A range-extender (known as a REx or REEV) is pleasingly simple: it's an electric vehicle with a smaller battery that can be topped up by a petrol engine when the state of charge drops. Traditionally, a REx is a type of series hybrid, which means the electric motor turns the road wheels and the engine works as a generator, charging the battery. Modern REx engines are small, light and tuned for high efficiency, and they run at a constant speed when the battery needs charging.