Most passenger vehicles still rely on conventional petrol or diesel engines, for which incremental improvements can be foreseen in terms of energy efficiency. Electric motors offer higher efficiency, either as battery electrical vehicles (BEVs), which are purely electrical, hybrid electrical vehicles (HEVs), which combine an internal combustion engine with an electric motor, or plug-in hybrid electrical vehicles (PHEVs), where electricity can be charged from the grid. The main challenge is the low energy density of available batteries, which limits the range between charges.