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BQ76940: Cell Connections for 13s Configuration

Pranit Pawar
Intellectual 520 points
Part Number: BQ76940
Other Parts Discussed in Thread: , TIDA-00792

Hi!

I'm developing a BMS for a 48.1v Li-ion battery pack. Since I'm using 13 cells connected in series, I want to know how to connect the cells to BQ76940 IC. 

Datasheet says for 13 cells, short VC14-VC13 and VC9-VC8. Does that mean I have to short pins VC14-VC13 and pins VC9-VC8 ?

Can I design the board for 15 cells and then while designing the battery pack, I will replace the required cell with a short? (in this case, replace cell 14 and cell 9 by a short)

Side A: short the Cell Input pins of the IC

Side B: replace the cells by a short

  • 0
    TI__Guru 54715 points
    Hi Pranit,
    Yes, the VCn and VCn+1 pins for the unused cell should be shorted. On a schematic with internal balancing like you picture above the board could be built for 15 cells, then the unused inputs shorted by populating a zero ohm resistor for the Cc of that cell. Omit one Rc for the unused cell input. When wiring the battery skip that unused input.
    When using external balancing the current from the balance FET path returns to the board connector, so it must be shorted also
    See the bq76940EVM user guide for example connections referring to specific component references.
    TIDA-00792 also has a discussion of reducing cell count, see figures 2 and 3 of the design guide www.ti.com/.../tiducn1.pdf and the related description.
  • 0 in reply to WM5295
    Intellectual 520 points

    What if I keep the input filter and the mosfet, but don't connect a cell across the unused cell inputs?

  • 0 in reply to Pranit Pawar
    TI__Guru 54715 points
    Hi Pranit,
    Evaluate the circuit with your planned connection in the different operating conditions. If I understand your proposal the external balancing MOSFETs would not have a strong current path but would connect back to the other VCn pin through the input filter resistor. I believe this would put the unused input filter resistor in parallel with the input filter controlling the MOSFET reducing its drive voltage. You may analyze or test that if interested, but it would not operate as intended and we would not recommend it.