Electric Vehicle Frequently Asked Questions

HYBRIDS AND RANGE EXTENDERS

Can I add a small generator? (APUs and generator trailers)

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What is a pusher trailer?

A pusher trailer is a type of range extender trailer. Pusher trailers are made from a clip of a car that has all of the drive components in one end (example: the rear of a VW bug, or the front of a front wheel drive car). It is hitched to an EV and controlled from inside the EV. The pusher trailer then provides the motive power for long trips, usually for cruising power on the highways.

A pusher trailer is generally (though not universally) considered to be more efficient than the other popular option for range extension. This option is to carry a generator used to turn the EV into a series hybrid. The pusher is usually considered more efficient because there is only one energy conversion step in the pusher(chemical to mechanical), but in a generator trailer or series hybrid, there are several conversions (chemical to mechanical to electrical to chemical (battery storage) to electrical to mechanical). Since each energy conversion step has an inefficiency associated with it, the simpler method of the pusher trailer yields the more efficient range extension method. It is also easier for the hobbyist to build.

Examples of pusher trailers are on Mr. Sharkey's website, and JB's EV and pusher trailer.

What is meant by series or parallel hybrid?

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Can I charge my EV with solar power?

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Can I put a generator on the un-driven wheels to charge batteries while driving?

Q: "Why can't you put a generator on a wheel to charge the batteries
while you drive? Then you could get more range, couldn't you?"

A. Most EV hobbyists are asked this question very often. It seems as if
it ought to work -- after all, that's (more or less) how you charge the
starting battery on a gas car. But if you think about it, you'll see
that in an electric vehicle, it would violate the laws of physics.

Where does the kinetic energy of the vehicle come from? In an electric
vehicle, it comes from the batteries. That's the only energy source,
unless you have a sail.

The EV's motor converts the batteries' electrical energy into mechanical
energy. A generator is the opposite of a motor -- it changes mechanical
energy into electrical energy.

Now suppose you drove the back wheels with a motor, and put a generator
on a front wheel. When you engaged your front generator, some of the
vehicle's kinetic energy would be turned back into electrical energy.
With less kinetic energy, the vehicle would slow down, unless you added
more energy drawn from the batteries to replace it.

If your generator on the EV's front wheel were 100% efficient, and so was
your motor, at best you would only break even -- your front generator
passing to the rear motor and batteries exactly the kinetic energy that
the vehicle already has.

That is, the generator would turn the vehicle's mechanical energy into
electrical energy at exactly the same rate the motor was turning the
electrical energy into mechanical energy.

In other words, you would gain nothing.

But in fact, neither the generator nor the motor is 100% efficient. Each
one is typically only 50% to 85% efficient. The result of this scheme is
a net ~loss~ in range, not a gain.

So, unless you want the vehicle to slow down, it's just a waste of energy
to put a generator on a front wheel.

However, there is some use for a generator in an EV. As the above
paragraph suggests, you can use it to help stop the vehicle. This allows
you to reclaim some kinetic energy, instead of wasting it as heat in the
brakes. This is regenerative braking (sometimes also called
recuperation). This really ~does~ increase your range, typically by 5 to
20 percent.

However, there's no need for the weight and expense of putting a
generator on a front wheel to perform regenerative braking. Instead, with
some extra control hardware, you can use the main traction motor. When
you take your foot off the accelerator or touch the brake pedal, the
vehicle's control system rewires the motor "on the fly" so it works as a
generator.

How about a windmill? (A discussion of basic physics. )

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