Range extender is an auxiliary power unit built-in or externally attached to a all-electric (BEV) or plug-in hybrid electric vehicle (PHEV) to increase its all-electric range (AER). The most commonly used range extenders are internal combustion engines that drive an electric generator which in turn supplies the battery and electric motor with electricity. The range extender can also be powered by a fuel-cell or other energy sources. The range autonomy is one of the main barriers for the commercial success of electric vehicles, and extending the vehicle’s range when the battery is depleted helps alleviate range anxiety concerns. Plug-in hybrids with a built-in range extender unit, such as the Chevrolet Volt, are also known as extended-range electric vehicles (EREV); and electric cars with a range-extender unit, such as the BMW i3 with such unit, are called range-extended battery-electric vehicle (BEVx) by the California Air Resources Board.