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 be later used to aid forward 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.
  • 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 ; 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 mode also lead to concerns over sudden unintended acceleration, 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 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
as explained in some detail by his son Alfred Raworth.
These included tramway systems at Devonport, Rawtenstall, Birmingham, Crystal Palace-Croydon, and many others. 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 wheel brakes and track slipper brakes which could stop the tram should the electric braking systems fail. In several cases the tram car motors were shunt wound instead of series wound, and the systems on the Crystal Palace line utilized series-parallel controllers. Following a serious accident at Rawtenstall, an embargo was placed on this form of traction in 1911; the regenerative braking system was reintroduced twenty years later.
Regenerative braking has been in extensive use on railways for many decades. The Baku-Tbilisi-Batumi railway started utilizing regenerative braking in the early 1930s. This was especially effective on the steep and dangerous Surami Pass. In Scandinavia the Kiruna to Narvik electrified railway, known as Malmbanan on the Swedish side and Ofoten Line on the Norwegian, carries iron ore on the steeply-graded route from the mines in Kiruna, in the north of Sweden, down to the port of Narvik in Norway to this day. The rail cars are full of thousands of tons of iron ore on the way down to Narvik, and these trains generate large amounts of electricity by regenerative braking, with a maximum recuperative braking force of 750 kN. From Riksgränsen on the national border to the Port of Narvik, the trains use only a fifth of the power they regenerate. The regenerated energy is sufficient to power the empty trains back up to the national border. Any excess energy from the railway is pumped into the power grid to supply homes and businesses in the region, and the railway is a net generator of electricity.
Electric cars used regenerative braking since the earliest experiments, but this initially required the driver to flip switches between various operational modes in order to use it. The Baker Electric Runabout and the Owen Magnetic were early examples, which used many switches and modes controlled by an expensive "black box" or "drum switch" as part of their electrical system. These, like the Krieger design, could only practically be used on downhill portions of a trip, and had to be manually engaged.
Improvements in electronics allowed this process to be fully automated, starting with 1967's AMC Amitron experimental electric car. Designed by Gulton Industries the motor controller automatically began battery charging when the brake pedal was applied. Many modern hybrid and electric vehicles use this technique to extend the range of the battery pack, especially those using an AC drive train.
An AC/DC rectifier and a very large capacitor may be used to store the regenerated energy, rather than a battery. The use of a capacitor allows much more rapid peak storage of energy, and at higher voltages. Mazda used this system in some 2018 cars, where it is branded i-ELOOP.

Electric railways

During braking, the traction motor connections are altered to turn them into electrical generators. The motor fields are connected across the main traction generator and the motor armatures are connected across the load. The MG now excites the motor fields. The rolling locomotive or multiple unit wheels turn the motor armatures, and the motors act as generators, either sending the generated current through onboard resistors or back into the supply. Compared to electro-pneumatic friction brakes, braking with the traction motors can be regulated faster improving the performance of wheel slide protection.
For a given direction of travel, current flow through the motor armatures during braking will be opposite to that during motoring. Therefore, the motor exerts torque in a direction that is opposite from the rolling direction.
Braking effort is proportional to the product of the magnetic strength of the field windings, multiplied by that of the armature windings.
Savings of 17%, and less wear on friction braking components, are claimed for British Rail Class 390s. Caltrain claims 23% of energy used by its Stadler KISS electric trains are recaptured and returned to the grid. The Delhi Metro reduced the amount of carbon dioxide released into the atmosphere by around 90,000 tons by regenerating 112,500 megawatt hours of electricity through the use of regenerative braking systems between 2004 and 2007. It was expected that the Delhi Metro would reduce its emissions by over 100,000 tons of per year once its phase II was complete, through the use of regenerative braking.
Electricity generated by regenerative braking may be fed back into the traction power supply; either offset against other electrical demand on the network at that instant, used for head end power loads, or stored in lineside storage systems for later use.
A form of what can be described as regenerative braking is used on some parts of the London Underground, achieved by having small slopes leading up and down from stations. The train is slowed by the climb, and then leaves down a slope, so kinetic energy is converted to gravitational potential energy in the station. This is normally found on the deep tunnel sections of the network and not generally above ground or on the cut and cover sections of the Metropolitan and District Lines.