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BQ76940: cell balancing timing

Electrician
Prodigy 10 points
Part Number: BQ76940

We are using bq76940 for passive balancing and we have a situation with the balancing timing.

We wish to do balancing for 3 cycles (250ms) and disable balancing for 1 cycle.

Our application code executed every 250ms scheduler as the bq76940 – but they are not necessary synchronized. So when the code disable the balancing, it sometimes cut the balancing period, causing us to lose balancing time on the 3th cycle.

Look like the command affect immediately on the device. I thought it will implement the command just after finishing the current cycle.

See below picture to emphasis the above, see that the first 2 balancing cycles are completed, and the third is cut off without finishing the balancing process:

 

  1. Is there a way we can know for sure the device has finished a cycle so we can somehow synchronize it with our code?
  2. When I configure the CELLBAL registers, will the device start balancing cycle immediately?
  3. I read Embedded Scheduler in Cell Battery Monitor of the bq769x0 – section 2.2 describes bq769x0 with single group. How it will be divided among the 3 groups? Will they all start balance together or it depends on the schedular of every group (the groups may not be synchronized according to bq769x0 Family Top 10 Design Considerations document)?

 

Thanks!

  • 0
    TI__Mastermind 25066 points

    Hello,

    The scheduler of the device is responsible for controlling the start and stop times of five voltage measurements, coulomb counting, cell balancing, and stacked communications. The scheduler takes 250-ms regardless of Cell Balancing ON or OFF. When cell balancing is OFF, the cell voltage measurements take 50-ms each. When cell balancing is ON, the cell voltage measurements take 12.5-ms each and the remaining time is reserved for cell balancing. This is best described in Figure 6, Figure 8, and Table 2 of the Embedded Scheduler in Cell Battery Monitor of the bq769x0 app note.

    1- Table-2 shows the sequence. But, when the first voltage measurement is taken, this implies the scheduler has started. Notice in table-2 how the first cycle ends at ti ck_cnt = 19 (time = ~250 ms). Then the scheduler starts again by measuring VC1 again, and so on. 

    1.1 

    Also possibly monitor the ALERT pin from CC to know when the scheduler starts and ends.

    2- Cell Balancing will occur after the first five voltage measurements have been taken every 250-ms immediately after enabling the CELLBALx register.

    2.1 These threads should clarify some doubts. Thread-1 and Thread-2

    3- The scheduler runs independently at each die and is not synchronized to each other. More details in Section 4 Scheduler in Stacking Interface of the Embedded Scheduler App note

    Regards,
    Jose Couso

  • 0 in reply to Jose Couso
    Prodigy 10 points

    Regarding #2 - after configuring the CELLBALx registers, when the device will start the balancing cycle (measure each cell for 12.5ms and then balance)? Will it start immediately after sending the command or its depend on the scheduler timeline? 

  • 0 in reply to Electrician
    TI__Mastermind 25066 points

    Hello,

    It depends on the scheduler timeline as it will measure five cells for 12.5-ms  at the beginning of the cycle and then it will balance the cells for the remaining time until it reaches 250-ms. 

    Regards,
    Jose Couso