What Is Regenerative Braking?

If you drive in an urban area, you probably realize you're continually stopping and starting on the road. It's a big waste of time, but you might not also realize that it's a huge waste of energy. Making a car move forward requires needs a large input of power, and every time you step on the brakes, all the energy you built up dissipates. According to the rules of physics, energy cannot be destroyed. That means when your car slows down, the kinetic energy that was moving it forward has to go somewhere -- it's lost in the brake pads and released as heat. But what if you could store up this energy and use it when you next begin to accelerate? That's the basic principle behind regenerative brakes, which are widely used in electric cars and trains.

Definition

Regenerative braking is a system in which the electric motor that normally drives a hybrid or pure electric vehicle is essentially operated in reverse (electrically) during braking or coasting. Instead of consuming energy to propel a vehicle, the motor acts as a generator that charges the onboard batteries with electrical energy that would normally be lost as heat through traditional mechanical friction brakes. As the motor “acts in reverse,” it generates electricity. The accompanying friction (electrical resistance) assists the normal brake pads in overcoming inertia and helps slow the vehicle.

Traditional vs. Regenerative

In a tr­aditional braking system, brake pads create friction with the brake rotors which stop or slow the car. Friction is also produced between the wheels and the road's surface. Both create heat from the car's kinetic energy.

However, with regenerative brakes, the system that drives the vehicle does most of the braking. When you depress the brake pedal on a hybrid or electric car, these brakes shift the automobile's electric motor into reverse which makes it run backward, in turn slowing the car's wheels. While running backward, the motor also acts as an electric generator by creating electricity that's delivered into the car's batteries.

Best Situations

Regenerative brakes are more effective at certain speeds. They are actually most useful in stop-and-go situations. Hybrids and electric cars also do have friction brakes that act as a type of back-up system in scenarios where regenerative braking can't supply enough power to stop. In these cases, drivers should aware that the brake pedal could respond differently to pressure. It will sometimes depress farther towards the floor than usual -- a feeling that can momentarily cause drivers to panic.

Hydraulic Regenerative Braking

Ford Motor Company and the Eaton Corporation have developed a newer type of regenerative braking system called Hydraulic Power Assist or HPA. When the driver depresses the brake with HPA, the cars kinetic energy powers a reversible pump which directs hydraulic fluid from a low-pressure accumulator (a type of storage tank) and into a high-pressure accumulator. Estimates for HPA indicate it could store 80 percent of the movements lost by deceleration and use it to move the car forward.