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  • Mobile solution for 36V eBike battery

    Hi!

    I'm looking to charge an ebike using a solar panel mounted on a bike trailer - off grid, while travelling with the entire setup. So the topic of this post is basically, how to get solar energy into your ebike battery using as little weight as possible, and of course without damaging your equipment?

    BACKGROUND
    European standard ebikes use 250W pedal assist motors that typically work off a 36V LiFePO4 battery with 10-15Ah (ca 400 Wh) worth of power. High end manufacturers have their own proprietary drive lines, which means you can't just charge a Bosch or Shimano battery with a generic battery charger, or easily attach a generic battery to such a motor - they take steps to make sure you use all their stuff all the way with special connectors etc. This can probably be circumvented in certain ways, but I expect warranties to be void and ugly or perhaps even unsafe solutions to result.

    Most manufacturers only provide chargers for on the grid. There are a few 12V chargers for those who want to charge their ebikes in their cars/RVs. These have the advantage of not having to go via an inverter, and being capped at reasonable amperages (since 12V car plugs are often max 10A, meaning a very transportable solar panel of roughly 100W would be sufficient). Modern on grid chargers often use 3-4 amps at 230V AC, especially for higher end systems, where more rapid charging is a constant development goal. Unfortunately, the 12V chargers are either sold out, hard to come by or unavailable for the systems I'm interested in, so it looks like high capacity AC chargers is what I'm stuck with for this project.

    TRADITIONAL SOLUTION
    The traditional way to charge a 36V bike battery using solar panels off grid would be:
    Solar panels -> regulator -> battery -> inverter -> ebike charger -> ebike battery

    This is safe, tested and it works. The problem is transportability. A 36V 10A LiFePO4 battery can be drained to almost all it's capacity, while - say - a 12V AGM battery can only be drained to about 50%. So, to safely put 36x10=360 Wh power into the bike battery, you'd need a 12V battery bank of roughly 700 Wh or 60 Ah. The battery alone would weigh 16 kg, adding panels and regulator on top of that. While the trailer + ebike solution does give me some room to travel with more weight, I'd like to keep it as low as possible.

    A costly but much more attractive option would be to use a LiFePO4 battery for the solar panel as well. This would allow me to use a sleek 30 Ah battery bank that would weigh half as much as a corresponding AGM battery, cutting the weight by a factor of 4. A 4kg battery is much more acceptable. The battery cost is probably 4 times higher too, and I'd need to use a special (more expensive) regulator for LiFePO4 batteries.

    However, this got me thinking: If I can charge a LiFePO4 battery bank with a solar panel, might I not be able to charge the ebike battery directly with solar panels too? This would have the cost and weight advantages of cutting out the intermediate battery. I'd need 3x 12 volt panels (of say, 50W each) and a regulator/charger that accepts at least 30-50 VDC input and output. Or is the bike battery so different that it really needs its own specially designed charger? Or is one LiFePO4 battery pretty much like the next one?

    MICROINVERTER SOLUTION
    A microinverter includes an MPPT regulator, attaches directly to a solar panel and outputs AC. When I first heard of these, it seemed to good to be true - and it probably was. The MIs I've found all seem to require grid installation - they simply don't work unless you connect them to the grid. Enphase (a manufacturer) is developing an off the grid MI to be launched in 2019. This would allow the following set up instead:
    Solar panels -> microinverter -> ebike charger -> ebike battery

    As long as the sun is shining, you basically have a mobile AC socket with you for all your needs - ebike battery included. One downside with this setup is that you would probably need to use (and travel with) more solar panels than you'd otherwise like. Microinverters are designed to be paired with run-of-the-mill 60 cell 250W panels. It's possible that an off grid model will be more flexible in this regard (to cater to the RV market for example), but who knows? Putting 250W of panels on the trailer might be doable, but kind of overkill for my needs.

    Another concern with such a set up is, how will the ebike charger handle a limited available amperage from the microinverter? If the charger wants 3 amps at 230 volts, but it only gets .8 to 1 amp at 230 volts, what will happen? Ideally, the charger will work fine, no equipment will be damaged, and the ebike battery will simply charge more slowly than if it had been connected to a normal on-grid socket. Worst case, it won't charge at all and/or equipment may be damaged. Ideally, the battery charger might provide options of different charging speeds to anticipate varying charging conditions, but such solutions are rare for small ebike batteries while they are common for electric cars.

    If it does work, it begs the question - why haven't we done this before in off grid applications? After all, there are simple regulators designed to be used without batteries. Why not connect a low capacity inverter directly to such a regulator and avoid batteries if you don't really need a 24h power source? I'm guessing the varying amperage would be problematic for almost all AC equipment you might be interested in connecting to such a setup, hence the battery bank is needed to provide steady current. But then again, if this is the case, why is Enphase developing a battery-less off grid microinverter?

    Bottom line, if I could get away with this, I'd take this solution in a heart beat - no extra battery weight or cost would be awesome. Not having to replace an intermediate battery would also be great in the long run.

    I'm basically wondering if I've missed any good alternatives and what you think about a solution without an intermediate battery?

  • #2
    If you have no charging source battery, charging the bike will depend on sunlight and keep you
    grounded in the mean time. If you have an extra bike battery, one could continuously be in
    charge mode while the other is in use (like lift truck batteries).

    With a trailer you must have a vehicle. The RV people have a second RV type battery with a
    battery isolator so it can be charged from the vehicle alternator. One of these could use a
    12V-36V step up converter to charge the bike battery, and have a solar source to keep it up
    as much as possible. Bruce Roe

    Comment


    • #3
      Originally posted by bcroe View Post
      If you have no charging source battery, charging the bike will depend on sunlight and keep you
      grounded in the mean time. If you have an extra bike battery, one could continuously be in
      charge mode while the other is in use (like lift truck batteries).
      Correct! When I'm using the ebike battery, I won't be charging it. However, travel time << non-travel time as it is. It's also pretty likely that I'll be traveling when the sun is down or at least not at peak efficiency, because that's generally when it's best to travel. If I go battery-less, I will also need a larger power bank to power lap top, phone and other stuff I take with me, and that will be attached when I'm pedaling (or when ebike battery is full) to ensure I always harvest all the sun. Its weight is minor compared to a setup with an intermediate battery though.

      Adapting my motorized travel pattern to accommodate for varying available sunlight is part of the concept and not a problem I really need to solve.

      With a trailer you must have a vehicle. The RV people have a second RV type battery with a
      battery isolator so it can be charged from the vehicle alternator. One of these could use a
      12V-36V step up converter to charge the bike battery, and have a solar source to keep it up
      as much as possible. Bruce Roe
      Not quite sure I follow here. The trailer is for the ebike, and the ebike is the vehicle for the trailer. There is no fossil fuel engine or alternator available anywhere here Inspirational video:
      https://www.youtube.com/watch?v=LNplw_ggn74

      Comment


      • #4
        OK, I thought the trailer was for carrying the bike (such as happens a lot here). Actually you say the
        bike is for pulling the trailer, cancel my RV comments.

        Micro inverters are designed to work only into grid power. All DC to DC conversions would be best,
        converting to AC and then back to DC is typically not very efficient, more so on a small scale.
        I would at least try to have a plug in charger on the trailer, for when an outlet is available. Bruce Roe

        Comment


        • #5
          Originally posted by bcroe View Post
          Micro inverters are designed to work only into grid power.
          Yup, that's a bummer, but off grid versions are in development at least.

          All DC to DC conversions would be best, converting to AC and then back to DC is typically not very efficient, more so on a small scale.
          Sure, but what setup are you suggesting then? An mppt regulator that can handle LiFePO4 batteries and then what - a 36V panel setup that can charge the ebike battery directly, or some regulator that can increase the voltage? Since the ebike battery is kinda expensive, I wonder if using a custom charging solution would be recommended. AC has the advantage of being able to use the supplied plug in charger.

          I would at least try to have a plug in charger on the trailer, for when an outlet is available.
          It will definitively be so. It comes with the bike, plus I reckon I will often take toors with just the bike to urban areas and leave the trailer behind, and top up the battery somewhere before I go back.

          Comment


          • #6
            About the only way to have an AC outlet, is to have a substantial battery and inverter, and
            a pretty big solar system behind them.

            There might be a switching solar charger for your battery, and the panels can be set up to
            supply the needed voltage at around their V max power. If your battery does get charged,
            you could connect a simple DC-DC converter to supply what laptop chargers, etc need.
            Bruce Roe

            Comment


            • #7
              Originally posted by urdjur View Post
              Yup, that's a bummer, but off grid versions are in development at least.
              Not really. They just pair hem with a bimodal inverter making an AC coupled system. The bimodal inverter does all the off grid work.

              Micros and off grid are just just a silly idea
              OutBack FP1 w/ CS6P-250P http://bit.ly/1Sg5VNH

              Comment


              • #8
                Just one little problem gets in the way of these pipe dreams called Conservation of Energy. It will require more energy to haul that panel around in a trailer than the panel can generate. You are loosing energy and making more work for yourself.
                MSEE, PE

                Comment


                • #9
                  Originally posted by Sunking View Post
                  Just one little problem gets in the way of these pipe dreams called Conservation of Energy. It will require more energy to haul that panel around in a trailer than the panel can generate. You are loosing energy and making more work for yourself.
                  Haha, I'm not using the trailer just so I can have a solar panel. I agree it would be much better to just ride your ordinary bike instead! I mean, it's a small camping bike trailer, I want to use it for camping. You can transport it with pedal power alone if you want to, but it weighs 40 kg so I thought some electrical assistance will make my life more pleasant. Of course, you don't *have* to charge your ebike with a solar panel, but with a camping set-up that might take you away from easy access to electricity, it makes sense. The added weight of a flexible 100W solar panel is very small compared to my weight, the trailer's weight, bike and gear. (Also, your reference to the first law of thermodynamics is not applicable, because this isn't an isolated energy system, but that's kinda beside the point

                  Not really. They just pair hem with a bimodal inverter making an AC coupled system. The bimodal inverter does all the off grid work.
                  But then you'd still need a battery bank, no?

                  Micros and off grid are just just a silly idea
                  Not that it matters much for my application, but wouldn't it save on battery life to be able to get AC directly from the microinverter to power your appliances when the sun is shining? Possibly other efficiency increases?

                  There might be a switching solar charger for your battery, and the panels can be set up to
                  supply the needed voltage at around their V max power. If your battery does get charged,
                  you could connect a simple DC-DC converter to supply what laptop chargers, etc need.
                  You mean something like this?
                  https://www.bioennopower.com/product...po4-sc-4830jud

                  And then using 36V panels since this regulator can't boost voltage. This would minimize conversion losses too of course. I'm just worried that the ebike battery might take damage when charged in this manner, but maybe I worry too much?

                  Comment


                  • #10
                    Originally posted by urdjur View Post
                    They should show this to young people entering the workforce, to inspire them to start saving for retirement early so they don't end up living in one of these.


                    Comment


                    • #11
                      Originally posted by sdold View Post
                      They should show this to young people entering the workforce, to inspire them to start saving for retirement early so they don't end up living in one of these.
                      That is what Millennial's want. Vote for Bernie Sanders and that is what you get.

                      MSEE, PE

                      Comment


                      • #12
                        Originally posted by urdjur View Post
                        Haha, I'm not using the trailer just so I can have a solar panel. I agree it would be much better to just ride your ordinary bike instead! I mean, it's a small camping bike trailer, I want to use it for camping. You can transport it with pedal power alone if you want to, but it weighs 40 kg
                        Try that here in the USA and you will be dead in 5 miles in 100 degree heat and hills.

                        We Americans are not too smart but 40 Kg = 88 pounds.

                        MSEE, PE

                        Comment


                        • #13
                          Originally posted by Sunking View Post
                          Try that here in the USA and you will be dead in 5 miles in 100 degree heat and hills.

                          We Americans are not too smart but 40 Kg = 88 pounds.
                          Indeed! Hence the need for motorized assistance IMO. Also you have to work with the limitations of this fairly simple climate shell. I don't intend to use it in too hot or too cold weather.

                          Anyway, might we get back on the topic of getting solar energy into ebike batteries now please?

                          Comment


                          • #14
                            Originally posted by urdjur View Post
                            You mean something like this?
                            That looks like a solution, except its 30A and with a 100W 50V panel you only need 3A or so. The
                            small panel is going to limit the possible charge current. Bruce Roe

                            Comment


                            • #15
                              Can you buy a spare battery for the bike and use a high end charge controller like a morningstar, that is programmable for 36V, and use a couple panels in series, to charge the battery directly? You will have to get battery specs, and be able to use a computer to program custom setpoints.
                              The TS-MPPT-30 https://www.morningstarcorp.com/products/tristar-mppt/ could do that, as could the Midnight Kid http://www.midnitesolar.com/products...lers%20-%20KID
                              Powerfab top of pole PV mount (2) | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
                              || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
                              || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

                              solar: http://tinyurl.com/LMR-Solar
                              gen: http://tinyurl.com/LMR-Lister

                              Comment

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