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BQ76925EVM: BQ76925 on the evaluation board BQ76925EVM

Arkadi shap
Intellectual 660 points
Part Number: BQ76925EVM
Other Parts Discussed in Thread: BQ76925,

Hello,

I would like to balance 3 cells, and i want to use BQ76925. The reason that i want to use BQ76925 is because of its communication (I2C), and because i have its evaluation board (BQ76925EVM)

I dont understand from the datasheet how to do the balancing.

Could you please instruct me what exactly i have to do, to perform the balancing on the evaluation board.

I mean, I would like to connect 3 cells to the evaluation board, and i would like to see the evaluation board balancing them. 

Best Regards,

Arkadi

  • 0
    TI__Guru 54715 points
    Hi Arkadi,
    The bq76925EVM can support 3 cells, see the EVM user guide section 7.6 and any needed schematics and figures.
    The bq76925 supports balancing, and that is available on the EVM and with the evaluation software, but neither the part, nor the EVM, nor the evaluation software provide the balancing algorithm. The part does not duty cycle balancing, and does not time out or turn off balancing on its own. The controller must monitor the progress and turn off balancing when needed.
    So to balance with the EVM you would:
    Connect the cells, the section 7.6 method uses the bottom 3 cells.
    Read the voltages of the cells. The passive balancing of the EVM can only balance down.
    See the cell balancing description in the bq76925 data sheet section 8.3.3.
    Decide which cells are high and that you want to balance. Assume for example cell 1 is 30 mV higher than cells 1 and 2. You might choose cell 1 to balance.
    Enable balancing on the high cell by setting the bit in the cell balance register. For the case with cell 1 high and you want to balance it, set register 0x02 to 0x01. Balancing and measurement can happen at the same time. If you measure while the bit is set, you will measure a low voltage on cell 1 and a high voltage on cell 2.
    Periodically disable balancing by setting register 0x02 to 0x00. Allow a few ms for the filter to settle. Measure the voltages to determine effect of the balancing action. Assume you still measure the same voltages as when you started. You may decide to balance again for the same amount of time to check progress or for a longer time.
    Again write register 0x02 to 0x01. Wait some time, write register 0x02 to 0x00 and measure voltages to decide what to do.
    Following this method you see the cell 1 voltage drop over time.
    After one of the balance periods, you see cell 1 is now only 5 mV higher than the other 2 cells. You might decide this is sufficient and stop balancing, or you may continue to achieve the criteria you decide.
    Remember that the bq76925 does not stop balance on its own. If you walk away with the bq76925 cell balance register 0x02 with a non-zero value, it will eventually deplete the cell associated with the bit set.
    Also remember that EVMs are evaluation tools, they are not finished products. Don't leave EVMs unattended in operation.