by Chris Woodford. Last updated: November 7, 2013.
Simple, convenient, cheap, and economical—bicycles are one of the world's favorite forms of transportation. But they're not for everyone. They can be hard to pedal up and down hills or with heavy loads, and elderly or disabled people may find them impossible to manage. In the last few years, a new generation of electric bicycles has begun to revolutionize our idea of environmentally friendly transportation. These new cycles have all the convenience of cars with all the simple economy of ordinary cycles. Let's take a closer look at how they work.
Photo: A typical electric bicycle parked at a railroad station. Electric bikes are heavier and more cumbersome than ordinary cycles, so it's harder to carry them on trains, buses, and so on on.
The basic concept of the electric bike
If you have dynamo-powered bicycle lights, you already own an electric-powered bicycle! Consider: as you pump your legs up and down on the pedals, you make the wheels rotate. A small dynamo (generator) mounted on the rear wheel produces a tiny current of electricity that keeps your back safety lamp lit in the dark. Now suppose you could run this process backward. What if you removed the lamp and replaced it with a large battery. The battery would kick out a steady electric current, driving the dynamo in reverse so that it spun around like an electric motor. As the dynamo/motor turned, it would rotate the tire and make the bike go along without any help from your pedaling. Hey presto: an electric bike! It may sound a bit far-fetched, but this is more or less exactly how electric bikes work.
Photo: This eZee Forte electric bicycle has a range of up to 48 km (30 miles) and a top speed of around 24 km/h (15 mph). Picture by kind permission and courtesy of 50 Cycles Electric Bikes.
Key parts of an electric bike
There are four key parts to an electric bike: the batteries, the motor, the sturdy frame and spokes, and the brakes.
The batteries are the most important parts of the bike, because (if you don't do any pedaling) they contain all the power that will drive you along. Typical electric bike batteries make about 350–500 W of power (that's about 35–50 volts and 10 amps), which is about a quarter as much as you need to drive an electric toaster. In theory, you could use any kind of battery on a bicycle. In practice, however, you want to use something that stores lots of power without being too heavy—or you'll be using half your power just moving the battery along! That tends to rule out heavy lead-acid batteries like the ones that start cars, though some electric bikes do use them. Lightweight lithium-ion batteries, similar to those used in laptop computers, mobile (cellular) phones, and MP3 players, are now the most popular choice, though they're more expensive than older rechargeable battery technologies such as nickel-cadmium ("nicad"). Typical batteries will give your bicycle a range of 10–40 miles between charges (depending on the terrain) and a top speed of 10–20 mph (which is about the maximum most countries allow for these vehicles by law). You can extend the range by pedaling or free-wheeling some of the time.
Photo: Electric bicycles give themselves away with their large battery packs, usually mounted somewhere on the frame between the wheels. In the photo of the eZee Forte up above, the battery is mounted vertically next to the seat tube.
In the theoretical electric bike we considered up above, we had the dynamo/motor driving the back wheel directly, simply by pressing on the tire. Most electric bikes work a different way. They have compact electric motors built into the hub of the back or front wheel (or mounted in the center of the bike and connected to the pedal sprocket). Take a look at the hub of an electric bike and probably you'll see it's much fatter and bulkier than on a normal bike. You can read more about how these motors work in our main article about hub motors.
Photo: Left: The hub motor of an electric bike with its workings revealed! Note the thick copper coils of wire that convert electric power from the battery into the movement that pushes you along. Picture by courtesy of Fabian Rodriguez, published on Flickr under a Creative Commons Attribution 2.0 License. Right: Usually, the coils are not exposed and the hub motor looks much like an ordinary hub, only somewhat more bulky.