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.