How Does Regenerative Braking Work in an Electric Car?

How is it better than the process of braking in a gas-powered car?

Low Angle View Of Car On Road
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As its name suggests, regenerative braking allows an electric or hybrid-electric vehicle to regenerate electricity as it decelerates. Slowing down or stopping a gas-powered car involves brake pads clamping down on discs attached to the wheels. In electric vehicles (EVs), regenerative braking is performed by the electric motor, not by the brakes. This allows EV drivers the ability to practice “one-pedal driving,” using their brakes at a minimum and saving on their wear and tear. Regenerative braking is especially useful during city driving, where stop-and-go traffic puts increased strain on disc brakes.

How It Works

In a gas-powered car, braking results in a lot of lost energy. Friction causes heat, and that heat escapes into the atmosphere. Friction braking also wears down brake pads, and the fine particles worn away are the source of approximately 20% of PM 2.5 traffic pollution, the particulate matter in the atmosphere linked to negative health outcomes. In electric vehicles, regenerative braking reduces the level of PM 2.5 pollution, and in hybrid-electric vehicles, “regen” also lowers fuel consumption and greenhouse gas emissions. 

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 on the rotor slows down the vehicle.

Electric vehicles do have disc brakes, but they are used less frequently. They are still necessary in many instances—as a backup in case the motor fails, or to slow the vehicle down faster than regenerative braking can provide. Below a certain speed (called the threshold speed), the torque (or rotational force) of the generator isn't strong enough to supply 100% of braking power, so disc brakes use friction power to bring the vehicle to a complete stop. And at higher speeds, sudden braking could shatter the driveshaft, break the motor, or cause other disastrous damage, so friction disc brakes are used. Electronics in the vehicle use “torque blending” to find the appropriate balance between friction braking and regenerative braking. EV drivers rarely notice the difference.

How Much Energy Is Stored?

Swiss companies are developing an electric truck that can generate more electricity than it uses. Why can't ordinary electric vehicles produce more electricity through regenerative braking than they consume while driving? If, hypothetically, an EV driver used 5 kilowatt-hours (kWh) to accelerate from 0 to 60, then decelerated (without using the brake pedal) from 60 mph to (nearly) 0, shouldn't the vehicle regain nearly all of those 5 kWh?

Basic physics says no. While an electric vehicle is far more efficient than a gas-powered one in converting fuel to kinetic energy, not all of those 5 kWh were sent from the battery to the motor. Some of it was lost as heat (from friction of wheels on the road, for example), as vibration, as sound energy, as aerodynamic drag, as energy used to run the car's electronics or heating/cooling system, and in the basic thermodynamic process of converting one form of energy into another.

If, hypothetically, three-quarters of those 5 kWh are converted to kinetic energy to accelerate to 60 mph, can regenerative braking regenerate 3.75 kWh? Alas, that same energy lost during acceleration to heat, sound, etc., is also lost during deceleration, just as a car put in neutral on a flat surface will eventually stop. Equally, some energy is lost in the round-trip conversion from kinetic to electrical to chemical energy (stored in the battery) and back to electrical and kinetic energy again.

Red Tesla descending a mountain in Kazakhstan
A downhill drive won't restore as much energy as it took to climb the hill.

Adil Abdrakhmanov/Getty Images

How much electricity gets regenerated and stored in the battery also depends on the types of electronics and capacitors the vehicle has, the temperature of the battery, and how full the battery already is. When the battery is already at full capacity, for example, there is no place to store any more electrons. In sum, studies show that up to roughly 70% 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.

Then how does that Swiss truck produce more energy than it consumes? Simply by being driven up a hill empty and carries a heavy load down the hill. The gravitational potential energy embodied in its cargo increases the energy available to be converted into battery energy.

When and Where Regenerative Braking Is Used

While the hybrid-electric Toyota Prius was the first commercially successful car to use regenerative braking, the technology is not new. 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. Long before electric and hybrid vehicles, however, regenerative braking was discussed in scientific and engineering circles, used on tramways in the first decade of the 20th century, and on railways such as the Transcaucasus Railway and those in Scandinavia by 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%.

e-bike rider's view of a bike trail from over the handlebars
E-bikes use regenerative braking, too.

Aaron Hawkins/Getty Images

In road transportation vehicles, however, regenerative braking is almost exclusive to electric and hybrid vehicles. By definition, an internal combustion engine is not regenerative: the flow of energy is one-directional only. 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.

In modern electric and hybrid vehicles, using regenerative braking is more beneficial 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, which 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.

Outlook of Regenerative Braking

Regenerative braking is a mature technology with over a century of use, but with the growing popularity of electric vehicles and other forms of e-mobility, research continues to refine its efficiency. Batteries inherently charge more slowly than they discharge electricity, but improving the rate at which batteries can charge will increase the amount of energy that regenerative braking can store. Improvements in the use of supercapacitors in braking systems is another avenue of research into improving energy storage rates.

Of all the motor vehicle laws on the books that drivers must obey, when it comes to regenerative braking, none are as important as the first two laws of thermodynamics. Energy can neither be created nor destroyed, and energy is lost as it is converted from one form to another. 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.

One-Pedal Driving

One-pedal driving takes getting use to, just as it takes drivers of standard transmission vehicles time to get used to the lack of a clutch in cars with automatic transmissions. But of all the benefits of regenerative braking—environmental and economic—the simplification that comes with using only a single pedal may be one that drivers enjoy the most.

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