At the most basic level, regenerative braking means re-capturing the kinetic energy of the vehicle's motion and turning it into another type of energy. Commonly, this is done by converting kinetic energy into electricity and recharging the car's battery with it. Click past the jump for more for a look at the way that regen brakes work and the different types of regenerative braking systems. Oh, and we'll discuss why you would want to do this in the first place.
How standard regenerative brakes work
When a car is cruising down the road, it has kinetic energy, which is simply defined as the energy something possesses because it is in motion. When you apply the brakes, instead of traditional brake pads clamping down and letting the kinetic energy dissipate as heat, regenerative braking systems use their electric motors to slow the car and generate electrical energy (hybrids still have conventional friction brake systems that are used at higher deceleration rates). Hybrid cars, where regenerative brakes are the most common, and electric cars can reverse the flow of power through their electric motors backwards to slow the car down. In one of those convenient engineering coincidences, electric generators are the same as electric motors.
When you apply a current to a motor it turns, converting electricity into mechanical torque. When you apply a mechanical torque to the motor it induces electric current so it can be used as a generator. Thus, using kinetic energy to turn the motor generates energy. Put this collected energy into the battery and, the next time you step on the accelerator, some of the energy you just saved is used to get you moving again. Of course, friction and other energy losses mean that you don't get to use all of the energy you captured (no potential for a perpetual motion machine here, sorry), but this is one reason the Prius and the Insight, for example, get such high mpg ratings.
Other types of regenerative braking systems
Instead of a battery, it is also possible to send the energy captured from the brakes into ultracapacitors (a sort of really fast charging, fast discharging energy storage system). While some ultracap technology remains shrouded in mystery (we're looking at you, EESTOR), many companies are testing ultracapacitors today and we expect them to make their way into more vehicles in the future. Hopefully. We've heard this claim before.
A very different regenerative braking system uses hydraulics to capture the kinetic energy. We have a detailed description of how hydraulic hybrids work here. Currently, this technology is mostly being tested in large delivery vans, such as those operated by FedEx and UPS.
F1 racing teams are currently at work on another type of regenerative braking system called KERS. Still in the early stages, KERS - aka kinetic energy recovery system - uses either batteries or a flywheel to take in the braking energy. Whether or not KERS ever makes it to production vehicles depends, in part, on how the safety concerns are dealt with on the track.
Why you would want to do this in the first place?
As stated, regenerative braking systems can dramatically increase a vehicle's fuel efficiency. This is the main reason to want them in your car. The systems do add cost and complexity to the drivetrain, but many hybrid owners think regenerative brakes are worth it. In pure electric cars, regenerative brakes are vital to increasing range and getting the most out of each charge. Regen brakes are not without complications, though. Most hybrid and electric vehicles use a more advanced electronic brake control unit that blends friction and regenerative braking adding another potential failure point to a safety critical system. In the all-electric MINI E, for example, the regenerative brakes are so strong that the rear brake lights will come on even if you don't step on the brake pedal. As the technology continues to mature, regenerative brakes will get better and better and we'll be able to feel green every time we step on the brakes.
Tire photo by rockstarassi. Licensed under Creative Commons license 2.0.