Alternative powertrain vehicles are those that use energy sources other than conventional fossil fuels like gasoline or diesel. Full battery electric vehicles rely solely on electricity stored in batteries. Hydrogen fuel cell vehicles use hydrogen gas reacted with oxygen to produce electricity, with water as a byproduct. Fuel cell-electric hybrid vehicles combine a fuel cell system with a battery, offering flexibility in energy usage. All three types represent distinct approaches to reducing or eliminating reliance on traditional combustion engines and their associated emissions.