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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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    • 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 2 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 3 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 4 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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