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BQ76940: BQ76940 balancing during fault

Martin Haberzettl
Prodigy 40 points
Part Number: BQ76940

Hello everyone,

I would like to use the BQ76940 Battery management IC for a 14 S Li-ion System.

I am concerned about how the chip handles the balancing while a fault condition is active:

  • Does it stop when an undervoltage fault is encountered?
  • would the same be true for an externally triggered OVRD_ALERT fault by the alert pin?

This is of course especially important if communication to the host controller was lost, as the balancing would damage the cells after some time.

I have not found the answer within the datasheets or this forum. Just that the CHG and DSG FETs are disabled during a fault.
It would be great if someone could clarify this.

best regards
Martin

  • 0
    TI__Guru 54715 points
    Hi Martin,
    The bq76940 will continue to balance during faults. Balance does not stop with under voltage or OVRD_ALERT. Balancing stops by command of the host or on shutdown.
  • 0 in reply to WM5295
    Prodigy 40 points

    Thank you very much for the clarification.

    Is there any other reliable way to shut it down or bring the IC in SHIP mode besides by the I2C interface?

  • 0 in reply to Martin Haberzettl
    TI__Guru 54715 points
    Hi Martin,
    If the bus is stuck with SDA low send up to 9 clocks for the device to release the SDA line. The part has no reset pin as you have noticed. It will shut down if the supply voltage goes below the VSHUT threshold, but if 1 cell is left balancing it is probable that that cell could deplete without the supply voltage reaching VSHUT. Opening the supply pins may not allow the supply voltage to drop sufficiently due to internal leakage paths. Forcing the supply pin low with cells attached may damage the IC, note the diodes in the section 8 schematics to prevent this situation. The way to reset the part is to disconnect cells and re-attach.
  • 0 in reply to WM5295
    Prodigy 40 points
    Thank you again for your kind and fast reply.
    This helps me a lot in my design process.

    I could imagine this may be an issue in many design considerations.
    If i may suggest: Stopping the balancing during a fault could be a nice additional feature for your future products
  • 0 in reply to Martin Haberzettl
    Genius 5010 points
    Hi TI Expert,
    I am facing the same problem~
    Cause BQ76940 and CPU 's supply voltage is seperate,when CPU is power off before sending commends to bring the IC to ship mode,
    what will be affected?
    Will the actual current of every cell be the same as each other, Or how much differences between them?
  • 0 in reply to Qianqian
    TI__Guru 54715 points
    Hi Fawn,
    If the MCU goes to sleep the bq76940 will continue to operate in the configuration it was left:
    OCD/SCD will continue to be monitored.
    If the ADC is enabled OV and UV will continue to be checked.
    If cell balancing is running it will continue and can completely discharge cells.
    If ALERT is set the bottom group will draw more current, see the dIALERT specification in the data sheet. Otherwise the difference in load on the cells could vary by the dINOM and ILKG specifications.