How Does Regenerative Braking Work?

Brake technology hasn’t changed a whole lot in the last hundred years, but regenerative braking does represent a sea change in the way that we think about braking. Instead of using brute force to stop vehicles by physically clamping down or pushing out like a component like a brake disc or drum, this technology cleverly exploits the way that hybrid and fully electric cars work to actually recover a little energy every time the vehicle slows down.

How Do Brakes Work in Cars and Trucks?

Advances in brake technology have been largely iterative rather than innovative over the years, like the transition from drum brakes to disc brakes. There have also been significant advances in the physical materials that brake pads are made of, which has resulted in friction materials that last longer, create less dust, and are less likely to make noise. Technologies like anti-lock brakes have also made brake technology safer, but the underlying principle of converting kinetic energy to heat has remained unchanged.

Traditional brakes work just fine, but they are tremendously wasteful by their very nature. Every time you push down on your brake pedal, you are effectively clamping down on your wheels with the force of thousands of pounds of hydraulic pressure. The precise mechanism involves disc-shaped metal rotors, which are sandwiched between each tire and wheel hub, being squeezed between organic, metallic, or ceramic brake pads.

In older vehicles, and the rear brakes of some light trucks, less efficient drums and brake shoes are used instead. In either case, the vehicle slows down due to the tremendous friction that’s generated between the pads and discs or shoes and drums. That friction essentially turns kinetic energy into heat energy (and sometimes a great deal of noise), and your car slows down as a result.

The problem with traditional brakes is that your engine had to expend a lot of fuel to built up that kinetic energy, and it’s essentially wasted when your brakes convert it into heat.

The basic idea behind regenerative braking is that a variety of technologies make it possible to recapture some portion of that kinetic energy, convert it into electricity, and then reuse it.

How Do Regenerative Brakes Work?

The most common form of regenerative brake technology re-purposes an electric motor as a generator, which is why regenerative brakes are often found in hybrid and electric vehicles.

During normal operation, the electric motor draws power from the battery and uses it to move the vehicle. When the brake pedal is depressed, the electric motor is able to reverse this process, become a generator powered by the rotating wheels, and feed electricity back into the battery.

Since regenerative brakes effectively charge the battery without plugging into a charger, in the case of an electric vehicle, or using the alternator in a hybrid, the overall efficiency of the vehicle is increased. That translates into more miles between charges or fuel-ups.

Since regenerative brakes effectively turn kinetic energy into electricity, they are able to slow a vehicle down in addition to charging the battery. However, there are limitations to the efficiency of a regenerative brake system. One of the main issues is that regenerative brakes don’t work as well at low speeds as they do at high speeds. Due to that inherent limitation in regenerative braking, most vehicles are also equipped with a supplemental traditional braking system.

In the same way that traditional steering, braking, and acceleration controls are often included as a backup for drive-by-wire systems, traditional brakes can serve as a backup for regenerative braking. Traditional systems may only kick in when there's an equipment failure, or they can be in used in concert with regenerative braking all the time.

Limitations of Regenerative Brakes

In addition to the natural fall-off of regenerative braking efficiency at low speeds, the technology also suffers from a number of other limitations. Some of the most notable include:

• Regenerative braking only works on drive wheels: If an electric vehicle isn't all wheel drive, with a motor for each wheel, then the wheels that lack rotors aren't able to benefit from regenerative braking.
• Problems with panic stops: Regenerative brakes typically don’t provide enough braking force under panic stop conditions. That's one area where traditional brakes still work much better.
• Battery and motor limitations: The efficiency of a regenerative system is limited by factors like the capacity of the energy storage system and the output of the electric motor.
• Only work with electric and hybrid vehicles: Traditional regenerative systems are incompatible with non-electric, non-hybrid vehicles. Since these vehicles lack electric motors, implementing regenerative braking is costly and complicated.
• Dynamic braking constraints: Some regenerative systems are forced to use supplemental “dynamic braking” that doesn’t store the reclaimed kinetic energy.

Capacitive Brakes and Traditional Combustion Engines

Since regenerative brake systems typically rely on their electric motors to generate electricity, they are inherently incompatible with vehicles that use internal combustion engines. However, there are some alternative regenerative technologies that can be applied to traditional internal combustion engines.

One such system uses large capacitors to rapidly store and release electricity, which is then passed through a step-down transformer. The 12-volt output is then fed into the electrical system of the vehicle, which takes some load off the engine. This technology is currently able to increase fuel efficiency by up to 10 percent, though it is still in its infancy.

What Cars Use Regenerative Brakes?

Most hybrid and electric vehicles use some type of regenerative braking system. OEMs like Chevrolet, Honda, Nissa, Toyota, and Tesla were all on board early with regenerative braking technology in their hybrid and electric vehicles. Non-hybrid vehicles that use some type of regenerative braking include are significantly less common, but BMW and Mazda were both early adopters of the technology in certain models.