anyone ever try fitting an alternator to an ebike?

Discussion in 'Electric Bicycles' started by zippinaround, Aug 6, 2013.

  1. zippinaround

    zippinaround Active Member

    just wondering if anyone out there has ever tried fitting a truck alternator to run friction drive from the back tire to recharge the batteries for extended range? just an idea that popped into my head maybe it wouldn't turn it fast enough to charge?

  2. professor

    professor Active Member

    What you desrcibe is a perpetual motion machine.
    Besides that an alternator is very inefficient.
  3. zippinaround

    zippinaround Active Member

    how is it inefficient? have you ever tried it?
  4. TheCrystalSkull

    TheCrystalSkull New Member

    Power required to turn the alternator at a useful speed (if you need to use gearing, there's added power needed), losses in rectification and battery charging circuitry.

    It can take one to a few horsepower to run an alternator, you produce _max_ one-third of a horsepower, your motor might use 100wh to 1500wh to produce it's rated 500w or 1000w output (typical setups anyways) depending on what's going on. But it's gonna go up dragging an alternator around. And now your motor has to work harder... which means it requires more current to produce the same output... which uses more current, which requires more mechanical power to be applied to the alternator to supply that current...

    See where this is going?
  5. bluegoatwoods

    bluegoatwoods Well-Known Member

    The history of physics is sort of fuzzy in my memory. So I might not have details perfect. But........

    Look up (by google, for instance) a fellow named Helmholtz. First name, maybe, Friederich or Ferdinand. Plus I think there's a Von in between. He lived in, probably, the nineteenth century. If not, then the eighteenth. Certainly a German. I'm assuming he lived in Germany.

    He came up with three laws of energy that are not seriously doubted by anyone who is seriously interested in the truth to this very day.

    The first states that energy --though Helmholtz himself did not realize it, this really applies to matter/energy-- can not be created nor destroyed.

    The second states that energy will move on it's own from areas where it is more concentrated to areas that are less concentrated.

    The third is less easy to state in a simple sentence or two. But it deals with a concept known as entropy.

    (And how am I going to get to the point without writing a long lecture that I'm only very partly competent to write? I don't know yet, but I'll try......)

    Entropy is a measure of just how 'diluted' the energy of a system is. That is diluted in the sense of the second law; how evenly spread out the energy is. A closed system --no energy entering or leaving-- at maximum entropy might be viewed as 'dead' in a physical sense. Nothing can possibly happen within that system. No movement, no chemical reactions, etc. Plus there is no way, even in theory, to separate the energy in the system. To put it back to the state where there is energy concentrated more in some parts than in others. So that movement or chemical reactions and such then could take place again, per the second law.

    Now imagine a dead battery. This would be an approximate, real world example of a closed system at maximum entropy. It is inert and it will not do the work that we normally expect of a battery. Now in the real world a dead battery does not need to remain a closed system; we can easily pour more energy into it. Using a battery charger, for instance. Or an alternator.

    But the third law makes it clear that you must pay a penalty for doing so. No matter what you do it will always, always, always cost you more in energy than you can expect to extract from the system.

    So imagine this alternator on your bike and charging your batteries. We can imagine that this setup will give to you X amount of energy from those batteries once they are fully charged. The third law tells us that no matter what you do you will have to pour more than X energy, substantially more in most cases, into that alternator. That energy will have to come from your own legs, perhaps. (Might as well forget that. Humans don't have the power to charge batteries like this in any reasonable time.) It might come from a gasoline engine. But the gasoline burnt would get you further by chain drive off of the engine than it will by charging batteries and having them turn a motor. The third law tells us so.

    I guess I don't need to go on. I'll only end up confusing the subject and myself. But I can promise you that if you were to build such a system and claim that you made it work, then every scientist in the world --who is worth the name-- will bet any odds, any odds at all, against you, feeling completely certain that they can count their winnings as money in the bank. And they really, really will be right.

    There's a reason I took the time and space to write all of this. It's in the hope that anyone who reads it will grow a bit curious. Maybe google Von Helmholtz and read what they (you) can. Gather an understanding of concepts like the conservation of mass/energy and entropy.

    The understanding that I referred to above forms protective armor for the mind. You'll be able to spot horsefeathers from a mile away. You'll be insured a great deal more against fraud. Both literal and figurative.
    zippinaround likes this.
  6. DrkAngel

    DrkAngel Member

    Grade School Science?

    Simply ...
    Any alternator, generator etc. requires more energy input than it will output!

    An eBike that cruises along with a 500w input ...
    Add a generator to produce 500w ...
    Your battery will be forced to supply the original 500w ... plus an additional 1000w to overcome the drag produced by the generator. After initial start the battery must supply 1000w through the motor to the generator to produce the 500w to power the motor. Effectively ... your battery must supply 2x the energy! 50% for motivation and 50% to spin and heat up your generator (alternator).
    So ... add an alternator ... if you want to cut your range in half!

    A permanent magnet generator might be 80-90 efficient in turning mechanical energy into electrical.
    A series wound generator, or alternator, uses additional energy to power electromagnets, dropping efficiency to 50-60% or lower! Drop this another 10-20% to offset the inefficiencies of a friction drive.
    Last edited: Aug 8, 2013
  7. zippinaround

    zippinaround Active Member

    ok i realise that it wouldn't work now thanks for all the feedback!
  8. bluegoatwoods

    bluegoatwoods Well-Known Member

    No, no shouldn't give up this easy. We need to have a an argument like those physics geeks on that TV show.:cool:

    You might challenge us nay-sayers on these grounds, for instance. "Then how come a Chevy Volt uses a gasoline engine to charge a battery? Wouldn't it be more efficient, by your reasoning, to simply have the engine power the wheels directly, like in a conventional car?"

    I've been pondering that question since writing that long winded post above. And I think I know the answer. But it's not a lot more than guesswork.
  9. BigBlue

    BigBlue Active Member

    Here's a possibility!

    AKA: BigBle
  10. siouxindian

    siouxindian Member

    are you just talking a bought charging up a battery for head lights and tail lights?
  11. zippinaround

    zippinaround Active Member

    hahhaha bigblue i actually have one of those little generators too!

    soo what about fitting a small wind turbine generator?
  12. professor

    professor Active Member

    Here is what I did when I thought e-bikes were legal in my state ( was untrue)-
    HF 79 spinning a Ford alternator, charging a set of batteries, with a scooter motor powering the bike. Would go just under 20 mph BUT the engine was flat out unless coasting. I found out an alternator is about 50 percent efficient ( did a lot of asking and looking on the net for that info but could not find it).
    Gas milage was dismal. Finally bought a scrap moped w/ registeration and went to full gas power and made the bike into the moped- registered and everything/ plate on the back- no problems.
    Here is the video I did-

    I don't know if that is clickable so search - gas/electric e-bike on youtube.
    Last edited by a moderator: Dec 15, 2015
  13. zippinaround

    zippinaround Active Member

    looks pretty cool!
  14. Pablo

    Pablo Motored Bikes Sponsor

    A solar panel roof on my ebike would be sweet.
  15. Anton

    Anton Administrator Staff Member

    Solar panel roof ebike is an interesting concept. I saw a guy riding through the centre of Australia with a working example!
  16. DrkAngel

    DrkAngel Member

    Solar Sails!

    Well ...
    Solar panel output between 6-10watts per square foot, under optimal conditions, direct overhead sun etc.
    To power a 500w eBike motor you would require between 600-1000w electrical input.
    At 10watts per square foot that would be 60-100sq/ft or 2-3 full 4' x 8' sheets of plywood in size.
  17. Pablo

    Pablo Motored Bikes Sponsor

    I was just thinking of charging, not running it...
  18. Dankoozy

    Dankoozy Member

    Maybe a solar panel to fill the area under the crossbar. You should be able to fit the bones of 30w in there. Buy loose cells, backing and cover, clips and custom make a panel to fill the entire area. On a good sunny day it should help nicely to fill the battery
  19. Traveler

    Traveler Member

    This would only work if you could engage the alternator when you are coasting downhill. Some electric cars work on this theory, not necessarily with an alternator. I think the motor it'self could generate electricity to help charge batteries while going downhill. All this would do though, is allow you to travel further on a charge in an area that has hills.
  20. craiger45

    craiger45 New Member

    When I was a kid we used small generators that had a wheel on top that pressed against the rear wheel on our bicycles. It would make the front headlight and rear red taillight shine. The faster you propelled the bike the brighter your lights would shine.