Regenerative Braking: How and Why It Works for Electric Cars

Regenerative braking allows an electric or hybrid-electric vehicle to collect electricity as it decelerates. Traditional braking results in a lot of lost energy, which in traffic leads to increased gas consumption and wear on brakes.

In electric vehicles (EVs), regenerative braking is performed by the electric motor, not by the brakes. This helps EV drivers use their brakes less.

How Regenerative Braking Works

In a gas-powered car, braking results in a lot of lost energy.

In regenerative braking, when an EV driver releases the accelerator pedal, the flow of electricity from the battery to the motor is stopped. Yet the spinning part of the motor (the rotor) still rotates along with the wheels of the still-moving car.

Without a continuous flow of electricity from the battery, the motor becomes a generator, sending the kinetic energy from the spinning rotor into the battery, while resistance to the rotor slows down the vehicle.

Electric vehicles still have disc brakes, but they are backups in situations like:

• In case of motor failures
• Below a certain speed, disk brakes supplement the generator since the torque (or rotational force) of the generator isn't strong enough to supply 100% of braking power
• At very higher speeds, when a short stop could break the motor.

Torque blending is how EVs find the appropriate balance between friction braking and regenerative braking. Like in an automatic car, EV drivers rarely notice the difference.

How Regenerative Are Electric Brakes?

Swiss companies are developing an electric truck that can generate more electricity than it uses. But this isn't possible for ordinary electric vehicles.

While an electric vehicle is far more efficient than a gas-powered one in converting fuel to kinetic energy, some energy is lost as heat, as vibration, as sound energy, as aerodynamic drag, etc.

The same forces that take up energy during acceleration are also lost during deceleration, just as a car put in neutral on a flat surface will eventually stop.

Other factors impact battery performance and how much braking energy it can save, including:

• The types of electronics and capacitors in the vehicle 
• The temperature of the battery
• How full the battery already is. 

Studies show that up to roughly 50% of the car's kinetic energy while braking can be used to accelerate the car again later. Anecdotal testimony from real-world driving, however, reports a range of 15% to 32% recapture of energy through regenerative braking.

History of Regenerative Braking

Regenerative braking is not new technology. In 1967, the American Motor Car Company introduced an ill-fated electric car, the AMC Amitron, with an impressive range of 150 miles and regenerative braking. Regenerative braking was also used on railways such as the Transcaucasus Railway and those in Scandinavia in the 1930s.

Today, Japan's highly efficient maglev trains and France's TGVs use regenerative braking, as do most electric trains and metro systems all around the world. Increasingly popular electric bicycles (e-bikes), scooters, and skateboards also use regenerative braking, with an efficiency of some 4% to 5%.

The hybrid-electric Toyota Prius was the first commercially successful car to use regenerative braking, and the technology is almost exclusive to electric and hybrid vehicles.

The Mazda 3 is one of the few gas-powered vehicles that use regenerative braking, in this case merely to power the car's auxiliary electronic functions.

When Is Regenerative Braking Best?

Regenerative braking is most effective at higher speeds and on long downhills, since more kinetic energy is available to be converted.

Yet in stop-and-go urban traffic, the benefit of regenerative braking comes less in the amount of energy recaptured than in the reduced wear-and-tear on the friction brakes. This, in turn, reduces the emission of particulate matter pollution. At a societal level, the health outcomes from regenerative braking may even outweigh the financial or climate benefits.

The Future of Regenerative Braking

Regenerative braking is a mature technology with over a century of use, but research continues to refine its efficiency.

Battery improvements will increase the amount of energy that regenerative braking can store. Additional improvements to supercapacitors will also improve braking efficiency.

Continued research can reduce the energy loss in the braking process in order to make electric vehicles more efficient, more economical, and more environmentally friendly.