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An electric car stores its energy on board—typically in batteries, but alternatively with capacitors or flywheel storage devices. Or it may generate energy using a fuel cell or generator. A fuel cell is a specialized form of battery that combines hydrogen with oxygen in a chemical reaction that produces electricity and water vapor. Unlike an electric cell or battery, a fuel cell does not run down or require recharging; it operates as long as the fuel and an oxidizer are supplied continuously from outside the cell. Most current versions of electric cars use some combination of these energy sources. “Pure” electric cars, however, run only on batteries and need a charger to replenish the battery's power from an electrical outlet. The hybrid electric vehicle (HEV) uses both an electric motor or motors and a gasoline or diesel engine to extend the car’s range and often to provide additional power. A conventional HEV, such as the Toyota Prius, uses battery power up to certain speeds and the gasoline engine for higher speeds, and can draw on both power sources if needed. The batteries are recharged by the gasoline engine, which acts as a generator, and in most models by the energy generated from braking. Another type of HEV, known as a plug-in hybrid electric vehicle (PHEV), uses an extra battery or batteries to extend the range of the vehicle. The PHEV can be plugged into a typical 120-volt electric outlet, like those found in most households, for a recharge. Regardless of the energy source, an electric car needs a controller, which is connected to the accelerator pedal, for directing the flow of electricity from the energy source to the motor. Most electric cars use lead-acid batteries, but new types of batteries, including zinc-chlorine, nickel metal hydride, and sodium-sulfur, are becoming more common. The conventional HEV uses nickel metal hydride, while some automakers contemplate using lithium-ion batteries; General Motors, for example, may use these in their hybrid electric vehicle prototype known as the Volt. The motor of an electric car harnesses the battery's electrical energy by converting it to kinetic energy. The driver simply switches on the power, selects “Forward” or “Reverse” with another switch, and steps on the accelerator pedal.