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BQ76952: Auto-Shutdown happening during discharge

Matthew Zilvetti
Prodigy 50 points
Part Number: BQ76952

Tool/software:

When waking our pack from shutdown mode, we have noticed that if there are no communications the pakc re-enters shutdown, even if there is a discharge current applied (2.8A is what was tested). We are able to replicate this with the following timing:

  1. Apply 33.6V, 0.05A setting to power supply
  2. Turn on power supply for 2 seconds, then turn off
  3. Turn on attached Electronic Load set at 2.8A
  4. Verify that current is being drawn
  5. Verify approximately 1min after discharge has started, the battery voltage drops to 0V

We are using the BQ76952 in an 8s1p battery pack with the following Shutdown settings:

  • Shutdown Cell Voltage: 2700mV
  • Shutdown Stack Voltage: 21600mV
  • Low V shutdown Delay: 1s
  • Shutdown Temperature: 85C
  • Shutdown Temperature Delay: 5s
  • FET Off Delay: 1s (4*0.25s)
  • Shutdown Command Delay: 0.25s
  • Auto Shutdown Time: 1 min
  • RAM Fail Shutdown Time: 1s

We also have the other settings that may be relevant:

  • Sleep Current: 20mA
  • TS2 Configuration: Thermistor (used for FET measurement)

According to the TRM, a discharge current above the sleep current threshold should prevent the BMS from going back into shutdown. Is there something else that we are missing here?

  • 0
    TI__Genius 17835 points

    Hello Matthew,

    If TS2 is being used as a thermistor, the device is not intended to enter SHUTDOWN mode. If you would like the device to enter SHUTDOWN mode, the TS2 pin should be left floating or else when the SHUTDOWN sequence is initiated, it will get stuck in a Soft Shutdown State. 

    Is the REG18 voltage measuring 0V when the pack re-enters shutdown? 

    More information can be found here regarding SHUTDOWN and SOFT SHUTDOWN:

    1) SHUTDOWN FAQ
    2    ) SOFT SHUTDOWN FAQ


    Best Regards,
    Alexis

  • 0 in reply to Alexis_H
    Prodigy 50 points

    Hi Alexis, thank you for the information.

    Probing the REG18 pin showed that it is measuring 1.8V, meaning that the pack is in Soft Shutdown state. However, I looked in the Soft Shutdown FAQ, and it said that it is not possible to exit the soft shutdown state unless a soft reset is performed. We have never performed a soft reset (Our battery pack is enclosed, so that pin is not accessible), yet our battery pack leaves the shutdown condition for some reason. Can you provide more information into what ways soft shutdown would be exited?

    Thanks

  • 0 in reply to Matthew Zilvetti
    TI__Mastermind 42060 points

    Hello Matthew,

    The only other way to exit soft-shutdown is for the device to be allowed to SHUTDOWN and then wake it up again.

    The FAQ shared above provides all known details on soft-shutdown.

    Best Regards,

    Luis Hernandez Salomon

  • 0 in reply to Luis H. S.
    Prodigy 50 points

    Hi Luis,

    If soft-shutdown mode can only be exited with a soft reset or a full shutdown, then our packs must be entering into some other mode. We have seen that the packs are able to fully function after receiving a wake up charge, and communicate over I2C. Are there another possible mode that we are entering instead?

    Thanks,

    -Matt Zilvetti