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SMBus addressing for situatuion where pack has more than one gas gauge

Mark Arsenault
Intellectual 420 points
Other Parts Discussed in Thread: BQ40Z60

If we are forced to use more than one battery management IC in our pack because we must balance charge in parallel cells, is it possible to combine the SMBus lines of two or more Battery Management Units, like bq40z60 and address each individually? 

  • 0
    TI__Guru 73830 points
    The SMBus address is fixed at 16, but you can use an SMBus multiplexer to support multiple devices. What is the concern with balancing parallel cells?
  • 0 in reply to ThomasCosby
    Intellectual 420 points
    Regarding balancing parallel cells, because this application must meet ATEX Directive (protect employees from explosion risk in areas with an explosive atmosphere) requirements, we must measure to guarantee that none of the cells exceeds its charge/discharge specs. This includes parallel cells, even though we believe they should reach equilibrium naturally. With what I know at this point, this means we must have 3 independent series strings, each with their own battery manager IC and secondary protection to meet ATEX directive with the battery pack capacity the customer needs. I'd rather not do it this way. Is there a better approach?
  • 0 in reply to Mark Arsenault
    TI__Guru 73830 points

    Mark,

    You can have three independent packs with a bq40z60 device in each pack. The packs can be connected in parallel with the PGND signals (on the J1 connector on the bq406z60EVM schematic) connected together. The VSYS signals could be connected together, but this would result in the packs being able to charge one another in an uncontrolled manner. You can prevent this by adding a series schottky diode to each VSYS output (J5 connector) and connecting the cathodes together and to the load. This will prevent reverse current and charging one another. The VAC signals can be connected together to charge the packs.

    Does this sound reasonable?

    Tom

  • 0 in reply to ThomasCosby
    Intellectual 420 points

    Sounds reasonable. Do you have component suggestions for the SMBus multiplexer?

    Thanks,

    Mark

  • 0 in reply to Mark Arsenault
    TI__Guru 73830 points
    Mark,
    TI has several SMBus Multiplexers on the web. I have not worked with them, but other customers have used them with the bq devices.

    www.ti.com/.../universalsearch.tsp;src=top

    Tom
  • 0 in reply to ThomasCosby
    Prodigy 20 points
    Hi Tom,
    This is also what I am trying to do. Can you clarify why you say you can have 3 independent packs? What the limitation causing 3 max? I assume you also want an independent ACFET for each BQ40z60?
  • 0 in reply to Lauren Ekkens
    TI__Guru 73830 points
    Lauren
    I did not realize that I limited the number of independent packs to three. The SMBus address is fixed at 16, so you can only have one pack on the bus. If you want to have more than one pack on the bus, then you will need to use an SMBus multiplexer. Also, i worked with another customer who was using several independent bq40z60 circuits in parallel and driven by a single charger. The device may get damaged, if you run the charger without any cells attached to the bq40z60, so you will need to add a FET switch to isolate the charging voltage, if the cells are not attached to the device.
    Tom
  • 0 in reply to ThomasCosby
    Prodigy 20 points
    Hi Tom,
    We will always have cells attached but what happens if there is a fault and the fuse blows? Is there potential for damage to the BQ40z60 in this case?
    Where are you proposing to add the FET switch? On the input next to the ACFET, the output of VSYS instead of the OR'ing diodes or with the CHG/DSG FETs? What is the mechanism causing this damage?

    Thanks,
    Lauren