How Does Regenerative Braking Work?

Gas And Brake Pedal

Regenerative braking and regenerative brakes are features that can be found in fully electric and gas-electric hybrid vehicles. Cars like the hybrid Toyota Prius, Ford Fusion Hybrid and Honda Insight, and the fully electric Tesla Roadster feature regenerative braking systems.

Regenerative braking systems capitalize on the similarities in construction of electric motors and electric power generators. An electric motor's interior is made up of copper windings. It uses an electromagnetic energy field to produce torque through its main shaft when power is applied to it. A generator or dynamo is also comprised of copper windings and makes use of an electromagnetic field; applying a force to turn its rotor allows it to produce electric power.

Regenerative braking is based on the principle of physics that states that energy cannot be destroyed; it can only be transformed from one form to another.

Traditional Braking Systems
In a traditional braking system, pressing on the brake pedal causes a pair of brake pads in each wheel to come into contact with the surface of a brake rotor. This contact produces friction, slowing down and eventually stopping the vehicle. The friction itself produces heat as an energy byproduct. Automotive engineers and designers generally perceive heat as a loss. This is the reason why, especially in high performance cars, brake cooling systems such as air dams are employed to dissipate heat from the brakes so that they can quickly regain their efficiency.

Regenerative Braking Systems
In a regenerative braking system, the objective is to recapture the energy byproduct that results when the brakes are applied.

In electric or hybrid automobiles, the electric motor that drives the car's wheels plays a major part during braking. When the brake pedal is pressed, the regenerative braking circuit switches the motor so that it now operates in reverse to counter the direction of the wheels. This reversal actually makes it perform like a power generator or dynamo that produces electrical energy. The electricity developed is routed towards the car's storage batteries to recharge them.

The efficiency of regenerative braking systems in use today has improved significantly. Some existing systems are able to capture and store as much as 70 percent of the energy that would otherwise have been lost.

At higher speeds, regenerative brakes still require the assistance of traditional brake systems to be applied as a backup.

This recapturing and storing of electrical energy may be likened to "trickle" charging of the batteries. This is because most of the time, the electric motor runs in torque producing mode to drive the vehicle. The recommended battery charging method still has to be performed to charge the batteries fully, although regenerative braking does translate to an increase in vehicle range.

Common Regenerative Brake Problems

The more that is going on with a brake system, the more can go wrong. Since the technology is relatively new, it is inherently going to be more prone to malfunction than the standard old disc brakes that have served for decades. Here you will find the two most commonly reported problems that are being reported.

Funny "Feel"
Many drivers report a funny or strange feel in the resistance of their brake pedal when engaging the brake. Others report feeling an actual acceleration before they begin to decelerate. The funny feel in the brake's pedal resistance can be due to air in the brake lines, and many repair facilities will tell you this is the case. However, it should only be a one time occurrence. If the feeling in the pedal persists, get the vehicle to a repair facility as soon as possible to properly diagnose the problem.

Pedal to the Floor
There have been several reports of drivers applying the brake pedal, only to find it goes all the way to the floor. There is no resistance, and the vehicle does not slow down or stop. Many times, if the driver again tries to engage the brake a couple of seconds later, the brake engages as normal. This can be a very scary experience. Some drivers have never gotten the brake to engage and when they needed them and, thinking quickly, used the emergency brake to safely stop the vehicle. Some have also been told when the vehicle was taken to the shop that the malfunction was due to air in the brake lines.

When it comes to dealing with a system on a vehicle that is as important as the brake system, if there is any question on the performance of the car it should be taken to a proper service center. There are numerous resources online for national highway safety, technical service bulletins, and many other places to find information or see if your particular vehicle is listed as having an issue.

Related Questions and Answers

Are there any Regenerative Brake Systems on Non-Hybrid Systems?

There have been a few car makers that have installed a regenerative brake system on non-hybrid cars. It is called an integrated starter-generator. Unfortunately, these systems are not able to generate enough energy to power the vehicle, or provide any real fuel savings. They have been largely abandoned at this point in time, but there is hope that future systems could help increase fuel mileage. On hybrids, regenerative braking systems take some of the energy put off by the brakes and convert it back into battery power. The electric motor is used to power the vehicle at low speeds and the gas motor takes over at higher speeds. This is why hybrids get such excellent gas mileage.

Do all Hybrid Vehicles Have Regenerative Braking Circuits?

Yes, all hybrid vehicles have regenerative braking circuits. These circuits control the regenerative braking system. When the brake pedal is pushed, these circuits reverse the electric motor. It is now operating counter to the direction of the wheels, which means it is performing like a power generator. This dynamo is actually producing energy, which is directed to the car batteries to recharge them. The batteries in turn power the electric motor that moves the car at slower speeds before the gas engine kicks in. This is what makes a hybrid such a high miles per gallon vehicle.

Do Hybrid Regenerative Braking Systems Require More Maintenance?

Actually, hybrid regenerative braking systems require less maintenance. Due to the lower heat levels involved with regenerative braking, the brake pads will normally last twice as long as regular brake pads. In some cases, owners are getting up to 85,000 miles on a single pair of brake pads. This is unheard of when it comes to brake pads on regular cars. The biggest expense when it comes to hybrid vehicles, is the batteries. If these need to be replaced or repaired, the cost can be substantial. Hybrid vehicles normally do not require any maintenance above and beyond the routine maintenance any vehicle requires.

Do You Get Better MPG with Regenerative Braking Systems?

Yes, regenerative braking systems help increase MPG. As they are primarily used on hybrids, you will see a big jump in MPG when compared to a normal gas-powered only vehicle. Regenerative braking converts the energy put off by the braking system back into power that is routed to a battery pack. These battery packs power the electric motor that runs the vehicle at lower speeds. When the speed increases, the gas-powered motor takes over. Using the electric motor is what helps increase the miles per gallon rating of hybrid vehicles, and the regenerative braking system is an integral part of that system.

Is a Regenerative Braking Motor Fully Electric?

Yes, the regenerative braking motor is fully electric. Regenerative brakes put the hybrid's electric motor into reverse when the driver steps on the vehicle's brake pedal. As the motor runs in reverse, it slows the wheels of the car. Running in reverse also makes the electric motor act as a generator, which creates electricity. This electricity is sent into the vehicles batteries, recharging them. These batteries run the electric motor, which powers the vehicle at lower speeds. A hybrid has a gas engine as well. This engine takes over, as the speed of the car climbs or the batteries run low.