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BQ77910A Balancing error

Tobias Ofenberger1
Prodigy 50 points
Other Parts Discussed in Thread: BQ77910A

Dear Texas Instruments team,

I'm a student at a federal higher technical educational institution of electronics in Austria. I'm working on a diploma project in a small team. We develop a prototype of a mobile Power supply for notebooks which works with batteries, accumulators or fuel cells. The battery pack require a Battery Management System. We have already developed a BMS circuit for the bq77910A IC.

Now we have a problem with the balancing algorithm. According to the datasheet, the IC will balance the cell with the highest voltage first.

We have been testing the cell balancing function of the BQ77910A. I have included a graph below showing how the balancing works. I also included the EEPROM registers.

I could not find a tolerance specification in the data sheet for the part. I understand the part compares all the cells and balances the highest cell, for dwell time then checks again. But in our case the IC balances also the lowest cell. This is unexplainable for us, because we have no influences to the balancing algorithm. This problem occurs by the EVM board as well by our own circuit.

Kind regards,
Tobias Ofenberger

  • 0 in reply to Tobias Ofenberger1
    TI__Guru 54715 points

    The part knows the cell voltages according to its internal calibration for each cell input, which will have some tolerance at the factory calibration point.  As the voltage moves away from the calibration point, other tolerances will come into play.  The most relevant specification would be the "OV detection threshold accuracy", notice how the tolerance increases as voltage drops, and the tolerance of the UV threshold is larger.  The allowed cell voltage spread would be large, practically it could be <10 mV at the calibration point, larger as the balance voltage moves further away from that point.  Your charted part looks like it has ~20 mV spread.

    The balance algorithm in the part is simplistic.  It will keep the cells from becoming severely imbalanced, but it will also take perfectly balanced cells and spread their voltages to the part's calibration differences.