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Charge two 2-series Li-ion battery packs in series with bq40z60

Chris Kimble
Intellectual 270 points
Other Parts Discussed in Thread: BQ40Z60

I'm considering using bq40z60 to charge two Ultralife UBBL20 battery packs, in series.  Would that work?  The UBBL20 has two cells in series, presumably well-balanced.  It looks like I could wire the "bottom" UBBL20's positive terminal to the bq40z60's VC2 pin, and thereby achieve balance between the two UBBL20s.  Does this idea make sense?   Thanks...

  • 0
    TI__Genius 10830 points
    Hi Chris,
    Your post has been assigned to one of our BMS experts.

    Regards,
    David
  • 0 in reply to David Hien
    Intellectual 270 points

    Correction--I should have said two UBBL07 batteries (3.6 V) in series.  (UBBL20 is 7.2 V.)   --Chris

  • 0
    TI__Genius 14985 points
    This will not work. The bq40z60 needs to be able to measure the cell voltage of every cell in the stack.
  • 0 in reply to dMax
    TI__Genius 14985 points
    I just saw your correction that you are using packs with cells in parallel, not series. Still, this would not be recommended for a couple of reasons. First, the UBBL07 appears to be a complete battery pack (which hopefully includes a protection circuit) and not bare cells. The bq40z family is intended to be used inside a battery pack design and connect directly to the series cell segments. The bq40z devices provide primary protection, cell balancing, and gauging. The protection inside the UBBL07 may interfere with the functionality of the bq40z60, especially if protection in one UBBL07 trips but the other one doesn't.
    Secondly, I would steer you away from the bq40z60 in general as it's tricky to use due to the integration and doesn't give you flexibility to choose the most appropriate and newer chargers and gauges.