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Re: [RoboMower] Switching to lithiums for RL500

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  • wolfbob
    Unless you have a better source, I recommend using the very popular 18650 LiIon cells these are available at 2600mAh for under $4 from a number of non-Chinese
    Message 1 of 8 , Jan 4, 2013
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      Unless you have a better source, I recommend using the very popular 18650 LiIon cells these are available at 2600mAh for under $4 from a number of non-Chinese manufacturers like Sanyo. For more money you can get them rated to 3200 mAh but ...that is a lot of hype. I have found in my re-celling business that the Chinese batteries will not stay balanced for more than two cycles, but the Japanese and Korean ones are good for several hundred cycles.

      I don't remember but I think the RL500 etc. draw around 5A for 2 hours so 4 strings will be useful and 6 strings will run "forever". With 7 cells /string and 4 strings you are looking at 28 cells or about $120 in cells. I've had no issues paralleling cells and recommend that the strings be strapped at each cell. Even at 6 strings of 8 cells you are looking at less than $200 for cells. When you figure the life is 300 cycles (like 3-6 years mowing) they are not a bad investment compared to SLAs.

      My application has a 1.5 A drain and two 18650 cells in series will work fine giving useful voltage between 4 and 3 volts per cell. They are hard to charge if they get under 2.5 v as the BMS will shut the thing down to protect against a shorted cell (no balancing available). The problem gets worse with 6 or 8 cells in series and I think you must balance every cycle at 18-24v (6 cells) or 21-28v (7 cells) or 24-32v (8 cells). My chargers are set for 1.5A cc to 4.2v cv to 20 mA per cell.

      I would over ride the internal charger to assure that you have a cc-cv charging system with balancing.

      There might be some temperature storage issues if you store the mower at outside temps. I seem to recall that LiIon batteries don't do well at low temps. My applications are all at room temps.

      WBob


      ----- Original Message -----
      From: Christopher Zach
      To: RoboMower@yahoogroups.com
      Sent: Friday, January 04, 2013 7:30 AM
      Subject: [RoboMower] Switching to lithiums for RL500



      Last year I tried running a set of 20ah A123 pouch cells in my RL500
      with a BMS on them. Not only did they work, the biggest problem was that
      I couldn't run them down. Mower just kept going and going....

      It convinced me that Lithium is a way to go, keeping one old dead lead
      battery in there as ballast. However the A123 cells are too tall and
      have *way* too much power, so I'm going to try something else.

      Anyone thought about building a pack of smaller 3ah A123 cylinder cells
      in parallel? A 17ah lead pack will put out only around 12ah under load
      tops, so 4 strings of 3ah cells would do it nicely. 3.6 volt cells would
      allow a 7 cell string to provide 24+ volts, 8 cells would make the
      motors *sing* at 28 volts but might be too big to charge with the
      on-board charger.

      I'm going to start work this winter, to have something for March/April.

      C




      [Non-text portions of this message have been removed]
    • Danny Miller
      Unfortunately the question is now of VOLUMETRIC energy density. And you have a problem. The 12v 20ah half-U1 lead acid batts you re replacing are 1.72166
      Message 2 of 8 , Jan 4, 2013
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        Unfortunately the question is now of VOLUMETRIC energy density. And you
        have a problem.

        The 12v 20ah half-U1 lead acid batts you're replacing are 1.72166
        watt-hr/cu in.

        Now a large-format rectangular lifepo4 cell can be 2.1 watt-hr/cu in.
        The 26650 2.3AH A123's come to 1.44 wh/cu in.

        The problem is the old case has almost 100% utilization of the volume
        inside the pack. Cylindrical cells have considerable airspace AND will
        have a remainder of space on the sides because the sizes won't line up
        just right with the volume.

        Rectangular cells will likely not be able to use all the space either.
        And you have to have exactly 8 cells, not 7, not 9. You can't just add
        as many cells as the pack can hold, if you can only fit 7 with the "big"
        size a mfg has, so you select a much smaller one and you see you can
        stack 8 but there's still room for 2 on the side and one flat across the
        top, you will still have to just use 8 and ignore the extra volume.

        It's just very likely you're going to have a significant volume problem
        with any arbitrary LiFePO4 shape (but rectangulars will usually make
        much better use of the space).
        So rectangulars are 22% denser by volume, but having a case that's only
        2/3rd battery and 1/3rd air would be a pretty lucky fit the way I see
        it. It will be less dense. Of course lead-acid always had trouble
        actually making the rated capacity repeatedly so the energy density for
        lead-acid was a bit optimistic.

        A123 LiFePO4 is 3.3v per cell not 3.6v, you need 8 for 24v. That's
        26.4v, or 13.2v per "batt" on the old lead-acid pair, a LITTLE high but
        that's the voltage you see when it's on the charger. Most other mfgs
        rate their cells at 3.2v though.

        For rectangulars go to Thundersky/Winston batt, or better yet CALB batt.

        I don't know of an A123 "3AH" cell. The common 26650 A123 is 2.3AH.
        They do make a AHR32113 cell which is 4.4AH in their gen-2 version of
        that cell, but those would be hard to obtain anyways, they're not sold
        on ebay or online or anything.

        Danny

        On 1/4/2013 9:30 AM, Christopher Zach wrote:
        > Last year I tried running a set of 20ah A123 pouch cells in my RL500
        > with a BMS on them. Not only did they work, the biggest problem was that
        > I couldn't run them down. Mower just kept going and going....
        >
        > It convinced me that Lithium is a way to go, keeping one old dead lead
        > battery in there as ballast. However the A123 cells are too tall and
        > have *way* too much power, so I'm going to try something else.
        >
        > Anyone thought about building a pack of smaller 3ah A123 cylinder cells
        > in parallel? A 17ah lead pack will put out only around 12ah under load
        > tops, so 4 strings of 3ah cells would do it nicely. 3.6 volt cells would
        > allow a 7 cell string to provide 24+ volts, 8 cells would make the
        > motors *sing* at 28 volts but might be too big to charge with the
        > on-board charger.
        >
        > I'm going to start work this winter, to have something for March/April.
        >
        > C
        >
        >
        > ------------------------------------
        >
        > Yahoo! Groups Links
        >
        >
        >
        >
      • wolfbob
        I don t understand the problem, Danny I took a picture showing all the 18650s I have (90) only filling about 60% of the RL500 battery compartment. Even with
        Message 3 of 8 , Jan 4, 2013
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          I don't understand the problem, Danny I

          took a picture showing all the 18650s I have (90) only filling about 60% of the RL500 battery compartment. Even with the charger, BMS and some lead weights in there it should be no problem to get 24 volts 20Ah. The SLAs as you point out only furnish around 10Ah out of the 17 Ah rating and have any life.

          Gotta run now, but I can post the picture when I figure out how..

          WBob

          ----- Original Message -----
          From: Danny Miller
          To: RoboMower@yahoogroups.com
          Sent: Friday, January 04, 2013 11:36 AM
          Subject: Re: [RoboMower] Switching to lithiums for RL500



          Unfortunately the question is now of VOLUMETRIC energy density. And you
          have a problem.

          The 12v 20ah half-U1 lead acid batts you're replacing are 1.72166
          watt-hr/cu in.

          Now a large-format rectangular lifepo4 cell can be 2.1 watt-hr/cu in.
          The 26650 2.3AH A123's come to 1.44 wh/cu in.

          The problem is the old case has almost 100% utilization of the volume
          inside the pack. Cylindrical cells have considerable airspace AND will
          have a remainder of space on the sides because the sizes won't line up
          just right with the volume.

          Rectangular cells will likely not be able to use all the space either.
          And you have to have exactly 8 cells, not 7, not 9. You can't just add
          as many cells as the pack can hold, if you can only fit 7 with the "big"
          size a mfg has, so you select a much smaller one and you see you can
          stack 8 but there's still room for 2 on the side and one flat across the
          top, you will still have to just use 8 and ignore the extra volume.

          It's just very likely you're going to have a significant volume problem
          with any arbitrary LiFePO4 shape (but rectangulars will usually make
          much better use of the space).
          So rectangulars are 22% denser by volume, but having a case that's only
          2/3rd battery and 1/3rd air would be a pretty lucky fit the way I see
          it. It will be less dense. Of course lead-acid always had trouble
          actually making the rated capacity repeatedly so the energy density for
          lead-acid was a bit optimistic.

          A123 LiFePO4 is 3.3v per cell not 3.6v, you need 8 for 24v. That's
          26.4v, or 13.2v per "batt" on the old lead-acid pair, a LITTLE high but
          that's the voltage you see when it's on the charger. Most other mfgs
          rate their cells at 3.2v though.

          For rectangulars go to Thundersky/Winston batt, or better yet CALB batt.

          I don't know of an A123 "3AH" cell. The common 26650 A123 is 2.3AH.
          They do make a AHR32113 cell which is 4.4AH in their gen-2 version of
          that cell, but those would be hard to obtain anyways, they're not sold
          on ebay or online or anything.

          Danny

          On 1/4/2013 9:30 AM, Christopher Zach wrote:
          > Last year I tried running a set of 20ah A123 pouch cells in my RL500
          > with a BMS on them. Not only did they work, the biggest problem was that
          > I couldn't run them down. Mower just kept going and going....
          >
          > It convinced me that Lithium is a way to go, keeping one old dead lead
          > battery in there as ballast. However the A123 cells are too tall and
          > have *way* too much power, so I'm going to try something else.
          >
          > Anyone thought about building a pack of smaller 3ah A123 cylinder cells
          > in parallel? A 17ah lead pack will put out only around 12ah under load
          > tops, so 4 strings of 3ah cells would do it nicely. 3.6 volt cells would
          > allow a 7 cell string to provide 24+ volts, 8 cells would make the
          > motors *sing* at 28 volts but might be too big to charge with the
          > on-board charger.
          >
          > I'm going to start work this winter, to have something for March/April.
          >
          > C
          >
          >
          > ------------------------------------
          >
          > Yahoo! Groups Links
          >
          >
          >
          >





          [Non-text portions of this message have been removed]
        • Danny Miller
          I would strongly caution you against building a large pack like that out of cobalt-cathode 18650s. That s just dangerous. Among 90 cells, the failure of a
          Message 4 of 8 , Jan 4, 2013
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            I would strongly caution you against building a large pack like that out
            of cobalt-cathode 18650s. That's just dangerous. Among 90 cells, the
            failure of a single cell is high, and a single-cell failure can
            definitely ignite the entire pack. Lithium packs burn hot and regular
            fire extinguishers may not be appropriate for burning lithium.

            Yes you can certainly get superior capacity over lead-acid, esp if you
            DON'T weight it back down. The lighter mower requires substantially
            less power to the drive motors. But you do need to consider the overall
            layout and space-efficiency because the volumetric density is only
            moderately better but the ability to utilize the volume is always much less.

            Volumetrically, when you're trying to parallel small cells into
            macrocells, there's a problem where you STILL want a macrocell to be
            rectangular for practical reasons. There's no law against making
            nonrectangular macrocells but it's just awkward to assemble Tetris-piece
            shapes into a pack.

            Danny

            On 1/4/2013 2:41 PM, wolfbob wrote:
            > I don't understand the problem, Danny I
            >
            > took a picture showing all the 18650s I have (90) only filling about 60% of the RL500 battery compartment. Even with the charger, BMS and some lead weights in there it should be no problem to get 24 volts 20Ah. The SLAs as you point out only furnish around 10Ah out of the 17 Ah rating and have any life.
            >
            > Gotta run now, but I can post the picture when I figure out how..
            >
            > WBob
            >
            > ----- Original Message -----
            > From: Danny Miller
            > To: RoboMower@yahoogroups.com
            > Sent: Friday, January 04, 2013 11:36 AM
            > Subject: Re: [RoboMower] Switching to lithiums for RL500
            >
            >
            >
            > Unfortunately the question is now of VOLUMETRIC energy density. And you
            > have a problem.
            >
            > The 12v 20ah half-U1 lead acid batts you're replacing are 1.72166
            > watt-hr/cu in.
            >
            > Now a large-format rectangular lifepo4 cell can be 2.1 watt-hr/cu in.
            > The 26650 2.3AH A123's come to 1.44 wh/cu in.
            >
            > The problem is the old case has almost 100% utilization of the volume
            > inside the pack. Cylindrical cells have considerable airspace AND will
            > have a remainder of space on the sides because the sizes won't line up
            > just right with the volume.
            >
            > Rectangular cells will likely not be able to use all the space either.
            > And you have to have exactly 8 cells, not 7, not 9. You can't just add
            > as many cells as the pack can hold, if you can only fit 7 with the "big"
            > size a mfg has, so you select a much smaller one and you see you can
            > stack 8 but there's still room for 2 on the side and one flat across the
            > top, you will still have to just use 8 and ignore the extra volume.
            >
            > It's just very likely you're going to have a significant volume problem
            > with any arbitrary LiFePO4 shape (but rectangulars will usually make
            > much better use of the space).
            > So rectangulars are 22% denser by volume, but having a case that's only
            > 2/3rd battery and 1/3rd air would be a pretty lucky fit the way I see
            > it. It will be less dense. Of course lead-acid always had trouble
            > actually making the rated capacity repeatedly so the energy density for
            > lead-acid was a bit optimistic.
            >
            > A123 LiFePO4 is 3.3v per cell not 3.6v, you need 8 for 24v. That's
            > 26.4v, or 13.2v per "batt" on the old lead-acid pair, a LITTLE high but
            > that's the voltage you see when it's on the charger. Most other mfgs
            > rate their cells at 3.2v though.
            >
            > For rectangulars go to Thundersky/Winston batt, or better yet CALB batt.
            >
            > I don't know of an A123 "3AH" cell. The common 26650 A123 is 2.3AH.
            > They do make a AHR32113 cell which is 4.4AH in their gen-2 version of
            > that cell, but those would be hard to obtain anyways, they're not sold
            > on ebay or online or anything.
            >
            > Danny
            >
            > On 1/4/2013 9:30 AM, Christopher Zach wrote:
            > > Last year I tried running a set of 20ah A123 pouch cells in my RL500
            > > with a BMS on them. Not only did they work, the biggest problem was that
            > > I couldn't run them down. Mower just kept going and going....
            > >
            > > It convinced me that Lithium is a way to go, keeping one old dead lead
            > > battery in there as ballast. However the A123 cells are too tall and
            > > have *way* too much power, so I'm going to try something else.
            > >
            > > Anyone thought about building a pack of smaller 3ah A123 cylinder cells
            > > in parallel? A 17ah lead pack will put out only around 12ah under load
            > > tops, so 4 strings of 3ah cells would do it nicely. 3.6 volt cells would
            > > allow a 7 cell string to provide 24+ volts, 8 cells would make the
            > > motors *sing* at 28 volts but might be too big to charge with the
            > > on-board charger.
            > >
            > > I'm going to start work this winter, to have something for March/April.
            > >
            > > C
            > >
            > >
            > > ------------------------------------
            > >
            > > Yahoo! Groups Links
            > >
            > >
            > >
            > >
            >
            >
            >
            >
            >
            > [Non-text portions of this message have been removed]
            >
            >
            >
            > ------------------------------------
            >
            > Yahoo! Groups Links
            >
            >
            >
            >
          • wolfbob
            Back for a moment... The LiIon have metal cases instead of the easily punctured and easily shorted LiFe etc. cells and are the one that fills all of those 50
            Message 5 of 8 , Jan 4, 2013
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              Back for a moment...

              The LiIon have metal cases instead of the easily punctured and easily shorted LiFe etc. cells and are the one that fills all of those 50 million laptops. There is no Lithium metal in LiIon cells just some compound of Lithium that takes a lot to ignite. Read "Battery University"...It is by far the safest of the Lithium cells. Unlike the LiFe cells that the manufacturer warns you to never charge them without physical protection and never leave them unattended. My helicopter manufacturer (China) specifically warns about charging the LiFe batteries near anything valuable like people.

              By all means rectangular are more space efficient but not very good for cooling as the middle cells have no exposure. The 18650s get all of 10 deg F temp rise at 1.5A, hardly warm.

              Australia has banned individuals from shipping any Lithium batteries in their post. You can still ship them but you must use commercial carriers. I have several customers working on trying to educate the POLs who made such an arbitrary ruling.

              WBob


              ----- Original Message -----
              From: Danny Miller
              To: RoboMower@yahoogroups.com
              Sent: Friday, January 04, 2013 1:20 PM
              Subject: Re: [RoboMower] Switching to lithiums for RL500



              I would strongly caution you against building a large pack like that out
              of cobalt-cathode 18650s. That's just dangerous. Among 90 cells, the
              failure of a single cell is high, and a single-cell failure can
              definitely ignite the entire pack. Lithium packs burn hot and regular
              fire extinguishers may not be appropriate for burning lithium.

              Yes you can certainly get superior capacity over lead-acid, esp if you
              DON'T weight it back down. The lighter mower requires substantially
              less power to the drive motors. But you do need to consider the overall
              layout and space-efficiency because the volumetric density is only
              moderately better but the ability to utilize the volume is always much less.

              Volumetrically, when you're trying to parallel small cells into
              macrocells, there's a problem where you STILL want a macrocell to be
              rectangular for practical reasons. There's no law against making
              nonrectangular macrocells but it's just awkward to assemble Tetris-piece
              shapes into a pack.

              Danny

              On 1/4/2013 2:41 PM, wolfbob wrote:
              > I don't understand the problem, Danny I
              >
              > took a picture showing all the 18650s I have (90) only filling about 60% of the RL500 battery compartment. Even with the charger, BMS and some lead weights in there it should be no problem to get 24 volts 20Ah. The SLAs as you point out only furnish around 10Ah out of the 17 Ah rating and have any life.
              >
              > Gotta run now, but I can post the picture when I figure out how..
              >
              > WBob
              >
              > ----- Original Message -----
              > From: Danny Miller
              > To: RoboMower@yahoogroups.com
              > Sent: Friday, January 04, 2013 11:36 AM
              > Subject: Re: [RoboMower] Switching to lithiums for RL500
              >
              >
              >
              > Unfortunately the question is now of VOLUMETRIC energy density. And you
              > have a problem.
              >
              > The 12v 20ah half-U1 lead acid batts you're replacing are 1.72166
              > watt-hr/cu in.
              >
              > Now a large-format rectangular lifepo4 cell can be 2.1 watt-hr/cu in.
              > The 26650 2.3AH A123's come to 1.44 wh/cu in.
              >
              > The problem is the old case has almost 100% utilization of the volume
              > inside the pack. Cylindrical cells have considerable airspace AND will
              > have a remainder of space on the sides because the sizes won't line up
              > just right with the volume.
              >
              > Rectangular cells will likely not be able to use all the space either.
              > And you have to have exactly 8 cells, not 7, not 9. You can't just add
              > as many cells as the pack can hold, if you can only fit 7 with the "big"
              > size a mfg has, so you select a much smaller one and you see you can
              > stack 8 but there's still room for 2 on the side and one flat across the
              > top, you will still have to just use 8 and ignore the extra volume.
              >
              > It's just very likely you're going to have a significant volume problem
              > with any arbitrary LiFePO4 shape (but rectangulars will usually make
              > much better use of the space).
              > So rectangulars are 22% denser by volume, but having a case that's only
              > 2/3rd battery and 1/3rd air would be a pretty lucky fit the way I see
              > it. It will be less dense. Of course lead-acid always had trouble
              > actually making the rated capacity repeatedly so the energy density for
              > lead-acid was a bit optimistic.
              >
              > A123 LiFePO4 is 3.3v per cell not 3.6v, you need 8 for 24v. That's
              > 26.4v, or 13.2v per "batt" on the old lead-acid pair, a LITTLE high but
              > that's the voltage you see when it's on the charger. Most other mfgs
              > rate their cells at 3.2v though.
              >
              > For rectangulars go to Thundersky/Winston batt, or better yet CALB batt.
              >
              > I don't know of an A123 "3AH" cell. The common 26650 A123 is 2.3AH.
              > They do make a AHR32113 cell which is 4.4AH in their gen-2 version of
              > that cell, but those would be hard to obtain anyways, they're not sold
              > on ebay or online or anything.
              >
              > Danny
              >
              > On 1/4/2013 9:30 AM, Christopher Zach wrote:
              > > Last year I tried running a set of 20ah A123 pouch cells in my RL500
              > > with a BMS on them. Not only did they work, the biggest problem was that
              > > I couldn't run them down. Mower just kept going and going....
              > >
              > > It convinced me that Lithium is a way to go, keeping one old dead lead
              > > battery in there as ballast. However the A123 cells are too tall and
              > > have *way* too much power, so I'm going to try something else.
              > >
              > > Anyone thought about building a pack of smaller 3ah A123 cylinder cells
              > > in parallel? A 17ah lead pack will put out only around 12ah under load
              > > tops, so 4 strings of 3ah cells would do it nicely. 3.6 volt cells would
              > > allow a 7 cell string to provide 24+ volts, 8 cells would make the
              > > motors *sing* at 28 volts but might be too big to charge with the
              > > on-board charger.
              > >
              > > I'm going to start work this winter, to have something for March/April.
              > >
              > > C
              > >
              > >
              > > ------------------------------------
              > >
              > > Yahoo! Groups Links
              > >
              > >
              > >
              > >
              >
              >
              >
              >
              >
              > [Non-text portions of this message have been removed]
              >
              >
              >
              > ------------------------------------
              >
              > Yahoo! Groups Links
              >
              >
              >
              >





              [Non-text portions of this message have been removed]
            • Danny Miller
              You ve got many misconceptions, and you misread something somewhere. You ve stated most things backwards from fact. Li-ion, the 18650 type, most of those are
              Message 6 of 8 , Jan 4, 2013
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                You've got many misconceptions, and you misread something somewhere.
                You've stated most things backwards from fact.

                Li-ion, the 18650 type, most of those are li-cobalt (they make a
                lithium-manganese-spinel type which is safer but 1/2 the capacity in the
                18650 case), are THE most dangerous cell. Hands down. They've got the
                highest initial storage density and the best cost-to-capacity ratio but
                don't easily safely scale up to large batts.

                LiFePO4 is not commonly used in laptops. Lithium cobalt or
                manganese-spinel are.

                Helicopter cells are li-polymer which is physically very similar to
                li-ion, and yes they're risky, but much of that comes from the very
                aggressive charging cycles in RC. They are flammable and do explode
                (pop) if overcharged.

                Li-cobalt is very very flammable, the metal case doesn't protect much
                against that. If there is a problem in the charging system where they
                get overcharged, they plate out metallic lithium in the anode and
                basically turn it into a little firebomb. And they're very vulnerable
                to catching fire when heated by another burning cell (thermal runaway).

                LiFePO4 doesn't "burn", but its energy will turn into heat if the
                separator fails. The don't react nearly as badly when overcharged, and
                they're much more resistant to thermal runaway (melting down because
                they were exposed to the heat of an adjacent batt melting).

                LiFePO4 is widely recognized as the safer chemistry by far. But that's
                not to say they're totally safe and could never catch fire. I don't
                know how you got the idea that the li-ion cobalt type was safer, but
                you're very much mistaken. No battery expert is going to back you on that.

                Danny


                On 1/4/2013 6:00 PM, wolfbob wrote:
                > Back for a moment...
                >
                > The LiIon have metal cases instead of the easily punctured and easily shorted LiFe etc. cells and are the one that fills all of those 50 million laptops. There is no Lithium metal in LiIon cells just some compound of Lithium that takes a lot to ignite. Read "Battery University"...It is by far the safest of the Lithium cells. Unlike the LiFe cells that the manufacturer warns you to never charge them without physical protection and never leave them unattended. My helicopter manufacturer (China) specifically warns about charging the LiFe batteries near anything valuable like people.
                >
                > By all means rectangular are more space efficient but not very good for cooling as the middle cells have no exposure. The 18650s get all of 10 deg F temp rise at 1.5A, hardly warm.
                >
                > Australia has banned individuals from shipping any Lithium batteries in their post. You can still ship them but you must use commercial carriers. I have several customers working on trying to educate the POLs who made such an arbitrary ruling.
                >
                > WBob
                >
                >
                > ----- Original Message -----
                > From: Danny Miller
                > To: RoboMower@yahoogroups.com
                > Sent: Friday, January 04, 2013 1:20 PM
                > Subject: Re: [RoboMower] Switching to lithiums for RL500
                >
                >
                >
                > I would strongly caution you against building a large pack like that out
                > of cobalt-cathode 18650s. That's just dangerous. Among 90 cells, the
                > failure of a single cell is high, and a single-cell failure can
                > definitely ignite the entire pack. Lithium packs burn hot and regular
                > fire extinguishers may not be appropriate for burning lithium.
                >
                > Yes you can certainly get superior capacity over lead-acid, esp if you
                > DON'T weight it back down. The lighter mower requires substantially
                > less power to the drive motors. But you do need to consider the overall
                > layout and space-efficiency because the volumetric density is only
                > moderately better but the ability to utilize the volume is always much less.
                >
                > Volumetrically, when you're trying to parallel small cells into
                > macrocells, there's a problem where you STILL want a macrocell to be
                > rectangular for practical reasons. There's no law against making
                > nonrectangular macrocells but it's just awkward to assemble Tetris-piece
                > shapes into a pack.
                >
                > Danny
                >
                > On 1/4/2013 2:41 PM, wolfbob wrote:
                > > I don't understand the problem, Danny I
                > >
                > > took a picture showing all the 18650s I have (90) only filling about 60% of the RL500 battery compartment. Even with the charger, BMS and some lead weights in there it should be no problem to get 24 volts 20Ah. The SLAs as you point out only furnish around 10Ah out of the 17 Ah rating and have any life.
                > >
                > > Gotta run now, but I can post the picture when I figure out how..
                > >
                > > WBob
                > >
                > > ----- Original Message -----
                > > From: Danny Miller
                > > To: RoboMower@yahoogroups.com
                > > Sent: Friday, January 04, 2013 11:36 AM
                > > Subject: Re: [RoboMower] Switching to lithiums for RL500
                > >
                > >
                > >
                > > Unfortunately the question is now of VOLUMETRIC energy density. And you
                > > have a problem.
                > >
                > > The 12v 20ah half-U1 lead acid batts you're replacing are 1.72166
                > > watt-hr/cu in.
                > >
                > > Now a large-format rectangular lifepo4 cell can be 2.1 watt-hr/cu in.
                > > The 26650 2.3AH A123's come to 1.44 wh/cu in.
                > >
                > > The problem is the old case has almost 100% utilization of the volume
                > > inside the pack. Cylindrical cells have considerable airspace AND will
                > > have a remainder of space on the sides because the sizes won't line up
                > > just right with the volume.
                > >
                > > Rectangular cells will likely not be able to use all the space either.
                > > And you have to have exactly 8 cells, not 7, not 9. You can't just add
                > > as many cells as the pack can hold, if you can only fit 7 with the "big"
                > > size a mfg has, so you select a much smaller one and you see you can
                > > stack 8 but there's still room for 2 on the side and one flat across the
                > > top, you will still have to just use 8 and ignore the extra volume.
                > >
                > > It's just very likely you're going to have a significant volume problem
                > > with any arbitrary LiFePO4 shape (but rectangulars will usually make
                > > much better use of the space).
                > > So rectangulars are 22% denser by volume, but having a case that's only
                > > 2/3rd battery and 1/3rd air would be a pretty lucky fit the way I see
                > > it. It will be less dense. Of course lead-acid always had trouble
                > > actually making the rated capacity repeatedly so the energy density for
                > > lead-acid was a bit optimistic.
                > >
                > > A123 LiFePO4 is 3.3v per cell not 3.6v, you need 8 for 24v. That's
                > > 26.4v, or 13.2v per "batt" on the old lead-acid pair, a LITTLE high but
                > > that's the voltage you see when it's on the charger. Most other mfgs
                > > rate their cells at 3.2v though.
                > >
                > > For rectangulars go to Thundersky/Winston batt, or better yet CALB batt.
                > >
                > > I don't know of an A123 "3AH" cell. The common 26650 A123 is 2.3AH.
                > > They do make a AHR32113 cell which is 4.4AH in their gen-2 version of
                > > that cell, but those would be hard to obtain anyways, they're not sold
                > > on ebay or online or anything.
                > >
                > > Danny
                > >
                > > On 1/4/2013 9:30 AM, Christopher Zach wrote:
                > > > Last year I tried running a set of 20ah A123 pouch cells in my RL500
                > > > with a BMS on them. Not only did they work, the biggest problem was that
                > > > I couldn't run them down. Mower just kept going and going....
                > > >
                > > > It convinced me that Lithium is a way to go, keeping one old dead lead
                > > > battery in there as ballast. However the A123 cells are too tall and
                > > > have *way* too much power, so I'm going to try something else.
                > > >
                > > > Anyone thought about building a pack of smaller 3ah A123 cylinder cells
                > > > in parallel? A 17ah lead pack will put out only around 12ah under load
                > > > tops, so 4 strings of 3ah cells would do it nicely. 3.6 volt cells would
                > > > allow a 7 cell string to provide 24+ volts, 8 cells would make the
                > > > motors *sing* at 28 volts but might be too big to charge with the
                > > > on-board charger.
                > > >
                > > > I'm going to start work this winter, to have something for March/April.
                > > >
                > > > C
                > > >
                > > >
                > > > ------------------------------------
                > > >
                > > > Yahoo! Groups Links
                > > >
                > > >
                > > >
                > > >
                > >
                > >
                > >
                > >
                > >
                > > [Non-text portions of this message have been removed]
                > >
                > >
                > >
                > > ------------------------------------
                > >
                > > Yahoo! Groups Links
                > >
                > >
                > >
                > >
                >
                >
                >
                >
                >
                > [Non-text portions of this message have been removed]
                >
                >
                >
                > ------------------------------------
                >
                > Yahoo! Groups Links
                >
                >
                >
                >
              • Lee Hart
                ... I agree with Danny. Your typical 18650 laptop cell is *highly* combustible. Extensive safety measures need to be taken to prevent them from ever catching
                Message 7 of 8 , Jan 5, 2013
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                  On 1/4/2013 3:20 PM, Danny Miller wrote:
                  > I would strongly caution you against building a large pack like that out
                  > of cobalt-cathode 18650s. That's just dangerous.

                  I agree with Danny. Your typical 18650 laptop cell is *highly*
                  combustible. Extensive safety measures need to be taken to prevent them
                  from ever catching fire, because if they do, it will be spectacular.
                  Think of them like a box of matches.

                  The LiFe cells still burn, but it's harder to get them started, and the
                  resulting fire spreads much slower. Think of them more like a box of
                  kindling.

                  > you need to consider the overall layout and space-efficiency because
                  > the volumetric density is only moderately better, but the ability to
                  > utilize the volume is always much less.

                  Again, I agree. The watthours per pound of lithium is about 3 times
                  better than lead acid, but the watthours per cubic foot is only slightly
                  more. When I do talks, I have three batteries that graphically
                  illustrate this: One Hawker lead-acid 12v 60ah, one GM/Ovonic nimh 12v
                  90ah, and one Thundersky 12v 90ah. All three are the same size. But the
                  lead-acid weighs 60 lbs, the nimh 40 lbs, and the lithium 22.5 lbs.

                  When you have a fixed size battery box, you'll wind up with about the
                  same watthour capacity no matter which battery chemistry you pick.

                  --
                  Results! Why man, I have gotten a lot of results. I know several
                  thousand things that won't work. -- Thomas A. Edison
                  --
                  Lee A. Hart, http://www.sunrise-ev.com/LeesEVs.htm
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