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Re: [RoboMower] Re: Time for new batteries

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  • Christopher Zach
    ... Or... lithium.... I m picking up a CleanPower BMS for my Robomower; since I plan to scale up this solution over the summer I wanted to try a
    Message 1 of 14 , Apr 30 7:11 PM
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      On 4/30/2012 4:30 PM, bkelley9401 wrote:
      >
      >
      > If you need a 20ah battery try these:
      > http://www.portablepower.com/12_volt_20_50/EnerSys_DataSafe_NPX-80FR

      Or... lithium....

      I'm picking up a CleanPower BMS for my Robomower; since I plan to scale
      up this solution over the summer I wanted to try a professional-ish BMS
      first.

      I'm also fiddling with how to mount the batteries; I think the best move
      will be to compress copper bars with plastic insulators, held under
      compression. Still working on the details, but a Robomower that can run
      for 6 hours and be lighter could be nice. Right now the 8 cells weigh a
      lot less than a single 17ah battery....

      Chris
    • bkelley9401
      Lithium cost too much with no warranty on life. The only drop-in for the robomower cost $600 for 19.2ah. These would be an exact replacement no wiring change.
      Message 2 of 14 , May 1, 2012
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        Lithium cost too much with no warranty on life. The only drop-in for the robomower cost $600 for 19.2ah.
        These would be an exact replacement no wiring change.
        http://www.all-battery.com/Tenergy12.8V19.2AhLiFePO4RechargeableBattery-31943.aspx

        If you would use these you would need to change the wiring to parallel.
        http://www.all-battery.com/Tenergy25.6V9.6AhLiFePO4RechargeableBattery-31913.aspx

        Or build your own. The parts to do so are getting to be plug and pray, but who do you blame when it fails.


        > Or... lithium....
        >
        > I'm picking up a CleanPower BMS for my Robomower; since I plan to scale
        > up this solution over the summer I wanted to try a professional-ish BMS
        > first.
        >
        > I'm also fiddling with how to mount the batteries; I think the best move
        > will be to compress copper bars with plastic insulators, held under
        > compression. Still working on the details, but a Robomower that can run
        > for 6 hours and be lighter could be nice. Right now the 8 cells weigh a
        > lot less than a single 17ah battery....
        >
        > Chris
        >
      • Chris Zach
        ... Oh for crying in the toast; I could have just BOUGHT a drop in solution? DRAT! So far I am in $240 for the cells, another $140 for the BMS, and probably
        Message 3 of 14 , May 1, 2012
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          On 5/1/2012 10:43 AM, bkelley9401 wrote:
          >
          >
          > Lithium cost too much with no warranty on life. The only drop-in for the
          > robomower cost $600 for 19.2ah.

          Oh for crying in the toast; I could have just BOUGHT a drop in solution?
          DRAT!

          So far I am in $240 for the cells, another $140 for the BMS, and
          probably another $100 for the little parts and such. And it will look
          like dirt compared to a solution like the one above.

          > Or build your own. The parts to do so are getting to be plug and pray,
          > but who do you blame when it fails.

          Well, you deal and move on. Maybe cry a little bit or something. But
          that's what we call progress.

          I can get 3+ years on a pack with the zener regs, most people get 1 year
          on a pack. I'm currently using Lithium batteries on my Scooba and even
          though it's twice the cost of a normal pack it has already lasted 3
          times as long as NiMH. Same with the Roomba and it's lithium pack.

          For $600 including BMS and all that is a nice deal. But I need to learn
          how to build these for my elecric tractor, then my electric car. Walk
          before run and all that.

          Chris
        • Lee Hart
          ... People like Valence will sell you a drop-in replacement for about $600. It includes a built-in BMS, and has a good warranty. ... Most consumer products
          Message 4 of 14 , May 1, 2012
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            On 5/1/2012 10:31 AM, Chris Zach wrote:
            > On 5/1/2012 10:43 AM, bkelley9401 wrote:
            > Oh for crying in the toast; I could have just BOUGHT a drop in solution?
            > DRAT!

            People like Valence will sell you a drop-in replacement for about $600.
            It includes a built-in BMS, and has a good warranty.

            > I can get 3+ years on a pack with the zener regs, most people get 1 year
            > on a pack. I'm currently using Lithium batteries on my Scooba and even
            > though it's twice the cost of a normal pack it has already lasted 3
            > times as long as NiMH. Same with the Roomba and it's lithium pack.

            Most consumer products provide the cheapest possible batteries, charger,
            and (with lithium) the minimum possible BMS that's barely good enough to
            avoid getting sued for starting fires.

            My Robomower came with el-cheapo Chinese batteries, and a crude charger
            that murdered them in one season. My replacement Hawkers and quality
            electric wheelchair charger are 3 years old and going strong.

            > For $600 including BMS and all that is a nice deal. But I need to learn
            > how to build these for my elecric tractor, then my electric car. Walk
            > before run and all that.

            Yes. We do have to pay for learning. Either in tuition and books, or
            extra time, or by wrecking things.

            --
            If we knew what we were doing, it wouldn't be called research, would it?
            -- Albert Einstein
            --
            Lee A. Hart, 814 8th Ave N, Sartell MN 56377, leeahart at earthlink.net
          • Danny Miller
            Ironically, I m pretty darn sure those batteries would NOT be a safe option. There s no proper charge controller on there that I can see, so they re
            Message 5 of 14 , May 1, 2012
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              Ironically, I'm pretty darn sure those batteries would NOT be a "safe"
              option. There's no proper charge controller on there that I can see, so
              they're literally slapping something together that a customer might
              want, and to hell with design prudence and safety.

              There's a lot of things you can CALL "a charge controller". But if it
              can't dissipate the heat of a normal charge rate, it's doomed to
              failure. And I can tell you, bad charge controllers are a FREQUENT
              complaint, and often involve total loss of the battery, if not a fire.

              It's not just about cost. There's design constrains involved with
              installing 8x 10W resistors. Or exposing 9 low-resistance terminals for
              an external controller. It's either a funky, awkward shape with a
              heatsink hanging off it that puts the buyer off, or "complex" enough to
              put the buyer off. So the market is more than willing to sell designs
              that are basically doomed to fail. And they DO.

              Danny

              On 5/1/2012 9:43 AM, bkelley9401 wrote:
              >
              >
              >
              >
              > Lithium cost too much with no warranty on life. The only drop-in for the robomower cost $600 for 19.2ah.
              > These would be an exact replacement no wiring change.
              > http://www.all-battery.com/Tenergy12.8V19.2AhLiFePO4RechargeableBattery-31943.aspx
              >
              > If you would use these you would need to change the wiring to parallel.
              > http://www.all-battery.com/Tenergy25.6V9.6AhLiFePO4RechargeableBattery-31913.aspx
              >
              > Or build your own. The parts to do so are getting to be plug and pray, but who do you blame when it fails.
              >
              >
              >> Or... lithium....
              >>
              >> I'm picking up a CleanPower BMS for my Robomower; since I plan to scale
              >> up this solution over the summer I wanted to try a professional-ish BMS
              >> first.
              >>
              >> I'm also fiddling with how to mount the batteries; I think the best move
              >> will be to compress copper bars with plastic insulators, held under
              >> compression. Still working on the details, but a Robomower that can run
              >> for 6 hours and be lighter could be nice. Right now the 8 cells weigh a
              >> lot less than a single 17ah battery....
              >>
              >> Chris
              >>
              >
              >
              >
              > ------------------------------------
              >
              > Yahoo! Groups Links
              >
              >
              >
              >
            • Danny Miller
              Well, you re an engineer too Lee. You tell me. If you have say a 3A charge current, an internal BMS , how s that gonna work? You shunt 3A at 3.65v, that s
              Message 6 of 14 , May 1, 2012
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                Well, you're an engineer too Lee. You tell me. If you have say a 3A
                charge current, an internal "BMS", how's that gonna work?
                You shunt 3A at 3.65v, that's 11W, potentially on multiple cells.
                Where's that gonna go inside a plastic case?

                In theory you could have a series pass transistor so if any ONE cell
                started bypassing, it drops the charge current down to like 5% while
                shunting & balancing. That will work, but I'd be surprised if they did
                that. And such a battery might not work well in series with another
                battery, I'm not sure.

                There's also a thing where you need the shutoff transistor. But that's
                a mess because the transistor needs to shut off in the opposite
                direction, and a MOSFET with its intrinsic body diode won't do that.
                You'd need a series pair of either an NMOS-PMOS combination or an
                NMOS-reversed NMOS pair to be able to control current in both
                directions. Again, it's possible, sure. But I'd be surprised if they
                did that. I wouldn't believe it without seeing a teardown of what they
                did, I have very low faith in LiFePO4 vendors doing this.

                Danny

                On 5/1/2012 1:48 PM, Lee Hart wrote:
                > On 5/1/2012 10:31 AM, Chris Zach wrote:
                >> On 5/1/2012 10:43 AM, bkelley9401 wrote:
                >> Oh for crying in the toast; I could have just BOUGHT a drop in solution?
                >> DRAT!
                > People like Valence will sell you a drop-in replacement for about $600.
                > It includes a built-in BMS, and has a good warranty.
                >
                >> I can get 3+ years on a pack with the zener regs, most people get 1 year
                >> on a pack. I'm currently using Lithium batteries on my Scooba and even
                >> though it's twice the cost of a normal pack it has already lasted 3
                >> times as long as NiMH. Same with the Roomba and it's lithium pack.
                > Most consumer products provide the cheapest possible batteries, charger,
                > and (with lithium) the minimum possible BMS that's barely good enough to
                > avoid getting sued for starting fires.
                >
                > My Robomower came with el-cheapo Chinese batteries, and a crude charger
                > that murdered them in one season. My replacement Hawkers and quality
                > electric wheelchair charger are 3 years old and going strong.
                >
                >> For $600 including BMS and all that is a nice deal. But I need to learn
                >> how to build these for my elecric tractor, then my electric car. Walk
                >> before run and all that.
                > Yes. We do have to pay for learning. Either in tuition and books, or
                > extra time, or by wrecking things.
                >
              • Lee Hart
                ... I m an EE, and design battery chargers and management systems professionally. One of my clients was building a replacement lithium battery, intended for
                Message 7 of 14 , May 1, 2012
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                  On 5/1/2012 1:52 PM, Danny Miller wrote:
                  > Ironically, I'm pretty darn sure those batteries would NOT be a "safe"
                  > option. There's no proper charge controller on there that I can see, so
                  > they're literally slapping something together that a customer might
                  > want, and to hell with design prudence and safety.
                  >
                  > There's a lot of things you can CALL "a charge controller". But if it
                  > can't dissipate the heat of a normal charge rate, it's doomed to
                  > failure. And I can tell you, bad charge controllers are a FREQUENT
                  > complaint, and often involve total loss of the battery, if not a fire.
                  >
                  > It's not just about cost. There's design constrains involved with
                  > installing 8x 10W resistors. Or exposing 9 low-resistance terminals for
                  > an external controller. It's either a funky, awkward shape with a
                  > heatsink hanging off it that puts the buyer off, or "complex" enough to
                  > put the buyer off. So the market is more than willing to sell designs
                  > that are basically doomed to fail. And they DO.

                  I'm an EE, and design battery chargers and management systems
                  professionally. One of my clients was building a replacement lithium
                  battery, intended for racers that wanted a super-light engine starting
                  battery. The racers will pay "big bucks" for such batteries as they can
                  easily take 10-50 pounds off the weight of their car or motorcycle.

                  They first tried selling them with no BMS (just lots of BS). They soon
                  had angry customers calling. Batteries would catch fire, sometimes
                  destroying entire vehicles!

                  I designed a BMS for them that made their lithium battery behave exactly
                  the same as a lead-acid battery, and that would fail safe. It would quit
                  working if you tried to run it dead, draw too much current, or
                  overcharge it; but it wouldn't start a fire.

                  Unfortunately, the BMS added cost, which reduced their sales.
                  Competitors (mainly fly-by-night outfits selling junk) undercut them.
                  They outsourced the design to India to cut costs. But then they had
                  quality problems.

                  What was their solution? They removed most of the safety features of the
                  BMS to minimize cost, and then sold their batteries without labels and
                  no guarantee to the fly-by-night resellers to market under their own
                  names. :-(
                  --
                  If you would not be forgotten
                  When your body's dead and rotten
                  Then write of great deeds worth the reading
                  Or do these great deeds, worth repeating.
                  -- Ben Franklin, from Poor Richard's Almanac
                  --
                  Lee A. Hart, 814 8th Ave N, Sartell MN 56377, leeahart at earthlink.net
                • Dan Barclay
                  Warnings also say: Do not let battery come in contact with water. Might be a problem in the rain? It s hard to know what they define as contact . Most of us
                  Message 8 of 14 , May 1, 2012
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                    Warnings also say:



                    Do not let battery come in contact with water.



                    Might be a problem in the rain? It's hard to know what they define as
                    "contact". Most of us have RL1000's that almost never come indoors. It
                    needs to withstand hard rain and wind.



                    In any case, $600 is a bit much. I assume that is for the pair!



                    Dan



                    From: RoboMower@yahoogroups.com [mailto:RoboMower@yahoogroups.com] On Behalf
                    Of Danny Miller
                    Sent: Tuesday, May 01, 2012 1:53 PM
                    To: RoboMower@yahoogroups.com
                    Subject: Re: [RoboMower] Re: Time for new batteries





                    Ironically, I'm pretty darn sure those batteries would NOT be a "safe"
                    option. There's no proper charge controller on there that I can see, so
                    they're literally slapping something together that a customer might
                    want, and to hell with design prudence and safety.

                    There's a lot of things you can CALL "a charge controller". But if it
                    can't dissipate the heat of a normal charge rate, it's doomed to
                    failure. And I can tell you, bad charge controllers are a FREQUENT
                    complaint, and often involve total loss of the battery, if not a fire.

                    It's not just about cost. There's design constrains involved with
                    installing 8x 10W resistors. Or exposing 9 low-resistance terminals for
                    an external controller. It's either a funky, awkward shape with a
                    heatsink hanging off it that puts the buyer off, or "complex" enough to
                    put the buyer off. So the market is more than willing to sell designs
                    that are basically doomed to fail. And they DO.

                    Danny

                    On 5/1/2012 9:43 AM, bkelley9401 wrote:
                    >
                    >
                    >
                    >
                    > Lithium cost too much with no warranty on life. The only drop-in for the
                    robomower cost $600 for 19.2ah.
                    > These would be an exact replacement no wiring change.
                    >
                    http://www.all-battery.com/Tenergy12.8V19.2AhLiFePO4RechargeableBattery-3194
                    3.aspx
                    >
                    > If you would use these you would need to change the wiring to parallel.
                    >
                    http://www.all-battery.com/Tenergy25.6V9.6AhLiFePO4RechargeableBattery-31913
                    .aspx
                    >
                    > Or build your own. The parts to do so are getting to be plug and pray, but
                    who do you blame when it fails.
                    >
                    >
                    >> Or... lithium....
                    >>
                    >> I'm picking up a CleanPower BMS for my Robomower; since I plan to scale
                    >> up this solution over the summer I wanted to try a professional-ish BMS
                    >> first.
                    >>
                    >> I'm also fiddling with how to mount the batteries; I think the best move
                    >> will be to compress copper bars with plastic insulators, held under
                    >> compression. Still working on the details, but a Robomower that can run
                    >> for 6 hours and be lighter could be nice. Right now the 8 cells weigh a
                    >> lot less than a single 17ah battery....
                    >>
                    >> Chris
                    >>
                    >
                    >
                    >
                    > ------------------------------------
                    >
                    > Yahoo! Groups Links
                    >
                    >
                    >
                    >





                    [Non-text portions of this message have been removed]
                  • Danny Miller
                    The A123 20AH pouch cells are a far better deal. Then you CAN put a proper BMS on it. But some hacker fu knowledge is required. Danny
                    Message 9 of 14 , May 1, 2012
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                      The A123 20AH pouch cells are a far better deal. Then you CAN put a
                      proper BMS on it. But some hacker fu knowledge is required.

                      Danny

                      On 5/1/2012 3:26 PM, Dan Barclay wrote:
                      > Warnings also say:
                      >
                      >
                      >
                      > Do not let battery come in contact with water.
                      >
                      >
                      >
                      > Might be a problem in the rain? It's hard to know what they define as
                      > "contact". Most of us have RL1000's that almost never come indoors. It
                      > needs to withstand hard rain and wind.
                      >
                      >
                      >
                      > In any case, $600 is a bit much. I assume that is for the pair!
                      >
                      >
                      >
                      > Dan
                      >
                      >
                      >
                      > From: RoboMower@yahoogroups.com [mailto:RoboMower@yahoogroups.com] On Behalf
                      > Of Danny Miller
                      > Sent: Tuesday, May 01, 2012 1:53 PM
                      > To: RoboMower@yahoogroups.com
                      > Subject: Re: [RoboMower] Re: Time for new batteries
                      >
                      >
                      >
                      >
                      >
                      > Ironically, I'm pretty darn sure those batteries would NOT be a "safe"
                      > option. There's no proper charge controller on there that I can see, so
                      > they're literally slapping something together that a customer might
                      > want, and to hell with design prudence and safety.
                      >
                      > There's a lot of things you can CALL "a charge controller". But if it
                      > can't dissipate the heat of a normal charge rate, it's doomed to
                      > failure. And I can tell you, bad charge controllers are a FREQUENT
                      > complaint, and often involve total loss of the battery, if not a fire.
                      >
                      > It's not just about cost. There's design constrains involved with
                      > installing 8x 10W resistors. Or exposing 9 low-resistance terminals for
                      > an external controller. It's either a funky, awkward shape with a
                      > heatsink hanging off it that puts the buyer off, or "complex" enough to
                      > put the buyer off. So the market is more than willing to sell designs
                      > that are basically doomed to fail. And they DO.
                      >
                      > Danny
                      >
                      > On 5/1/2012 9:43 AM, bkelley9401 wrote:
                      >>
                      >>
                      >>
                      >> Lithium cost too much with no warranty on life. The only drop-in for the
                      > robomower cost $600 for 19.2ah.
                      >> These would be an exact replacement no wiring change.
                      >>
                      > http://www.all-battery.com/Tenergy12.8V19.2AhLiFePO4RechargeableBattery-3194
                      > 3.aspx
                      >> If you would use these you would need to change the wiring to parallel.
                      >>
                      > http://www.all-battery.com/Tenergy25.6V9.6AhLiFePO4RechargeableBattery-31913
                      > .aspx
                      >> Or build your own. The parts to do so are getting to be plug and pray, but
                      > who do you blame when it fails.
                      >>
                      >>> Or... lithium....
                      >>>
                      >>> I'm picking up a CleanPower BMS for my Robomower; since I plan to scale
                      >>> up this solution over the summer I wanted to try a professional-ish BMS
                      >>> first.
                      >>>
                      >>> I'm also fiddling with how to mount the batteries; I think the best move
                      >>> will be to compress copper bars with plastic insulators, held under
                      >>> compression. Still working on the details, but a Robomower that can run
                      >>> for 6 hours and be lighter could be nice. Right now the 8 cells weigh a
                      >>> lot less than a single 17ah battery....
                      >>>
                      >>> Chris
                      >>>
                      >>
                      >>
                      >> ------------------------------------
                      >>
                      >> Yahoo! Groups Links
                      >>
                      >>
                      >>
                      >>
                      >
                      >
                      >
                      >
                      > [Non-text portions of this message have been removed]
                      >
                      >
                      >
                      > ------------------------------------
                      >
                      > Yahoo! Groups Links
                      >
                      >
                      >
                      >
                    • Lee Hart
                      ... You re right. The cheapest (and thus most popular) BMS is a simple shunt regulator that bypasses excess charging current, and burns it up as heat. You
                      Message 10 of 14 , May 1, 2012
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                        On 5/1/2012 2:05 PM, Danny Miller wrote:
                        > Well, you're an engineer too Lee. You tell me. If you have say a 3A
                        > charge current, an internal "BMS", how's that gonna work?
                        > You shunt 3A at 3.65v, that's 11W, potentially on multiple cells.
                        > Where's that gonna go inside a plastic case?

                        You're right. The cheapest (and thus most popular) BMS is a simple shunt
                        regulator that bypasses excess charging current, and burns it up as
                        heat. You can't have very many cells doing this in a pack, or for very
                        long before something gets very hot. And we know that heat significantly
                        shortens the life of lithium cells.

                        > In theory you could have a series pass transistor so if any ONE cell
                        > started bypassing, it drops the charge current down to like 5% while
                        > shunting & balancing. That will work, but I'd be surprised if they did
                        > that.

                        That is the next step up in BMS systems. The individual shunt regulators
                        have some kind of buss. When a cell begins shunting current, the bus
                        signals the charger to cut back to a low current. It roughly doubles the
                        cost and number of wires to be installed on the BMS. You see them;
                        they're just less common.

                        It's a challenge to make it fail-safe and reliable. Many of these bus
                        systems send a signal on the buss when there's a problem. That means if
                        a wire breaks or a regulator fails, there is *no* signal to tell the
                        charger to cut back or stop. But if they make it send signals for "OK",
                        then the regulators are drawing current all the time from the cells to
                        run the buss. The cells can be run dead and destroyed just from the "OK"
                        power requirements of the BMS.

                        Reliability starts to become a problem because you have hundreds of
                        parts and connections in these BMS systems. When they get built with the
                        cheapest possible parts and wiring, you get a high failure rate.

                        > And such a battery might not work well in series with another
                        > battery, I'm not sure.

                        Most BMS ICs are designed for laptops and other applications where the
                        total pack voltage is no more than 24v. Over this, you need multiple
                        ICs, and some way for them to cooperate.

                        There are problems dealing with failure modes. For instance, a cell can
                        fail open. That puts the full pack voltage across its terminals. The
                        parts between that cell and its BMS gets "hit* with full pack voltage.
                        The 300v pack in an EV can put 300v across any input to the BMS!

                        > There's also a thing where you need the shutoff transistor. But that's
                        > a mess because the transistor needs to shut off in the opposite
                        > direction, and a MOSFET with its intrinsic body diode won't do that.
                        > You'd need a series pair of either an NMOS-PMOS combination or an
                        > NMOS-reversed NMOS pair to be able to control current in both
                        > directions.

                        Right again. The BMS I designed for the starting battery (see earlier
                        post) had two anti-series NMOS MOSFETs to act as a switch. It could
                        handle the high starting current (like 500a for 10 sec), and the smaller
                        steady state currents (like 50a for an hour, or 10a indefinitely).

                        But that was just a single 12v battery, so I could use 30v MOSFETs. If
                        you put more than two batteries in series, these would have to be be
                        changed to higher-voltage MOSFETs, which would significantly increase
                        the cost. For higher voltage setups, you need to use a contactor or
                        relay to disconnect the battery in case of trouble.

                        > I have very low faith in LiFePO4 vendors doing this.

                        Same here. I read all the engineering journals and data sheets and
                        patents on BMS systems. I also reverse-engineer a lot of stuff, just to
                        see what's being sold and how it works.

                        The overwhelming majority of stuff is cheap junk that is *barely* safe
                        enough for the seller to dodge lawsuits if something goes wrong. And,
                        there are no concessions for longer battery life. This is why it is rare
                        to find anything with lithium cells more than a few years old where the
                        cells aren't shot.

                        This attitude on the part of venders worries me about lithiums in
                        electric cars. Will the automakers do it "right"? Or, will they use the
                        cheapest possible system that will barely get past their lawyers? From
                        the automaker's point of view, a $10,000 battery pack that must be
                        replaced every few years is a *big* profit opportunity for them!

                        The scheme I like best is the one I use in my own Battery Balancers (
                        See http://www3.telus.net/nook/balancerland). It uses relays so a single
                        central controller can select any cell, measure its voltage, and charge
                        or discharge it as needed, using power from the pack as a whole. This
                        way, no power is lost in wasted heat, and you don't have dangerous
                        failure modes.
                        --
                        First they ignore you; then they mock you; then they fight you; then you
                        win.
                        -- Mahatma Gandhi
                        --
                        Lee A. Hart, 814 8th Ave N, Sartell MN 56377, leeahart at earthlink.net
                      • Danny Miller
                        Well I think the strategy they re using is assuming that there will only be a trivial imbalance in the cells, and the charger will be tapering off because
                        Message 11 of 14 , May 1, 2012
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                          Well I think the "strategy" they're using is assuming that there will
                          only be a trivial imbalance in the cells, and the charger will be
                          tapering off because the charge is nearly full.

                          So they're probably betting the shunt will take 10% of the full charge
                          current and/or only for a minute or two as it tops off. But:
                          a) there's no guarantee the balance will be "close" like that. It
                          depends on differences in cell leakage and time since last balancing.
                          It can be significant.
                          b) it's assuming the charger will taper off. Not all charging schemes
                          do, in fact "constant current" which doesn't taper is a common design.
                          An algorithm of "3A until voltage= 3.65v * 4 = 14.6v", then "shut off
                          entirely".
                          c) the mfg generally doesn't control the charger that's put on it.
                          Tenergy for example is making a rather bold claim that it works with any
                          12v lead-acid charger. Well WTF does that limit the possible situations
                          to? There's many different charger types.

                          With a series transistor putting it in trickle-balance mode, that
                          transistor could itself be burning a LOT of heat, assuming it doesn't
                          have an inductor and use switching mode. The situation gets more
                          serious when you try to put multiple batts in series, because the
                          voltage margin needed to limit the current gets higher.

                          The series transistor could conflict with a SLA charging algorithm.
                          3.45v * 3 cells + 3.65v * 1 cell= 14v, but the transistor tries to limit
                          current to 150mA to avoid overheating the shunts. In doing so it allows
                          the batt voltage to rise above 14.8v and the charger drops out of Bulk
                          charge and into Float, which could be anything below 100mA, it could be
                          10mA, which will take days and days to complete a balancing.

                          Danny

                          On 5/1/2012 4:05 PM, Lee Hart wrote:
                          > The overwhelming majority of stuff is cheap junk that is *barely* safe
                          > enough for the seller to dodge lawsuits if something goes wrong. And,
                          > there are no concessions for longer battery life. This is why it is
                          > rare to find anything with lithium cells more than a few years old
                          > where the cells aren't shot. This attitude on the part of venders
                          > worries me about lithiums in electric cars. Will the automakers do it
                          > "right"? Or, will they use the cheapest possible system that will
                          > barely get past their lawyers? From the automaker's point of view, a
                          > $10,000 battery pack that must be replaced every few years is a *big*
                          > profit opportunity for them! The scheme I like best is the one I use
                          > in my own Battery Balancers ( See
                          > http://www3.telus.net/nook/balancerland). It uses relays so a single
                          > central controller can select any cell, measure its voltage, and
                          > charge or discharge it as needed, using power from the pack as a
                          > whole. This way, no power is lost in wasted heat, and you don't have
                          > dangerous failure modes.
                        • Lee Hart
                          ... Exactly right. They assume all cells are good, new, and well matched. But what you ll find in an old laptop pack is that as the pack aged, one or more
                          Message 12 of 14 , May 1, 2012
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                            On 5/1/2012 4:54 PM, Danny Miller wrote:
                            > Well I think the "strategy" they're using is assuming that there will
                            > only be a trivial imbalance in the cells, and the charger will be
                            > tapering off because the charge is nearly full.
                            >
                            > So they're probably betting the shunt will take 10% of the full charge
                            > current and/or only for a minute or two as it tops off.

                            Exactly right. They assume all cells are good, new, and well matched.

                            But what you'll find in an old laptop pack is that as the pack aged, one
                            or more cells has drifted away from the others. The amount of current or
                            time needed to try to balance it exceeded some limit, and the BMS shut
                            it down. The back is "bad" and must be replaced, even if the only
                            problem is that one cell has 80% of the amphour capacity of the others.

                            > a) there's no guarantee the balance will be "close" like that. It
                            > depends on differences in cell leakage and time since last balancing.
                            > It can be significant.
                            > b) it's assuming the charger will taper off. Not all charging schemes
                            > do, in fact "constant current" which doesn't taper is a common design.
                            > An algorithm of "3A until voltage= 3.65v * 4 = 14.6v", then "shut off
                            > entirely".

                            All correct.

                            > c) the mfg generally doesn't control the charger that's put on it.

                            Most products with lithiums *do* provide the charger, for exactly that
                            reason.

                            > Tenergy for example is making a rather bold claim that it works with any
                            > 12v lead-acid charger. Well WTF does that limit the possible situations
                            > to? There's many different charger types.

                            Exactly. Many 12v lead-acid chargers will happily output 17v forever!
                            They are nothing but a transformer and rectifier, and the light-load
                            voltage is very high.

                            > With a series transistor putting it in trickle-balance mode, that
                            > transistor could itself be burning a LOT of heat, assuming it doesn't
                            > have an inductor and use switching mode. The situation gets more
                            > serious when you try to put multiple batts in series, because the
                            > voltage margin needed to limit the current gets higher.
                            >
                            > The series transistor could conflict with a SLA charging algorithm.
                            > 3.45v * 3 cells + 3.65v * 1 cell= 14v, but the transistor tries to limit
                            > current to 150mA to avoid overheating the shunts. In doing so it allows
                            > the batt voltage to rise above 14.8v and the charger drops out of Bulk
                            > charge and into Float, which could be anything below 100mA, it could be
                            > 10mA, which will take days and days to complete a balancing.

                            Like I said, this is why people sell these batteries on eBay through
                            anonymous "fronts" so they can disappear when things go wrong.
                            --
                            Ring the bells that still can ring
                            Forget your perfect offering
                            There is a crack in everything
                            That's how the light gets in.
                            -- Leonard Cohen, from "Anthem"
                            --
                            Lee A. Hart, 814 8th Ave N, Sartell MN 56377, leeahart at earthlink.net
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