# Battery re-powered from wheels of a car?

## Main Question or Discussion Point

Hi,

I was just learning about Electric Vehicles, and how they are powered. Let us consider a BEV (Battery Electric Vehicle), and not a hybrid. So we just have the battery powering the drive train and the wheels.

The other day a friend of mine asked why the each wheel of the car can't in turn be connected to an axle (going into the car parts, not outward), with fan-like blades at the end. As the wheels moved, the fan-blades would move too and if these blades were placed within the magnetic field of a strong permanent magnet, by Faraday's law this should generate electric current. This current could e used to charge the battery again.

The idea seems great, but probably reveals a lack of basic understanding of a certain law. I am aware of the energy conservation principle and all, but I can't figure what would actually happen in such a set-up. I have had similar idease earlier (of using motion between a magnetic field to get electric enegry "again"), but I understand there is a flaw in it, which is why it hasn't been used till date. Can anyone explain?

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davenn
Gold Member
2019 Award
yes you will produce some power
But you you cannot produce as much or more power from the system that that being used by the system. So in time the battery is still going to go flat.

Dave

Drakkith
Staff Emeritus
Inefficiencies! You will lose power in many different areas, meaning that only a small part of the power will be put back into the batteries.

Bobbywhy
Gold Member
There is a system called "regenerative braking", where the energy (momentum) of a moving car is captured by reversing the process: generate electric power by switching a moving wheel into a generator. Google it.

There is a system called "regenerative braking", where the energy (momentum) of a moving car is captured by reversing the process: generate electric power by switching a moving wheel into a generator. Google it.
Yes, this is used. For starters, see:
http://en.wikipedia.org/wiki/Regenerative_brake

Last edited:
Borek
Mentor
Regenerative braking let's you reuse some of the energy that would be otherwise lost during braking, I have a feeling OP asks about producing energy all the time. That would just increase power needed to move the car - TANSTAAFL.

mfb
Mentor
To add that specific point: The moving blades would be braked by the field they are moving in - you would need more power to drive the wheels if you try to extract energy like that. With 100% efficiency, this "more power" is exactly the power you get back in your additional setup.

Staff Emeritus
2019 Award
He is trying to produce energy by driving with the brakes on. That, of course, will not work.

CWatters
Homework Helper
Gold Member
Jay...

What you propose amounts to the following circuit..

Battery---> motor ---> wheel ---> generator--> back to the battery

Lets suppose the generator is delivering 100W back to the battery and each step in the chain is 90% efficient.

That means the wheel must supply the generator with..

100*100/90 = 111W

But the wheel is being turned by the motor so the motor must supply the wheel with an extra..

111*100/90 = 123W

but the motor is being powered by the battery so the battery must supply the motor with an extra...

123*100/90 = 137W

So the net result is that the battery is supplying 137W and getting 100W back.

This is a practical example of this theory in operation...

http://en.wikipedia.org/wiki/Conservation_of_energy

To add that specific point: The moving blades would be braked by the field they are moving in - you would need more power to drive the wheels if you try to extract energy like that. With 100% efficiency, this "more power" is exactly the power you get back in your additional setup.
Okay, this made me understand it. As in moving against the field would mean moving against "something" and that would actually slow the wheels down, right?

And in corollary, IF we need to slow down, we can activate such a mechanism to recharge the battery - which is regenerative braking.

Is that right? Thanks guys :)

mfb
Mentor
As in moving against the field would mean moving against "something" and that would actually slow the wheels down, right?
The movement in the magnetic field from one side induces a magnetic field in the other side as well, and the interaction between both slows the rotation.

IF we need to slow down, we can activate such a mechanism to recharge the battery - which is regenerative braking.
Right.

CWatters