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18438Re: [RoboMower] Lithium time!

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  • Lee Hart
    Feb 10, 2013
      On 2/10/2013 11:01 AM, The CZ Unit wrote:
      > Well, it's time to start building the packs for my Robomowers. I'm
      > thinking Lithium, 10-15ah capacity at 24 volts. Over the winter I have
      > converted my Roombas and Scooba to 18650 type cells, and they work *great*.
      > The BMS I am using is simple monitoring, it's up on Ebay, up to 8ah load
      > capability with per cell monitoring. Since the Robomower doesn't pull
      > that much current, a pair of 4 cell monitors should do it.
      > Anyone with thoughts?

      Hi Christopher,

      It seems like a good idea. It's something I should do myself! I actually
      have a pair of Enersys/Hawker "Genesis" AGMs in my Robomower now, and
      they're likely to last for years. But lithium plays right into people's
      "I want it" mentality. So, it's something I need to get experience with,
      if I ever expect to sell anything to anyone.

      The trouble is, stuff for lithium sells no matter how badly done it is.
      :-/ They make big promises, it fails in a year, and the seller sneaks
      off into the night with your money.

      Is this a project you're just doing for yourself, or something you want
      to sell to others?

      If it's to sell to others, is this going to be a cheap "make a fast
      buck" product? Or something that costs more and is supposed to last?

      Most of the 18650 cells I've tested are only good for 1-3 years of life.
      Is that OK? They seem to be the best option for a quick-n-dirty project.
      They also burn ferociously if misused. How much consideration do you
      want to give to safety?

      The A123 cells last a lot longer, but also cost more. We could build a
      really nice battery pack with them that could last 5-10 years.

      Frankly, I think the Robomower would be better done with a few large
      cells rather than many small ones. This greatly simplifies the charger
      and BMS design. In the extreme, it is perfectly practical to use *one*
      3.2v 100ah cell with a 3v-to-24v boost converter. This makes the charger
      and BMS issues essentially disappear!

      An engineer can do for a nickel what any damn fool can do for a dollar.
      -- Henry Ford
      Lee A. Hart, http://www.sunrise-ev.com/LeesEVs.htm
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