Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

E-bike with generator

  1. Jun 30, 2014 #1
    I want to build an electric bike that has a small lightweight power generator for maximum range. The law here in Norway prohibit gasoline powered bicycles and electric bikes are limited to a maximum effect of 250w. Recharging the battery takes a long time and therefore having a power generator charging the battery constantly while riding is more convenient. I talked to the authorities and the only legal way to do it is to have the generator mounted to a trailer behind the bike. So thats the way i will go.

    I have talked to the guys at the Endless Sphere forum and they recommend the use of SLA batteries. The ebike system is a 48v system with a 250w geared electric hub motor. Should four 12v 7ah batteries be appropriate?

    As for the generator and charger assembly, thats where i will need help from someone who is experienced with electronics. My knowledge in that field is close to nothing. I need help on choosing the right dynamo and so on.

    The engine that will turn the dynamo is a Sachs Stamo 33cc two stroke from the 60s. It has a pulley wheel mounted to it which makes it suitable.

    Motor specs

    1.5 hp / 1.1 kW at 6,500 rev / min
    1.7 hp / 1.3 kW at 7,000 r / min

    Operating speed:
    (4.000 **) 5000-7500 U / min

    0.22 kpm at 4,500 rev / min

    So, what should the power output of the dynamo be? It must not be too powerful or the engine may not be able to turn it. Do i need a regulator to keep the voltage stable? What voltage is required to charge the four SLA batteries? Will i need some kind of charger to charge the batteries from or can i direct to power directly from the dynamo to the batteries?

    Any help would be greatly appreciated!!

    I must wait until i have all the answers before i can order the parts for the electric system. Today i will start building the generator trailer. It shall be a bob trailer that leans with the bike. I will weld it from steel tubes. It will have a swing arm with suspension to protect the generator from mechanical shocks from the terrain. It will all be enclosed in a metal box with ventilation and sound isolating foam to protect it from water and dampen the noise. I intend to use this for daily commuting to work.
  2. jcsd
  3. Jun 30, 2014 #2


    User Avatar

    Staff: Mentor

    Welcome to the PF.

    Wow, that sure seems like a roundabout way to make an electric bike with longer range. Using a gas-powered generator in a towed trailer to recharge the batteries during the ride. Yikes.

    What is the smallest gas-powered scooter that is available, and how much do used ones cost? That seems like a more economical option, IMO...
  4. Jun 30, 2014 #3
    Indeed it would be a much more affordable option. However, it would not be legal to ride offroad. And i also do not have the drivers licence.

    The trailer option is however more affordable than purchasing batteries for long range. Those are extremely expensive.

    And also, its always fun to take on a new project and learn new things!
  5. Jun 30, 2014 #4
    Essentially you are making a trailer powered bike and you are probably breaking a number of other, different laws that you don't know about. Do you know of anyone else that does anything like this?

    and you've said gasoline bikes are illegal, but what about natural gas or methane?
  6. Jun 30, 2014 #5
    No, it is perfectly legal. I already checked that.

    No combustion engine propelled vehicle is allowed to ride unregistered and off the streets. Only 250w ebikes or segways are legal to ride without registration. The generator does not propel the bike, and therefore its still considered an electric bike. However, it cannot be built in to the bike itself, it has to be on a trailer or else it will be considered a hybrid vehicle, which then again must be registered.

    What I need to know is what electrical components i must have for charging the four SLA batteries. The generator must generate more power than the electric motor consumes so that i would only have to refuel the generator.
  7. Jun 30, 2014 #6
    Having to pull a trailer behind you all the time seems impractical. I would simply add a bigger battery and get a good charger that can recharge them quickly.
    You may be able to use cheap chinese lithium batteries instead of the expensive ebike battery packs. For example on ebay I found LI batteries called "UltraFire" that offer about 1kWh capacity for 100$. However that sounds too good to be true. Who knows how much capacity those things really have.
  8. Jun 30, 2014 #7

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    Here's an interesting article on the humble and under-appreciated "Lundell" automobile alternator.
    They'll make substantially more than 12 volts.

    You'd need one that does NOT have an internal regulator.
    At the speed of your engine you could get a few kilowatts from a modern 100 amp one, see curves in fig 4 & 5 for an actual Ford alternator. But that would overload your little engine.

    Older cars had 30 amp alternators. 30 amps at 48 volts is 1440 watts which still more than your engine but not by much. Check you local old car graveyard..
    Metal recycling yards sell them by the pound, i get them for around two dollars locally.
    They're only about forty bucks for a nice clean rebuilt one at most auto parts stores.

    You'd have to come up with a 48 volt regulator for it..

    old jim
    Last edited: Jul 1, 2014
  9. Jul 1, 2014 #8


    User Avatar

    Staff: Mentor

    Bravo! I admire such non-defeatist resolve.

    With that attitude, I'm sure you'll go far! :smile:
  10. Jul 1, 2014 #9

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    Thanks Z S for that message..

    Motorcycles have an interesting alternator indeed - permanent magnet rotor on end of crankshaft surrounded by a stationary armature.
    If you have access to a small lathe you might be able to use your ingenuity to make a very compact power unit.

    Some riding lawnmowers have a permanent magnet starter motor that'd make a convenient size dynamo, but i dont know how much voltage they make or how they'd stand up in continuous operation. Probably you'd need to improve ts ventilation. Check salvage yard for junked Briggs&Stratton engines in the 10 to 20 hp range, a starter shouldn't cost much at all.

    I like your motorcycle parts idea better. Alternators inherently limit their own current so are easier to regulate.

    old jim
  11. Jul 4, 2014 #10
    My budget has gotten tighter after some problems with a motorcycle restoration...

    Anyway, I think i might have a better solution than using SLA batteries. Or let alone any battery.
    The idea is to find a small dynamo and connect it to the fuel engine and then connect a 48v regulator to it. If i can find a dynamo that can produce as much as 48 volts.. If not i will find a 24v and purchase a 24v hubmotor instead.

    As for the pedal sensor (which is required for legal purposes) i was thinking of getting a regular motion sensor (like one from a yard lamp) and insert it into a black tube so that it will only detect motion that is right in front of it. I will mount this next to the bicycle crank and sprocket. As for the sprocket i will print a circle with black lines onto it. When this starts spinning the motion sensor activates and opens the circuit from the regulator to the hub motor and the amount of power is adjusted by the speed of the generator.

    Sounds like a plan?

    I will protect the hub motor from overheating by filling it with ATF oil, it has been tested before on the same motor with good results.

    However, i wonder what would happen if i skipped the voltage regulator. Would that hurt the motor? Or is it the amps that do?
    Last edited: Jul 4, 2014
  12. Jul 4, 2014 #11
    the motor has a controller doesn't it?
  13. Jul 4, 2014 #12

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    A DC machine's voltage with constant excitation is linear with its speed.
    You need only about seven amps at 48 volts for 350 watts.

    Metal recycle yards are good for the man on a budget..

    Automobile heater fan motors are ten or so amps. If you can fine a permanent magnet type, spin it and plot volts versus RPM. I've never run that test on such a motor - would be interesting. You might want to reinforce the ends of the windings against centrifugal force, i have used 40 lb tournament grade fishing line for that purpose. Apply liberal varnish to hold it in place.

    1960's outboards had a ten amp generator with belt drive that'd be an interesting candidate, already has a pulley affixed.
    I've heard of automobile radiator fan motors being used for generators but i've never done that..

    It's the amps that overheat your windings.

    Good luck - old jim
  14. Jul 4, 2014 #13
    Thanks a lot Jim, you sure know allot!

    I am actually going to the nearest scrap yard next week to salvage parts for my project. I have already started making the trailer, i have welded the frame so that it fits in the bottom of an old metal box. This way the electronics and motor stays enclosed, I also welded on the rear end of a bicycle to attach the rear wheel, it will be a one wheel trailer. I will also cut a hole in both ends and install a fan to ensure cooling.

    The hub motors come in 24, 36 and 48 volts. I suppose the 48v motor is the fastest.

    I actually found this 36v motor on Ebay, it is 36V 350w 2500RPM 13A. Do you think that will work? Its fairly cheap actually. My little sachs engine is geared down, im not sure by how much but it was once used as an engine for a water pump. At the shaft it has 7000 rpm. It might be 3600 rpm on the pulley wheel as that is the the industrial standard, at least nowdays anyways. Hopefully it is also governed.

    If the voltage does increase a lot, perhaps it would be appropriate with the 48v hub motor instead of the 36v? Even if the 36v "dynamo motor" is 36v at 2500rpm i suppose the voltage will increase as the engine spins at 3600 rpm or above. Apparently as seen on ebike forums the 250w hub motors can be "hotrodded" with as much as 1kw as long as they have the appropriate cooling.

    So do you think thats a safe setup and that it will work once i assemble it? The car fan heater motors seem a bit fragile, im afraid the small bearings wont be able to cope with the drag force of the belt... Ill definitly check out the outboard motor, if they have one at the scrap yard!
  15. Jul 4, 2014 #14

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    Having no idea as to your mechanical skill, i'm afraid to say.

    Spinning that 2500 rpm motor as a generator at 7,000 rpm is in my opinion pushing things too far.
    Centrifugal force goes up as square of speed.
    An oversped DC motor usually just slings the wires out of its rotor which brings it to an abrupt halt when they jam in the air gap. That ruins it. And the sudden stop can wrench it from its mount and throw it across the room..

    Worst case it'd start slinging pieces of pulley which would be really dangerous.
    That's why i suggested automobile or motorcycle alternators that're made for such speeds, or the outboard generator.

    agreed. You'd want some sort of inline flexible coupling. I've seen people use plastic hose to join the two shafts..

    and dont give up on the motorcycle alternator.

    Have you a photo of your motor and its output shaft?

    Also bear in mind i've never done what you're attempting.. just brainstorming with you and trying to help explain motor/generator basics.

    old jim
  16. Jul 4, 2014 #15
    What i meant was that i believe the engines pulley wheel spins at 3600 rpm, as it has a small gearbox with the ratio 3,0 or 3,7. Im not sure exactly how much that is however the crank spins at 7000. I think 3600 rpm is the standard for small engines, or so i heard anyway.

    I dont actually have a motorcycle alternator, i was hoping to find one at the scrap yard. I dont have a lathe, so i must have the engine block with the alternator, and i could cut around the block to get the back plate to hold the spindle in place.

    Sounds like a good idea to join the shafts with a flexible coupling! Ill definitely look into that! Perhaps i can put the tube right over the fastening nut of the pulley wheel, then things would become even simpler, without the belt and belt tensioner etc..

    Here is a link to the motor i found on the web http://www.racemotorparts.com/product_info.php?products_id=667

    And here is a link to the sachs motor and all its specifications http://www.sachs-stationaermotoren.de/st30.html

    Thanks for the ideas!
  17. Jul 4, 2014 #16
    Wait.. I just realised that the force on the engine output shaft is about as strong as the force on the wheels pulling it all forwards. Dont you think the plastic hose coupling would spin and melt right away as for the high load?

    And Mr. Sparkle. I will not have a controller as it cost quite a lot and is sensitive to too much power.
    I will be sure to get a hub motor without hall sensors as there are very sensitive to high voltage

    This is the motor i will be using http://www.elifebike.com/peng/iview.asp?KeyID=dtpic-2013-9W-RT62.470PM
    Last edited: Jul 4, 2014
  18. Jul 4, 2014 #17
    you realize that when you apply 250 watts of power to a 250 watt motor, you are going to apply 250 watts of extra power whether you want 250 or only 25. Try to imagine driving a car where the throttle only has 2 position, off and pedal-to-the-metal. I don't know anything about the route you take, but if its hilly, has a lot of corners or slowdowns, I would be greatly annoyed without a controller.
  19. Jul 5, 2014 #18
  20. Jul 5, 2014 #19


    User Avatar

    Staff: Mentor

    This is getting too dangerous. Thread is closed.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook