Regenerative Braking is an energy recovery mechanism which slows a vehicle by converting its kinetic energy into another form, which can be either used immediately or stored until needed. The braking energy is converted into electric power, which is used to charge the vehicle battery. This contrasts with conventional braking systems, where the excess kinetic energy is converted to heat by friction in the brake linings and therefore wasted.

Regenerative braking is used on hybrid gas/electric automobiles to recoup some of the energy lost during stopping. Regenerative brake involves using an electric motor as an electric generator. The generated electricity is fed back into the supply system. In battery electric and hybrid electric vehicles, the energy is stored in a battery or bank of capacitors for later use. Energy may also be stored mechanically via pneumatics, hydraulics or the kinetic energy of a rotating flywheel.

Electric Taxi VehicleUnderstanding 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.

Hybrid gas/electric automobiles now use a completely different method of braking at slower speeds. While hybrid cars still use conventional brake pads at highway speeds, electric motors help the car brake during stop-and-go driving. 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.

But regenerative braking does more than simply stop the car. 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 or dynamo. This generated electricity is fed into a chemical storage battery and used later to power the car at city speeds. Only if the required deceleration is not sufficient, then the conventional brake system is activated.

Traditional friction-based braking is also used in conjunction with mechanical/electromechanical regenerative braking in the latest hybrid vehicles because the regenerative braking effect can only decelerate the vehicle to lower speeds; therefore the friction brake is still required in order to bring the vehicle to a complete halt. Physical locking of the rotor is also required to prevent vehicles from rolling down hills. The friction brake is a necessary back-up in the event of failure of the regenerative brake.