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Re: [RoboMower] Lithium time!

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  • Kevin Turner
    Is that English? On 10 Feb 2013, at 17:01, The CZ Unit wrote: Well, it s time to start building the
    Message 1 of 7 , Feb 10, 2013
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      Is that English?

      On 10 Feb 2013, at 17:01, "The CZ Unit" <cz@...<mailto:cz@...>> 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?

      C



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    • Lee Hart
      ... 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,
      Message 2 of 7 , Feb 10, 2013
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        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
      • Danny Miller
        That d be my concern, working with li-manganese 18650 cells is pretty dangerous in larger packs. The chance of a single cell failing goes up proportionately
        Message 3 of 7 , Feb 10, 2013
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          That'd be my concern, working with li-manganese 18650 cells is pretty
          dangerous in larger packs. The chance of a single cell failing goes up
          proportionately with a great number of cells. Normally, with a
          separator failure in a single cell, the most heat it can generate is the
          energy from what's in that cell alone. But with a macrocell of
          paralleled cells, it'll absorb the energy of all of them, and fire is
          more likely. A thermal runaway where one cell ignites its neighbors and
          inevitably the whole pack is also likely.

          Once you build a pack out of >50 cells of cheap li-mn, the chance of
          catastrophic, intense fire is just too likely.

          Danny

          On 2/10/2013 1:26 PM, Lee Hart wrote:
          > 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!
        • The CZ Unit
          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
          Message 4 of 7 , Feb 10, 2013
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            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.

            However for the Robomower I would go with larger size cells, I was
            thinking Headways at first, a 10ah cell would work pretty well, 8
            needed. Or I could go with 10ah-15ah prismatics, not sure what would be
            best there.

            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?

            C
          • Lee Hart
            ... 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
            Message 5 of 7 , 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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