the GND of the bq76952 chip.

     It seems that the GND line is too far away from the B- of the battery pack, so when the discharge FET is turned off, the GND is oscillating.

     I now connect B- and the 33rd pin (RST_SHUT) with a copper wire, and the problem does not occur.

     In the same way, three BMSs were modified, and there was no problem when installed to the battery pack test.

  • 0 in reply to jmyang
    TI__Mastermind 42060 points

    Hello Jmyang,

    I am glad that you were able to fix that issue. However, we still would recommend you to lower the SHUTDOWN cell and stack voltage to what we recommended earlier. If left as is, it could lead to problems down the road.

    Best Regards,

    Luis Hernandez Salomon