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18442Re: [RoboMower] Lithium time!

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  • Lee Hart
    Feb 10, 2013
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      On 2/10/2013 7:22 PM, The CZ Unit wrote:
      > For the Roombas (14.4 volt packs) I went with eight Sony 18650 type
      > cells with 3.0ah capacity. These came out of some older Sony laptop
      > battery packs; "dead" usually means the batteries just drifted a bit.

      More often, I find one of the cells (or parallel pairs of cells) has
      gone bad. The rest may still be good if you catch them soon enough
      (before they go dead too, just from sitting).

      > I like Lee's thought of using one big battery and a DC-DC, but the
      > problem there is the primary side would need to handle something like
      > 80-100 amps to provide 10a on the output side. Does there exist a DC-DC
      > that could do this?

      100 amp MOSFETs are only a couple dollars, and there are dozens of
      switchmode controller ICs that could be used. The main parts that
      require careful selection are the MOSFET, output diode, and inductor.

      If I were designing it, I'd build it as two twin converters, running out
      of phase so one was on while the other was off. This minimizes input and
      output ripple.

      Also, since I know the boost ratio, the inductor would be tapped to act
      as an autotransformer. This keeps the duty cycle on the MOSFETs and
      diodes around 50% even though you're boosting about 8:1.

      --
      If we knew what we were doing, it wouldn't be called research, would it?
      -- Albert Einstein
      --
      Lee A. Hart, http://www.sunrise-ev.com/LeesEVs.htm
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