Perpetual Motion

A common question I get about electric cars is something along the lines of “Couldn’t you just attach an alternator to recharge the car as it drives?” Sometimes the question specifies that the alternator is attached to the wheels or that you couldn’t drive the car continuously, but could at least extend its range significantly.

The short answer is “No.”

For the longer answer, we always have to think about where energy comes from, and where it goes.

According to the Laws of Thermodynamics, energy can be neither created nor destroyed, but changes from one form to another. It also tends to become less useful. For example, coasting in a car that is already at speed is a pretty good way of moving people around. When we press the brakes, the inertia of the moving mass gets converted to heat through friction. The energy has been converted from kinetic (movement) energy into heat energy. However, the heat in the brakes isn’t very useful for moving the car. The energy has been converted from a practical or useful form of energy into a less useful one. Because of that, we constantly need a new source of energy. In an automobile, that’s typically in the form of chemical energy as gas or diesel fuel.

In an electric car, the main battery holds the energy for the system. It’s not actually the energy source – that’s the power plant where the electricity came from. (That could be coal power, nuclear power, solar, wind, or other sources.) The battery provides energy to spin the electric motor. That electric motor propels the vehicle down the road.

What happens when we add an alternator or generator?
A generator creates electrical power from mechanical power. The source of that power could be moving water (hydro-electric dams,) coal (fossil-fuel burning power plant,) or the gasoline engine of a car. What they all have in common is that the generator needs POWER IN to create POWER OUT. All generators create a drag or resistance on whatever powers them. The process used by a generator is that of creating an electro-magnetic field. The larger the magnetic field, the more electric current is generated and harder the generator resists its mechanical power source.

If you have ever used a stand-alone gasoline generator, you know how the engine slows down when you plug in and turn on a heavy electrical load. The more electricity that needs to be generated, the harder the engine has to work, and the more gasoline it burns.

Now imagine that we have a generator in an electric car.
The Electric Motor (powered from the battery) pushes the car down the road. A generator (whether connected to the motor or to the wheels) running to recharge the battery would put a load on the motor, making it work harder and draw even more electricity. No matter how much electricity the generator would produce, it would require even MORE electricity for the electric motor powering the car. Adding the resistance of the generator could only slow the car down. It’s essentially like trying to drive with the parking brake on.

In other examples, people sometimes ask about adding a Wind Turbine to the top of a car to generate power while driving. Just like with the alternator, you can’t get any more power out than what you put in. For any wind power the turbine makes, the wind resistance makes it even harder to drive down the road, and you can only end up with lost energy. It’s a bit like trying to fly by picking yourself up by your belt.

But what if we WANTED to slow the car down!?

Aha! Now you are thinking! If the electrical generator were ONLY running when you wanted to slow down the car anyways, it makes perfect sense! Convert the kinetic energy of the car into electrical energy to charge the battery, thus getting some energy AND slowing the car. Perfect if you want to come to a stop!

But it gets even better!
Many electric motors actually work VERY WELL as electric generators. Since we only want to generate power when we are NOT trying to use the electric motor to move the car, the electric motor can do double-duty as both motor and generator! This process is used on almost all modern hybrid and battery-electric cars and is known as “Regenerative Braking”. Using the motor as a generator eliminates the space, weight, and mechanical losses of a separate generator.

In addition, most modern cars are front-wheel drive. When a vehicle brakes, the weight gets shifted forwards, and the majority of the braking is done by the FRONT brakes. Using the electric motor on the front wheels to generate electricity while braking is not only efficient, but also saves wear on the brake pads. Our 2004 Toyota Prius has over 150,000 miles on it, but is still on its first set of brake pads!

So, there you have it. While you can’t drive an electric vehicle having it perpetually power itself, you CAN recapture some energy when you want to slow down anyways.

There ARE other ways to power plug-in vehicles long distance, but they all require an EXTERNAL power source. One example is with solar panels. The sun shining on the photovoltaic panels is an energy input to the system. Solar panels on electric cars have been well used on college engineering competitions, but aren’t practical for most on-road vehicles at the moment. (They can work well for boxy, slower vehicles, such as golf carts, pontoon boats, and parked Recreational Vehicles.) For now, solar is best mounted on a house or garage for charging electric vehicles. See our Solar Garage for one such example.

In the real-world, there’s no such thing as a perpetual motion machine. Keeping a car tuned-up, having correct tire pressure, and other regular maintenance can help keep it efficient, but it will never be an infinite range vehicle without some other external source of power.

PS: The history of perpetual motion is quite interesting. Look up perpetual motion machines on Wikipedia for a good time!