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BQ78350: BMS won't re-enable FETs

Christian Johnson
Prodigy 20 points
Part Number: BQ78350
Other Parts Discussed in Thread: BQ76940,

We're having a problem with a BMS BQ78350 gas gauge paired with a BQ76940 analog front end connected to a 14 cell pack.  The problem is that we had a working serial production battery pack that seemed to be working fine.  We had an "unknown  event" that tripped the BMS protection and opened the protection FETs.  It's likely that it was an over current event, but I don't know that for a fact at this point.  When the pack was removed from the system, the BMS will not recover and will not close the charge or discharge FETs. 

Things we've tried:

Connecting with battery management studio doesn't show any obvious issues: 

We've tried resetting the BQ76940 with the boot button.

We've tried manually controlling the FETs by issuing the FET_EN command followed by a 0x001F (CHG FET) and 0x0020 (DSG FET) with no results.

Can someone help point me in the right direction with regards to:

  1. What would be some common problems that would cause a working pack to exhibit this kind of failure?
  2. It there anything in Battery Management Studio that will tell us what's going on with the BMS and why it won't close the FETs?
  3. Is there anything we can set/reset through the Battery Management Studio that would reactivate the normal operating mode?

 

  • 0
    TI__Guru 73830 points
    The SAFE flag is set ad this will disable the FETs. You can try sending a RESET command to see whether it clears. Could you have accidentally sent the SAFE TOGGLE (1D) command?
  • 0 in reply to ThomasCosby
    Prodigy 20 points

    As I read the datasheet, the SAFE signal is a indication of the state of the SAFE pin.  On this design the SAFE pin is not connected on the PCB, and none of the PF SAFE A,B,C bits are set in the program memory.  Regardless, I used the manufacturing modes to toggle the SAFE status with a SAFE_EN (0x0026) command followed by a SAFE_TOGGLE (0x001D) to clear the signal and another SAFE_EN (0x0026) to clear the mode.  There was no change in behavior from the pack.

    Finally, I send the RESET command through battery management studio and still no change in behavior.  Do you have any other suggestions on things to try?

  • 0 in reply to Christian Johnson
    TI__Guru 73830 points
    The screenshot does not yield any more clues as to why the FETs are disabled. Please export an srec from the pack and I can check it to see whether there is any useful lifetime or PF data that will help.