Regenerative braking is a system that slows down a moving vehicle by changing its movement energy or stored energy into a form that can be saved or used right away.

This system usually works by using an electric motor in reverse to capture energy that would normally be lost as heat when braking. This process turns the motor into a generator, which sends power back into the vehicle’s energy storage, such as a battery or capacitor. The stored energy can later help the vehicle move forward. Because of the special electrical setup needed, regenerative braking is most often used in hybrid and electric vehicles.

This method is different from traditional braking systems, which waste energy as heat from friction in the brake parts. Similarly, rheostatic brakes also use generators to recover energy, but they release the energy as heat in resistors instead of storing it.

Regenerative braking improves a vehicle’s efficiency and helps the braking system last longer. This happens because parts like brake discs, calipers, and pads—used when regenerative braking alone is not enough to stop the vehicle—experience less wear compared to vehicles that only use traditional brakes.

General principle

Regenerative braking most often uses an electric motor that acts as a generator. In electric trains, the electricity created is sent back to the power supply for the train. In battery-powered and hybrid vehicles, energy is stored in a battery, in capacitors, or in a spinning flywheel. Hydraulic hybrid vehicles use hydraulic motors to store energy as compressed air. In hydrogen fuel cell vehicles, the electricity made by the motor is stored in a battery, like in battery and hybrid vehicles.

Regenerative braking alone cannot stop a vehicle completely or slow it enough for safety. It must work with another braking system, such as friction-based braking.

• Regenerative braking becomes less effective at low speeds and cannot stop a vehicle quickly with current technology. However, some cars, like the Chevrolet Bolt, can stop completely on flat surfaces if the driver knows how far the regenerative braking will stop the car. This is called One Pedal Driving (OPD).
• Some regenerative brakes do not keep a vehicle from moving when stopped, so physical locks are needed to prevent rolling, such as on hills. Some cars, like the Chevrolet Bolt, can stay still on small slopes using only the motor.
• Many vehicles with regenerative braking do not have motors on all wheels (like two-wheel drive cars), so regenerative braking only works on wheels with motors. For safety, braking all wheels is necessary.
• Regenerative braking has limits, and mechanical braking is still needed for large speed reductions or to stop completely.
• On steep hills, the energy recovered during a slow descent is much greater than the energy recovered when stopping from high speeds. For example, bicyclists lose about three hair dryers' worth of power or two horsepower to air drag at high speeds while going downhill.

Both regenerative and friction braking must be used together, requiring control systems to manage both. The GM EV-1 was the first car to do this. In 1997 and 1998, engineers Abraham Farag and Loren Majersik received patents for this brake-by-wire technology.

Early regenerative braking systems had safety issues. In many early electric vehicles, the same controls were used for acceleration and regenerative braking, with a manual switch changing the function. This caused accidents, such as the 1948 Wädenswil train crash in Switzerland, which killed 21 people.

In the 2020s, most vehicles with regenerative braking can stop quickly in One Pedal Driving mode. However, some cars do not turn on brake lights during regenerative braking, causing safety concerns. Most rules do not require brake lights to turn on during regenerative braking. One Pedal Driving also caused concerns about sudden unintended acceleration, as drivers might mistake the accelerator for the brake in stressful situations. The GB 21670-2025 vehicle standard later required brake lights to turn on during regenerative braking if the deceleration exceeds 1.3 m/s².

History

In 1886, the Sprague Electric Railway & Motor Company, started by Frank J. Sprague, created two important inventions: a motor that keeps a steady speed and doesn't spark, and a way to slow down a vehicle by using its motor as a generator.

In the 1890s, Louis Antoine Krieger in Paris made the first road vehicles with regenerative braking. His electric landaulet had motors in each front wheel and a special coil to help slow the car down by using the motor as a generator. During World War I, Ransomes, Sims & Jefferies in England introduced the Orwell Electric Truck, which used regenerative braking that the driver could activate manually.

In England, John S. Raworth's Traction Patents from 1903 to 1908 helped tramway operators use regenerative braking. This system allowed trams to slow down or stay in control on steep hills by using the motor as a generator. Trams also had wheel brakes and track slipper brakes as backup in case the electric braking failed. Some trams used motors with shunt windings instead of series windings, and the Crystal Palace-Croydon line used controllers that could switch between series and parallel settings. After a serious accident in 1911, regenerative braking was banned for a time but returned in 1931.

Regenerative braking has been used on railways for many years. The Baku-Tbilisi-Batumi railway began using it in the early 1930s, especially on the steep Surami Pass. In Scandinavia, the Kiruna to Narvik railway, called Malmbanan in Sweden and Ofoten Line in Norway, carries iron ore down from mines in Kiruna to the port of Narvik. These trains use regenerative braking to generate electricity, which powers the return trip and even supplies energy to the power grid.

Electric cars used regenerative braking from the start, but early systems required drivers to manually switch between modes. The Baker Electric Runabout and the Owen Magnetic were early examples that used complex switches and a costly "black box" or "drum switch" to control the system. These systems could only be used on downhill roads and needed manual activation.

Improvements in electronics made regenerative braking fully automatic. In 1967, the AMC Amitron experimental car used a motor controller that automatically charged the battery when the brake pedal was pressed. Modern hybrid and electric vehicles use this technique to extend battery range, especially those with AC drive systems.

Instead of using a battery, some systems use an AC/DC rectifier and a large capacitor to store energy. Capacitors can store energy quickly at high voltages. Mazda used this system in some 2018 cars, calling it i-ELOOP.

Electric railways

During braking, the connections of the traction motors are changed to turn them into electrical generators. The motor fields are connected to the main traction generator (MG), and the motor armatures are connected to the load. The MG activates the motor fields. As the train moves, the wheels turn the motor armatures, and the motors act as generators. The generated electricity is either sent through onboard resistors (dynamic braking) or returned to the power supply (regenerative braking). Compared to electro-pneumatic friction brakes, braking using traction motors can be controlled more quickly, improving the performance of wheel slide protection.

For a specific direction of travel, the flow of current through the motor armatures during braking is opposite to the flow when the train is moving forward. This causes the motor to create a force that resists the train’s movement.

Braking force depends on the strength of the magnetic fields in the motor windings and the armature windings.

British Rail Class 390s report a 17% energy saving and less wear on friction braking parts. Caltrain states that 23% of the energy used by its Stadler KISS electric trains is recovered and sent back to the power grid. Between 2004 and 2007, the Delhi Metro reduced carbon dioxide (CO₂) emissions by about 90,000 tons by regenerating 112,500 megawatt hours of electricity through regenerative braking. It was expected that the Delhi Metro would reduce emissions by over 100,000 tons of CO₂ annually once its phase II was completed using regenerative braking.

Electricity generated by regenerative braking can be sent back to the traction power supply. It can be used immediately to meet other electrical needs on the network, used for power in train cars, or stored in systems near the tracks for later use.

A type of regenerative braking is used on some parts of the London Underground. This is achieved by having small slopes near stations. The train slows as it climbs the slope and then speeds up as it descends, converting kinetic energy into gravitational potential energy. This method is typically used in deep tunnel sections of the network and not generally above ground or on the cut-and-cover sections of the Metropolitan and District Lines.

Comparison of dynamic and regenerative brakes

Dynamic brakes, also known as rheostatic brakes in British English, are used in electric vehicles like forklift trucks, diesel-electric locomotives, and trams. Unlike regenerative brakes, which reuse energy, dynamic brakes convert electrical energy into heat by sending electricity through large resistors. This heat can either warm the inside of the vehicle or be released through large, radiator-like covers that surround the resistors.

In 1936, General Electric tested steam turbine locomotives that used a method called true regeneration. These trains directed steam water over resistor packs instead of using air to cool them. This process saved energy by reducing the need to burn oil to heat the water, allowing the energy to be reused for movement.

Regenerative brakes have a main drawback compared to dynamic brakes: they require the electricity produced to match the power system’s needs, which increases costs. For direct current (DC) systems, the voltage must be carefully controlled. Alternating current (AC) systems, developed by engineer Miro Zorič, allow regenerative braking by matching the power supply’s frequency, especially in trains that convert AC to DC for their motors.

In places where power is needed constantly, such as for heating or air conditioning in trains, regenerative braking can send recovered energy to these systems. This method is widely used by North American railroads, where power needs for heating and cooling are about 500 kW all year. Because of this, newer train designs like the ALP-46 and ACS-64 no longer use resistor grids for dynamic braking and do not need external power systems, allowing self-powered trains to use regenerative braking.

Some steep railways use 3-phase power and induction motors. These systems keep train speeds nearly constant during both movement and braking because the motors spin at the same rate as the electricity supply.

Kinetic energy recovery systems

Kinetic energy recovery systems (KERS) were used in the motor sport Formula One during the 2009 season. These systems are also being developed for use in road vehicles. KERS was not used in the 2010 Formula One season but returned in the 2011 season. By 2013, all Formula One teams used KERS, with the Marussia F1 team beginning use in the 2013 season. One reason some cars did not use KERS immediately is that it raises the car’s center of gravity and reduces the amount of ballast available to balance the car for better handling during turns. Rules from the FIA also limit how much the system can be used. The idea of capturing a vehicle’s kinetic energy using flywheel storage was suggested by physicist Richard Feynman in the 1950s. This concept is used in systems like Zytek, Flybrid, Torotrak, and Xtrac in Formula One. Other systems, such as the Cambridge Passenger/Commercial Vehicle Kinetic Energy Recovery System (CPC-KERS), use a differential instead of a flywheel.

Xtrac and Flybrid are companies that use technologies from Torotrak. These technologies include a small, advanced gear system that works with a continuously variable transmission (CVT). The CPC-KERS is similar, as it is also part of the driveline assembly. However, in the CPC-KERS, the entire system, including the flywheel, is placed inside the vehicle’s hub, which looks like a drum brake. In this system, a differential replaces the CVT and transfers torque between the flywheel, drive wheel, and road wheel.

Motor sports

The first system introduced was the Flybrid. It weighs 24 kg and has an energy capacity of 400 kJ after considering internal losses. It can provide a maximum power boost of 60 kW (82 PS; 80 hp) for 6.67 seconds. The flywheel inside the system has a diameter of 240 mm and weighs 5.0 kg. It spins at up to 64,500 rpm. The system can produce a maximum torque of 18 Nm (13.3 ftlbs). It takes up 13 liters of space.

Formula One supports solutions to environmental challenges, and the FIA allowed the use of 60 kW (82 PS; 80 hp) KERS in the 2009 season. Teams tested KERS systems in 2008. Energy could be stored as mechanical energy, like in a flywheel, or as electrical energy, like in a battery or supercapacitor.

Before KERS was introduced, McLaren used an early regenerative braking system in their 1998 MP4/13 race car. This system did not send power directly to the wheels. Instead, it used energy from the brakes to power auxiliary pumps on the engine. This helped reduce losses and added 30 to 40 horsepower for a short time.

In 2008, two incidents occurred during KERS testing. The first happened when Red Bull Racing tested their KERS battery in July. The battery malfunctioned, causing a fire scare and leading to the team’s factory being evacuated. A week later, a BMW Sauber mechanic received an electric shock after touching Christian Klien’s KERS-equipped car during a test at the Jerez circuit.

In the 2009 season, four teams used KERS: Ferrari, Renault, BMW, and McLaren. Renault and BMW stopped using the system during the season. McLaren Mercedes became the first team to win a Formula One Grand Prix with a KERS-equipped car when Lewis Hamilton won the 2009 Hungarian Grand Prix on July 26. Their second KERS car finished fifth. At the next race, Lewis Hamilton took pole position with a KERS car, and his teammate, Heikki Kovalainen, qualified second. This was the first time both drivers on the front row used KERS. On August 30, 2009, Kimi Räikkönen won the Belgian Grand Prix with his KERS-equipped Ferrari. This was the first time KERS directly contributed to a race victory.

KERS remained legal in 2010, but teams agreed not to use it. New rules for the 2011 season raised the minimum car and driver weight to 640 kg. FOTA teams also agreed to use KERS again, so it returned in 2011. Teams could still choose not to use it, and three teams did not in 2011. In 2012, only Marussia and HRT raced without KERS. By 2013, all 11 teams on the grid used KERS.

In 2014, the power output of the MGU-K (a replacement for KERS that includes a turbocharger waste heat recovery system) increased to 120 kW. It could recover 2 mega-joules of energy per lap. This change balanced the shift from 2.4-liter V8 engines to 1.6-liter V6 engines. The fail-safe settings of the brake-by-wire system, which supports KERS, were examined after the fatal crash of Jules Bianchi at the 2014 Japanese Grand Prix.

Bosch Motorsport Service is developing KERS for racing. These systems include a lithium-ion battery or a flywheel, an electric motor weighing 4 to 8 kg (with a maximum power of 60 kW or 80 hp), and a KERS controller for power and battery management. Bosch also provides hybrid systems for commercial and light-duty vehicles.

Automakers like Honda have tested KERS systems. At the 2008 1,000 km of Silverstone, Peugeot Sport introduced the Peugeot 908 HY, a hybrid electric version of the diesel 908. Peugeot planned to use this car in the 2009 Le Mans Series, though it could not score championship points. Peugeot also plans a compressed air regenerative braking system called Hybrid Air.

McLaren tested their KERS in September 2008 at the Jerez track for the 2009 season. At that time, it was unclear whether they would use an electrical or mechanical system. In November 2008, Freescale Semiconductor partnered with McLaren Electronic Systems to improve the KERS system for future Formula One cars. Both companies believed this collaboration would help advance KERS technology for road vehicles.

Toyota used a supercapacitor for energy recovery on a Supra HV-R hybrid race car that won the Tokachi 24 Hours

Civilian transport

Regenerative braking can be used on electric bicycles in theory. However, as of 2024, it is not commonly used on bicycles. This is because regenerative braking requires a direct-drive hub motor, while many bicycles use a mid-drive motor that powers the chain. Also, regenerative braking cannot work with a freewheel mechanism. Additionally, the amount of energy recovered through regenerative braking is usually too small to be useful.

Regenerative braking is also possible on non-electric bicycles. The United States Environmental Protection Agency, along with students from the University of Michigan, created a system called the hydraulic Regenerative Brake Launch Assist (RBLA).

Many hybrid electric and fully electric vehicles use regenerative braking together with friction braking. Regenerative braking systems (RBS) cannot fully replace traditional braking systems, but improvements are being made. The settings that control when energy is recovered and when friction braking is used affect how drivers feel the braking action.

Regenerative braking systems are important for electric vehicles to capture energy during braking. This technology greatly influences the cost, emissions, safety, and other features of electric vehicles. Improving RBS can increase the amount of kinetic energy recovered and improve vehicle stability.

In Oranjestad, Aruba, regenerative braking helps reduce power use in streetcars (AE) or trams (CE). These vehicles, designed and built by TIG/m Modern Street Railways in Chatsworth, USA, use hybrid or electric technology. They are powered by lithium batteries and hydrogen fuel cells, not by external power sources like overhead wires.

Thermodynamics

The energy stored in a flywheel can be described using a general energy equation, assuming the flywheel is the system being studied. During braking, it is assumed that the potential energy, enthalpy, pressure, and volume of the flywheel do not change. This means only the kinetic energy of the car is considered. When a car brakes, the flywheel does not lose energy, and the only energy it receives is the initial kinetic energy of the car. The equation can be simplified to focus on this kinetic energy.

The flywheel captures a portion of the car’s initial kinetic energy. This portion is represented by η fly, which is a measure of how much energy the flywheel can store. The flywheel stores this energy as rotational kinetic energy, which is energy from spinning motion. Since the energy remains as kinetic energy and is not changed into another form, this process is efficient. However, the flywheel can only store a limited amount of energy, which depends on its inertia (how resistant it is to changes in motion) and its angular velocity (how fast it spins). When the car is not moving, the flywheel loses very little energy over time, so the energy stored in it can be assumed to remain the same when it is used again. The amount of energy the flywheel releases is therefore equal to the energy it stored.

Regenerative braking follows a similar energy equation to the flywheel system. Regenerative braking involves two steps: first, the car’s kinetic energy is converted into electrical energy by a generator, and then this electrical energy is stored as chemical energy in a battery. This process is less efficient than the flywheel system. The efficiency of the generator is represented by a value that shows how much electrical energy is produced from the car’s initial kinetic energy.

The efficiency of the battery is described by how much energy it can store and release. The energy produced by regenerative braking is the work output of the battery.

A diagram from the United States Department of Energy (DoE) shows that cars with internal combustion engines typically have an efficiency of 13% in urban driving and 20% on highways. Braking accounts for about 46% of the useful mechanical energy in cities and 10% on motorways.

The DoE also states that electric cars convert over 77% of electrical energy from the grid into power at the wheels. However, when considering losses from the electric network, heating, and air conditioning, the overall efficiency of an electric vehicle is about 50%, according to Jean-Marc Jancovici.

The efficiency of an electric motor is η eng = 0.5, and the braking proportion in cities is p = 0.46, while on motorways it is p = 0.1. The recuperated proportion of braking energy is η recup = 0.6.

Using these values, the energy flux (flow of energy) arriving at the electric motor is E, the energy lost during braking is E braking, and the energy recovered is E recup. These values are related by the equations:

E braking = (E + E recup) × η eng × p
E recup = η recup × E braking

Solving these equations shows that E braking = (E × η eng × p) / (1 – η eng × p × η recup). This means the original energy flux E is effectively replaced by a new value of E × (1 – η eng × p × η recup).

The expected gain in energy recovery is calculated as η eng × p × η recup. Higher values of recuperation efficiency, motor efficiency, and braking proportion lead to greater energy recovery.

On motorways, the expected gain is 3%, and in cities, it is 14%.