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Electricity Regained

  1. Feb 3, 2006 #1
    A friend of mine wondered if- you had two batteries in an all electric car (big ones), both hooked up to a generator on the wheels. The car drives for a bit using only the power from one battery (battery#1) to power the car. Then, when battery#1 runs out of power the car switches to battery#2. Now, while battery#2 is powering the car, battery#1 uses the spinning action of the wheels (along w/ the generator) to put some power back into battery#1.

    Now, I realize that you could never get all the power back from both batteries, but wouldnt it be able to go farther than one huge battery that holds the same amount of power as batteries#1 and #2 combined due to the generator replacing some of the lost energy of one battery while the other is working?:confused:

    If it does work, will my friend be able to not worry about his idea getting stolen since i've posted this before anyone else, and i know the date too?
    Last edited: Feb 3, 2006
  2. jcsd
  3. Feb 3, 2006 #2
    Nope its not efficient because the energy used from the batteries to spin the wheels would also be used to power the generator that charges the batteries. Hence useless.

    Put it simply, the only source of energy is the battery and you cannot make more energy from what you have.
  4. Feb 4, 2006 #3
    What do you mean its not efficient? One battery is charging the other (indirectly), while powering the car. I KNOW that you cant get all the energy back, but it would make the car go farther on the same amount of "starting" energy as one big battery. There are 2 batteries, not just one, so the generator isnt hooked up to the same battery that energy is coming from.
    Last edited: Feb 4, 2006
  5. Feb 4, 2006 #4
    The electrical generator works like this. A rotor with magnets is spun inside a coil which produces the electric current to charge the battery in your example.

    But, eddy currenst are also created in the coil which cross the magnetic flux of the magnets producing mechanical resistance of the rotor. So the faster you spin the generator, the more resistance is created to slow it down.

    Assuming your car was driving on a flat surface, the electrical generator would start producing resistance slowing the car down. Then Battery #1 would have to supply more power to compensate for the resistance to maintain current speed. Battery #2 would then receive at most the power being compensated by Battery #1.

    Put is simply, some energy from Battery #1 was transefered to Battery #2.

    Therefore you cannot go further because your only source of energy is Battery #1.

    Now if the car was driving down hill then you could gain more power due to earth's gravitational potential energy. But if you go up hill then you lose even more.

    Hope that helps.
  6. Feb 4, 2006 #5


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    You should consider a regenerative drive. That consumes energy from the battery when accelerating or maintaining speed, but recharges the battery during deceleration/braking. You should have mechanical friction brakes as a back-up, but your car should be designed to use the resistance of the regenerative drive for most of your braking. A PIV transmission would be helpful, too, since it could be designed to act like a super-low gear during start-up and a super-high gear (overdrive) when cruising. This is probably about as efficient as you can get with today's technology. Of course you could make your vehicle very sleek and flat and cover its upper surfaces with photovoltaic panels for extra charging.
    Last edited: Feb 4, 2006
  7. Feb 4, 2006 #6


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    Since energy is being taken out via the generator, and the generator isn't 100% efficient, the car wouldn't be able to go as fast or as far as without the generator. All you are doing is adding inefficiency.
  8. Feb 4, 2006 #7
    But once battery #1 runs out it switches to battery #2, however much it is charged. It leapfrogs back and forth.
  9. Feb 4, 2006 #8


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    This will not help you gain efficiency. You will lose efficiency by lugging around twice as many batteries (heavy things!) AND a generator, AND it will cost you more battery power fight the resistance of the generator than you can extract from it to charge the second battery. There is no free lunch.

    Your best bet is a regenerative drive in which your electric motor powers the wheels when accelating or maintaining speed, but acts as a generator when you are slowing down. The electromagnetic resistance provides the braking force you need to slow down. The reason I suggested adding a PIV (positive infinitely variable) type of transmission is that it can be tuned (with the regenerative drive) to provide a wide range of braking forces.
  10. Feb 4, 2006 #9


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    You do understand that the amount of energy you get out of a battery by running a motor is exactly the same as the amount of energy you get back by connecting it to a generator (minus the inefficiencies, of course), right? There'd be nothing left to actually move the car!

    Any energy that goes to charging the battery is energy that isn't available to move the car. So if you are able to charge the second battery to half the charge of the first(again, minus efficiency losses), you lose half the distance the car could have traveled.

    Again, even if everything were perfectly efficient, you'd gain nothing at all.
  11. Feb 4, 2006 #10


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    Orion's Thought - it takes POWER to run a generator. If you can easily spin a generator, then you know its output is easily small as well.

    The power to generate electricity has to come from somewhere, and if you're sucking it out of the other battery its going to go dead that much faster on an exponential curve. So switch all you want, but with Peukret factored in the one big battery will win everytime even if the capacities are matched beforehand.
  12. Feb 5, 2006 #11
    Yes, but one battery isnt powering the other battery, its powering the wheels. In turn, the wheels are powering the other batter through the generator, so the first battery isnt interacting at all with the second battery, its only job is to move the wheels.

    Whats this about a regenerative drive? I'll google it, but any additional info would be useful.

  13. Feb 5, 2006 #12


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    Ask yourself where the energy to charge battery 2 is coming from.

    Its only job is to move the wheels. Yes. But the wheels are much harder to turn, since they're attached to a generator which is trying to charge battery 2. Just because battery 1 and battery 2 aren't electrically connected doesn't mean that they're independent.

    A regenerative drive (as you've been told) turns kinetic energy into electrical energy on overrun and braking, - ie when you're not running your drive motors.
  14. Feb 5, 2006 #13


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    Huh? No. Push a car yourself and find out if that's true!! Push it in neutral and it'll be easy. Then put it in gear - is it still easy? You're not interacting with the motor, just the car, right...?

    You really don't seem to be putting any effort into understanding this. Please try harder - this is very simple.
  15. Feb 5, 2006 #14


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    Ok I've watched this long enough and I can no longer keep myself from posting. Let me ask you this orion:
    Take an electric motor and a generator and hook their shafts together. Now take a battery and hook it to the motor so it spins. Then take the generator and hook it so it charges the battery. Now think about what is happening here. Will this scenario run longer than if you just hooked the battery to the motor without using a generator at all? If you believe that it does then shouldn't we be able to remove the battery spin the assembly up by hand and shouldn't it spin for a longer time than just spinning the motor or generator up by hand alone? I assume you believe that perpetual motion is not possible. If my assumption is correct then what I've just proposed to you should cause you to see the light. Just because we have no car or wheels touching the road doesn't mean a thing. The scenario I proposed is the car and road removed which are ONLY loss to the system. There is no power gain by having a motor/generator attached to a car/road/wheels.
  16. Feb 5, 2006 #15
    This is simple here is an analogy.

    Suppose you have an imiginary car that runs on water and can get 100 miles on full tank, thats your Battery#1 call it Tank #1.

    You also have a second Tank#2 analogous to Battery #2 which is empty.

    So to charge Tank#2 what do you do?

    You would have to transfer some of the water from Tank#1 to Tank#2.

    Now Tank#1 has less water as a result of charging Tank#2 and therefore you wouldn't get as far would you?

    Now if Tank#1 empty and you switch to Tank#2 How far will you go?
  17. Feb 5, 2006 #16
    First- please dont get angry with me, I've been letting my friend ask the questions, and second- he asks, what if there was a generator attached to the back and front axels, to each is hooked up one battery. Then, when you start out, its front wheel drive, then, you switch to rear wheel drive when the first one runs out of E. Is this still presenting the same problem?
  18. Feb 5, 2006 #17


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    Yes. The front wheels and the back wheels are connected to each other via the ground, so the first battery is just charging the second. There is no extra energy anywhere to capture.
  19. Feb 5, 2006 #18


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    Okay, it seems folks are still busy spinning their wheels here (bad pun intended), so I'll venture in and give it a try as well.

    First, let's just forget battery #2 and focus on battery #1 and build from there. Let's further assume you've designed a very efficient vehicle, so as you use energy from the battery to run the wheels, it just goes from battery to wheels to motion of the vehicle without any other losses.

    Now, you add a connection to a generator to those wheels, and that generator is intended to produce as much energy as is required to turn a set of wheels. Now, the energy from that battery has to not only turn the wheels, but also power the generator; it's going to take twice as much energy from the battery to do this (again, assuming a perfectly efficient system, which is not reality). So, all the energy from battery #1 that is going into compensating for the increased friction on the wheels for powering the generator goes to battery #2. The alternative, without your generator, would mean instead of expending the extra energy from battery #1 to charge battery #2, that energy would just stay in battery #1 until needed. So, the end result is 0 improvement. When you add in the losses that naturally occur because there is no such thing as a perfectly efficient system, the net result is less battery power to get where you're going because more is wasted in the process of transferring it from battery #1 to battery #2.

    In your example of putting the generator on the rear wheels instead of the front wheels, it would be like driving with your foot on the brake, so more work has to go into turning the front wheels to keep the car moving so you can power a generator off the back wheels.
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