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BQ76940: (bq769x0) - Is it possible that cell balancing depletes cell completely?

Juhamatti Nikander
Prodigy 90 points
Part Number: BQ76940

Hello,

bq769x0 datasheet (SLUSBK2G) does not mention how cell balancing is controlled by the chip.

Only the following is mentioned on page 29:

All cell balancing control bits in CELLBAL1, CELLBAL2, and CELLBAL3 are automatically
cleared under the following events, and must be explicitly re-written by the host
microcontroller following clearing of the event:

• DEVICE_XREADY is set
• Enters NORMAL mode from SHIP mode

Question 1:

Does the bq769x0 chip disable cell balancing if chip enters from NORMAL mode to SHIP mode?

Question 2:

Does the bq769x0 chip disable cell balancing if fault is detected: for example, UV, OV, SCD, OCD, or are the balancing transistor controlled only by software commands?

Question 3:

Is it possible to keep cell balancing on until cell voltage becomes zero volts?

I am trying to figure out if there is a such failure mode that would leave the balancing transistor on that would cause the battery cell to be discharged fully and, thus, brick the whole battery. This could happen if there is, for example, 1) software bug that stops communication between bq769x0 chip and host microcontroller, or 2) software bug in the balancing algorithm, or 3) host microcontroller failure.

Best regards,

JM

  • 0
    TI__Guru* 81035 points

    Hi JM,

    Cell balancing is enabled / disabled by the host controller.

    Question 1:

    Does the bq769x0 chip disable cell balancing if chip enters from NORMAL mode to SHIP mode? The device is in full Shutdown when in SHIP mode. The only active circuits are the VSTUP power supply and the boot detector. No cell balancing can take place in SHIP mode.

    Question 2:

    Does the bq769x0 chip disable cell balancing if fault is detected: for example, UV, OV, SCD, OCD, or are the balancing transistor controlled only by software commands? The host controller must decide when to balance. Balancing is not automatically disabled during UV, OV, SCD, or OCD.

    Question 3:

    Is it possible to keep cell balancing on until cell voltage becomes zero volts? The device will enter SHIP mode due to a POR event before cell reach zero volts. However, it is really not safe to discharge cells to this level and it is not recommended to operate the device near POR.

    These document might help to clarify some understanding of cell balancing for the BQ769x0:

    Section 9 of the FAQ: http://www.ti.com/lit/an/slua749a/slua749a.pdf

    Section 4 of the Top Ten Design Considerations app note: http://www.ti.com/lit/ug/sluub41/sluub41.pdf

    Embedded scheduler app note: http://www.ti.com/lit/an/slua775/slua775.pdf

    Best regards,

    Matt