Regenerative breaking is used on hybrid gas/electric automobiles to recoup some of the energy lost during stopping. This energy is saved in a storage battery and used later to power the motor whenever the car is in electric mode.
Understanding how regenerative braking works may require a brief look at the system it replaces. Conventional braking systems use friction to counteract the forward momentum of a moving car. As the brake pads rub against the wheels (or a disc connected to the axle), excessive heat energy is also created. This heat energy dissipates into the air, wasting up to 30% of the car's generated power. Over time, this cycle of friction and wasted heat energy reduces the car's fuel efficiency. More energy from the engine is required to replace the energy lost by braking.
Regenerative braking utilizes the fact that an electric motor can also act as a generator. The vehicle's electric traction motor is operated as a generator during braking and its output is supplied to an electrical load. It is the transfer of energy to the load which provides the braking effect.
As the driver applies the brakes through a conventional pedal, the electric motors reverse direction. The torque created by this reversal counteracts the forward momentum and eventually stops the car.
At the most basic level, regenerative braking means re-capturing the kinetic energy of the vehicle's motion and turning it into another type of energy. Commonly, this is done by converting kinetic energy into electricity and recharging the car's battery with it.The energy captured in the battery is used moving the car later and due to the energy capture and reutilization the mpg for the car improves drastically.