Regenerative braking is used on automobiles to recoup some of the energy that is lost while the vehicle is stopping. This technology is used on hybrid vehicles that use both gas and electricity as sources of power. The energy that is recouped during braking is saved in a storage battery and used later to power the motor whenever the vehicle is using its electric power source.
Conventional Braking Systems
In braking systems on conventional vehicles, friction is used to counteract the forward momentum of a moving vehicle. As the brake pads rub against the wheels or a disc that is connected to the axles, excessive heat energy is created. This heat energy dissipates into the air, wasting as much as 30 percent of the vehicle's generated power. Over time, this cycle of friction and wasted heat energy reduces the vehicle's fuel efficiency. More energy from the engine is required to replace the energy that was lost by braking.
Regenerative Braking Systems
Hybrid gas/electric automobiles use a completely different method of braking at slower speeds. Hybrid vehicles still use conventional brake pads at highway speeds, but electric motors help the vehicle brake during stop-and-go driving at slower speeds. As the driver applies the brakes by pressing down on a conventional brake pedal, the electric motors reverse direction. The torque created by this reversal counteracts the forward momentum and eventually stops the car.
Regenerative braking does more than simply stop the car, however. Electric motors and electric generators — such as a car's alternator — are essentially two sides of the same technology; both use magnetic fields and coiled wires, but in different configurations. Regenerative braking systems take advantage of this duality. Whenever the electric motor of a hybrid car begins to reverse direction, it becomes an electric generator. This generated electricity is fed into a chemical storage battery and used later to power the vehicle at city speeds.
Some Loss of Energy
The technology employed during regenerative braking takes the energy that is normally wasted during braking and turns it into usable energy. A hybrid vehicle is not, however, a type of perpetual motion machine. Energy is still lost through friction with the road surface and other drains on the system. The energy collected during braking does not restore all of the energy that is lost during driving. It does improve energy efficiency, however, and it assists the main alternator.