Regenerative braking

Regenerative braking is an energy recovery mechanism that slows down a moving vehicle or object by converting its kinetic energy or potential energy into a form that can be either used immediately or stored until needed. Typically, regenerative brakes work by driving an electric motor in reverse to recapture energy that would otherwise be lost as heat during braking, effectively turning the traction motor into an electric generator. Feeding power backwards through the system like this allows the energy harvested from deceleration to resupply an energy storage solution such as a battery or a capacitor. Once stored, this power can then later be used again to aid propulsion. Because of the electrified vehicle architecture required for such a braking system, automotive regenerative brakes are most commonly found on hybrid and electric vehicles. This method contrasts with conventional braking systems, where excess kinetic energy is converted to unwanted and wasted heat due to friction in the brakes. Similarly, with rheostatic brakes, energy is recovered by using electric motors as generators but is immediately dissipated as heat in resistors. In addition to improving the overall efficiency of the vehicle, regeneration can significantly extend the life of the braking system. This is because the traditional mechanical parts like discs, calipers, and pads – included for when regenerative braking alone is insufficient to safely stop the vehicle – will not wear out as quickly as they would in a vehicle relying solely on traditional brakes.

General principle The most common form of regenerative brake involves an electric motor functioning as an electric generator. In electric railways, the electricity generated is fed back into the traction power supply. 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. In a hydrogen fuel cell powered vehicle, the electrical energy generated by the motor is stored chemically in a battery, similar to battery and hybrid electric vehicles.

Practical regenerative braking Regenerative braking is not by itself sufficient as the sole means of safely bringing a vehicle to a standstill, or slowing it as required, so it must be used in conjunction with another braking system such as friction-based braking.

The regenerative braking effect drops off at lower speeds and cannot bring a vehicle to a complete halt reasonably quickly with current technology. However, some cars, like the Chevrolet Bolt, can bring the vehicle to a complete stop on level surfaces when the driver knows the vehicle's regenerative braking distance. This is referred to as one-pedal driving (OPD). Some current regenerative brakes do not immobilize a stationary vehicle; physical locking is required, for example, to prevent vehicles from rolling down hills. Some cars, like the Chevrolet Bolt, can remain stationary on small slopes using only the motor. Many road vehicles with regenerative braking do not have drive motors on all wheels (as in a two-wheel drive car); regenerative braking is normally only applicable to wheels with motors. For safety, the ability to brake all wheels is required. The regenerative braking effect available is limited, and mechanical braking is still necessary for substantial speed reductions or to bring a vehicle to a stop. On steep hills with real traffic speeds, the magnitude of potential energy recoverable during the descent of a vehicle at a slower speed than its terminal speed is substantially greater than that recoverable by bringing the vehicle from even the terminal speed to a complete stop. An example used by bicyclists is that during the descent of a hill, approximately three hair dryers' worth of power or some two horsepower is lost to air drag at terminal speeds. Regenerative and friction braking must both be used, creating the need to control 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. Early applications commonly suffered from a serious safety hazard: in many early electric vehicles with regenerative braking, the same controller positions were used to apply power and to apply the regenerative brake, with the functions being swapped by a separate manual switch. This led to a number of serious accidents when drivers accidentally accelerated when intending to brake, such as the runaway train accident in Wädenswil, Switzerland in 1948, which killed twenty-one people. In the 2020s, most vehicles equipped with regenerative braking can completely halt reasonably quickly in one-pedal driving mode. Some car models do not illuminate the braking light when engaging in regenerative braking, leading to safety concerns. Most regulations do not mandate the illumination of a braking light when the vehicle decelerates through regenerative braking. The one-pedal driving (OPD) mode also leads to concerns over sudden unintended acceleration (SUA), as the driver could confuse the accelerator as the brake in stressful situations when the latter is seldomly used during OPD operation. The GB 21670-2025 vehicle standard later mandated that brake lights must turn on during regenerative braking when deceleration exceeds 1.3 m/s2.

History In 1886 the Sprague Electric Railway & Motor Company, founded by Frank J. Sprague, introduced two important inventions: a constant-speed, non-sparking motor with fixed brushes, and regenerative braking. Early examples of this system in road vehicles 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 (bifilar coil) for regenerative braking. The Orwell Electric Truck introduced by Ransomes, Sims & Jefferies in England during WW1 used regenerative braking switched in by the driver. In England, "automatic regenerative control" was introduced to tramway operators by John S. Raworth's Traction Patents 1903–1908, offering them economic and operational benefits

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