A regenerative brake is an energy recovery mechanism which slows a vehicle or object by converting its kinetic energy into a form which can be either used immediately or stored until needed. This contrasts with conventional braking systems, where the kinetic energy is converted to unwanted and wasted heat by friction in the brakes. In addition to improving the efficiency of the vehicle, regeneration can greatly extend the life of the braking system as its parts do not wear as quickly. The most common form of regenerative brake involves a motor as an electric generator. In electric railways the electricity generated is fed back into the supply system, in battery electric and hybrid electric vehicles, the energy is stored chemically in a battery, electrically in a bank of capacitors, or mechanically in a rotating flywheel. Hydraulic hybrid vehicles use hydraulic motors to store energy in the form of compressed air, the regenerative braking effect drops off at lower speeds, and cannot bring a vehicle to a complete halt reasonably quickly. A regenerative brake does not immobilise a vehicle, physical locking is required. Many road vehicles with regenerative braking do not have drive motors on all wheels, for safety, the ability to brake all wheels is required. The regenerative braking effect available is limited, and insufficient in many cases, the friction brake is a necessary back-up in the event of failure of the regenerative brake. Regenerative and friction braking must both be used, creating the need to them to produce the required total braking. The GM EV-1 was the first commercial car to do this, in 1997 and 1998 engineers Abraham Farag and Loren Majersik were issued two patents for this brake-by-wire technology. Electric motors, when used in function as generators, convert mechanical energy into electrical energy. Vehicles propelled by electric motors use them as generators when using regenerative braking, braking by transferring energy from the wheels to an electrical load. Early examples of this system were the front-wheel drive conversions of horse-drawn cabs by Louis Antoine Krieger in Paris in the 1890s, the Krieger electric landaulet had a drive motor in each front wheel with a second set of parallel windings for regenerative braking. These included tramway systems at Devonport, Rawtenstall, Birmingham, Crystal Palace-Croydon, slowing the speed of the cars or keeping it in control on descending gradients, the motors worked as generators and braked the vehicles. The tram cars also had brakes and track slipper brakes which could stop the tram should the electric braking systems fail. In several cases the car motors were shunt wound instead of series wound. Following a serious accident at Rawtenstall, an embargo was placed on this form of traction in 1911, Regenerative braking has been in extensive use on railways for many decades
Mechanism for regenerative brake on the roof of a Škoda Astra tram
The box extending sideways from the roof directly over the word "operation" allows air to freely flow through the resistors of the dynamic brakes on this diesel-electric locomotive.