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Energy Consumed Versus Energy Returned

  1. Jun 14, 2010 #1
    I am trying to work out a way mathematically if it's worthwhile to place a dynamo or generator on an electric bicycle to convert the bikes movement to electricity. Some dynamos create very little drag. By doing this I hope to prolong the time before charging the batteries again.

    What I think I need to take into account is how much electricity the dynamo returns offset against the extra energy consumed from the battery to compenstate for load placed on the wheel by the dynamo.

    Is there any easy way to do this. I know no formulas except for how to work out kinetic energy. Even if someone could tell me what formulas would be required to mathematically test my idea would be good.
    Last edited: Jun 14, 2010
  2. jcsd
  3. Jun 14, 2010 #2
    Yeah, well, no one has come up with perpetual motion yet, which is required for your idea to work. Even worse, folks around here get very hot and steamy at the idea, and will generally throw the rule book at you for even looking in its direction.
  4. Jun 14, 2010 #3
    I find that strange as for if the device did work I can't see how it would be perpertual energy as the batteries would still need to be recharged at some point. Say if you connected a large generator to a small one I would of thought momentum from the larger generator, not just the power it consumes would help drive the smaller one.
  5. Jun 14, 2010 #4
    It would help your cause if you explained what you want to power from your batteries.
    You can already obtain various forms of generator to directly power lights etc and I have seen some schemes to store the energy first in rechargeable batteries.
  6. Jun 14, 2010 #5
    The bike (electric bike) is what I want to power from the batteries. I would be mounting a bottle generator to run off one wheel. Hoping to return some electricity to the battery instead of none at all. Or would the friction created by the bottle generator rubbing against the wheel be greater than the amount of electricity returned?
    Last edited: Jun 14, 2010
  7. Jun 14, 2010 #6
    This is really about practicality v theory.

    To recover any useful amount of the motional energy from a vehicle, including a bike, you are faced with several practical real world difficulties. Bikes add additional problems of their own.

    Firstly there is the simple question of the weight of the batteries. Batteries capable of producing a small amount of power, to say the lights, do not add much weight to a bike. But even with the advances in technology batteries capable of powering a motor are quite heavy.

    Then there is the weight of the motor itself.

    Hybrid cars can accept these weight penalties because the % addition to the weight is much smaller and there is more opportunity to save weight elsewhere.

    Power assisted bikes also suffer from this penalty but battery technology is now such that useful assistance can be obtained from batteries that are charged (frequently) via the mains.
    The amount of additional charge that can be input when freewheeling downhill is minimal, although I believe there is at least one assisted bike on the market that offers this - more for advertising than any real benefit.

    Then there is the wide variation of voltage available from hub or rim dynamos due to bike speed variation. Much of the time the bike just isn't going fast enough to produce useful voltage, since any charging voltage must be greater than the battery voltage, which you want at a reasonable value for motor efficiency.

    If you go for an ac generator than you have to add the additional weight penalty and conversion voltage loss to DC for charging, of the rectification circuitry.

    Engineering is about "theory meets practice = compromise"
  8. Jun 14, 2010 #7
    Thanks very much for that. I did wonder about how much weight the batteries and electric motor would add compared to a motor vehicle. It sounds very much like this theory would be far better off tested on a car. I was also thinking about trying to make a car alternator act as some sort of KERS or kinetic energy recovery system.

    Only the "brakes" would be permanently on due to the extra load the alternator places on the vehicle in all aspects. Kind of like driving your car with the handbrake permanently slightly on. Any idea if the amount of energy needed to overcome this "braking" could ever be less than the ouput of the alternator?
    Last edited: Jun 14, 2010
  9. Jun 14, 2010 #8


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  10. Jun 14, 2010 #9
    Whatever amount of energy the generator returns to the battery is exactly equal to how much more the motor will have to drain from the battery....plus inefficiencies. It doesn't matter how big how small how light how heavy or anything the generator/ motor is, this will ALWAYS hold true.

    In other words, the more you try to mess with it, the faster the battery will drain.

    If conservation of energy is something you can't grasp, here's another way of looking at it: the generator will tend to slow the bike down, by pushing it back. This means that the motor will have to push forward, in a perfect world, exactly that much harder. In the real world...probably at least twice as hard. You can try to mess with momentum and sizes and weight, but you can NEVER overcome this simple fact.
  11. Jun 14, 2010 #10
    If the generator was not physically connected to the motor or transmission of the car, would it be possible to recover energy at a gain not loss? I'm not trying to create perpetual energy, just theorising about running an EV further before the batteries drain completely.

    The reason I ask is because of http://www.freelights.co.uk/product.html" [Broken] There is no physical connection between the bike and the generator that I am aware of, but the website does refer to magnetic drag.
    Last edited by a moderator: May 4, 2017
  12. Jun 14, 2010 #11
    This is an interesting thread because most of you are quick to jump on conservation of energy and whatnot, but he is not trying to gain free energy. Bikes (and all other vehicles) have a lot of energy going to waste when they are going down hill and braking and there are a lot of online sites that offer regenerative braking rigs for electric bikes or discuss their viability. This is also not a new idea.
  13. Jun 15, 2010 #12


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    Though alluded to, I don't think the issues here were explained clearly enough above:
    The term "perpetual motion" is a colloquial term which really refers to any device that violates the laws of thermodynamics. A type 1 pmm is then merely a device that violates the first law of thermodynamics: conservation of energy.

    The device you describe is a type 1 pmm because it violates conservation of energy. Adding a generator to an electric bike at best would require exactly as much extra power from the motor as is generated by the generator. Hence, there is no left over energy to charge the battery.

    It is not a question of friction and other losses, it is a question of conservation of energy.
    Specifically, what you are missing is that a 1kW generator requires 1kW of mechanical power to turn it.
  14. Jun 15, 2010 #13


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    Yes he is talking about free energy, very much so. I wanted to bring up regenerative braking but was hoping that he could get over the free energy part first. Also you normally wouldn't want to use a seperate generator for regenerative braking, you're much better off implementing it with the existing electric motor on an electric bike.
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