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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 9:27 AM
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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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    • wolfbob
      Be very leery of Chinese cells. If you choose to use them be sure to balance the cells both before you put them in series and before you put the strings in
      Message 2 of 7 , Feb 10 9:54 AM
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        Be very leery of Chinese cells. If you choose to use them be sure to balance the cells both before you put them in series and before you put the strings in parallel. They tend to come with all levels of charge and will really mess up a pack of 30 or so cells. I have found that Sanyo cells are ready to go without balancing but they cost around $4/cell. I suspect that Panasonic and Sony cells are also of this quality, but I have not used them. Using a balancing charger will take quite a bit longer to charge and you really don't need to do it every cycle. Balance every 10-20 cycles.

        Also don't be fooled. Some "balancing" chargers don't really balance they just shut off the entire charge when one cell gets to an overcharge voltage. The battery will only be partially charged in this case and a real balancing charger will load or discharge that over voltage cell and continue to charge the remaining cells and won't quit until the are all under overload and charged. That is why it takes longer to charge.

        Another idea is to use a 12 volt model airplane charger designed to charge most anything up to 10S (37 volts):
        http://www.hobbypartz.com/75p-1220-charger.html
        and power it from a deep discharge 12 storage battery (Costco $90) charging from a 45 watt solar system (Harbor Freight $150). This will allow the automatic mowers to operate autonomously without any AC power.

        WBob

        ----- Original Message -----
        From: The CZ Unit
        To: RoboMower@yahoogroups.com
        Sent: Sunday, February 10, 2013 9:01 AM
        Subject: [RoboMower] Lithium time!



        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




        [Non-text portions of this message have been removed]
      • 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 3 of 7 , Feb 10 11:26 AM
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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 4 of 7 , Feb 10 3:21 PM
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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 5 of 7 , Feb 10 5:22 PM
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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 6 of 7 , Feb 10 6:10 PM
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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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