Hybrid cars use an electric motor to assist the engine during start-off and acceleration. Fuel cell cars and electric cars are driven by the electric motor alone. Cars that are equipped with a generator, use that generator to recharge the battery as required, even while the car is traveling. During braking, either the electric motor or the generator generates braking force through electric generation load and stores the power thus generated back into the battery for use when the car starts out again, which improves the fuel economy. These motors require compact size, high output, and the ability to stand up to heat, so they use rare earth magnets.