Regenerative braking is any technology that allows a vehicle to recapture and store part of the kinetic energy that would ordinarily be lost when braking. A simpler technology that can only convert the energy to heat but which uses similar principles is known as dynamic braking. Both are most commonly seen on electric or hybrid vehicles. Braking is accomplished by electrically switching motors to act as generators that convert motion into electricity instead of electricity into motion. Traditional friction-based brakes must also be provided to be used when rapid, powerful braking is required. Estimates currently see 30% efficiency; however, the actual efficiency depends on numerous factors, such as the state of change of the battery, how many wheels are equipped to use the regenerative braking system, and whether the topology used is parallel or series in nature.
Electric railway vehicles feed the recaptured energy back into the grid, while road vehicles store it for re-acceleration using flywheels, batteries or capacitors. Older dynamic brake systems generally used the electricity to provide heat or just passed it through large banks of resistors to dissipate the energy.