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BQ77908 Doubt

Douglas Malouf
Intellectual 405 points
Other Parts Discussed in Thread: BQ77908A, BQ76PL536A, BQ34Z100, BQ76PL536, BQ29200, BQ77910A, BQ77PL157A4225

Dear All,

I am designing a battery pack composed of 15 series LiFePO4 cells, each one of 20Ah for a small UPS system.

As for the cell balancing function, I intend to employ two IC BQ77908A to monitor the 15 cells. Nevertheless, as the IC supports up to 8 series cells, I would like to know if is possible to configure a stackable arrangement of both ICs, in order to monitor the 15 cells.

I have checked the possibility to use IC BQ76PL536A to perform such monitoring, but this last one needs external components to balancing, which may increase the costs.

The design takes into account IC 77PL157A4225 for secondary protection and BQ34Z100 for pack fuel gauge.

So, please let me know if the stackable configuration of BQ77908 is possible.

Thank you in advance.

  • 0
    TI__Guru 54715 points

    You might look at www.ti.com/lit/pdf/slua637 linked in the product folder for ideas.  The 2 main challenges are combining the outputs of the individual devices into a single FET drive and balancing across the groups.. 

    A high side switch may be most desired when used with a gauge which needs communication during protection.

    The 2 devices shown in the apnote operate independently for balancing.  Good balance is important for gauging on the bq34z100, You will need some way to maintain balance between the 2 groups.  Some users have considered overlapping a cell, this will load that cell with both parts operating current.  Overlapping each group with a bq29200 may be an option, but it may need some control. A bq76PL536 design would address the balance across all cells, but would need a separate current control if that is needed.  There may be future devices depending on your timeframe.

    For a secondary protector circuit with other options, you may also consider the bq771800 series devices..

     

  • 0 in reply to WM5295
    Intellectual 405 points

    Thank you WM!

    Please help me with the follwing issues:

    1. When applying two bq77910A for 15 cells balancing, is it necesary to have individual ground references for differents chips?

    2. The I2C signal of both bq77910A can be connected to a same isolator, such as ISO7421DW? 

    3. For each cell, a Rin and Cin for bq77PL157A4225 and a Rvcx and Cvcx can be connected in parallel?

    Thank you again for you support.

  • 0 in reply to Douglas Malouf
    TI__Guru 54715 points

    1. Yes, each IC will have its own reference.  The upper IC's "ground" reference will be at a significant potential from the lower IC's.

    2. The bq77908A or bq77910A are standalone protectors and do not operate I2C in the system, so you may not need the isolation on the battery board but only in the test/programming fixture.  TI has the ISO154 which will work for I2C.  Since the IC's operate at different reference potentials, the pins can not be connected together and to a common isolator, the isolator would need to be switched or one per IC.  If you maintain one device at your external ground reference, the other would need the isolator.  If you can connect to and talk to only one device at a time with your fixture and all other equipment is isolated, perhaps you don't need the isolator.  Consider your requirements carefully and make appropriate choices.

    3. When used with a secondary protector, you typically want the circuits to share as little circuitry as possible.  Both ICs would monitor the cells with their own filter networks; separate Rvcx and Cvcx.  If there was damage to or failure of the primary protector, the secondary would still be functional to protect the battery from overvoltage if the need arose.