A regenerative brake is an apparatus, a device or system which allows a vehicle to recapture part of the kinetic energy that would otherwise be 'lost' to heat when braking and make use of that power either by storing it for future use or feeding it back into a power system for other vehicles to use.
Regenerative brakes have most recently been introduced to production battery electric and hybrid electric vehicles. Electric regenerative brakes descended from dynamic brakes (rheostatic brakes in the UK) which have been used on electric and diesel-electric locomotives and streetcars since the mid-20th century. In both systems, braking is accomplished by switching motors to act as generators that convert motion into electricity instead of electricity into motion, but in the earlier systems the electrical power was dissipated through banks of resistors rather than being stored for future use. This means the system was no more efficient than conventional friction brakes, but it reduced the use of contact elements like brake pads, which eventually wear out. Traditional friction-based brakes must also be provided to be used when rapid, powerful braking is required.
Like conventional brakes, dynamic brakes convert rotational energy into heat energy, but this is accomplished by passing the generated current through large banks of resistors that dissipate the energy. If designed appropriately, this heat can be used to warm the vehicle interior. When the energy is meant to be dissipated externally, large radiator-like cowls can be employed to house the resistor banks.
Electric railway vehicles feed recaptured energy back into the grid, while road vehicles store it for re-acceleration using flywheels, batteries, or capacitors. It is estimated that regenerative braking systems currently see 31.3% efficiency; however, the actual efficiency depends on numerous factors, such as the state of charge of the battery, how many wheels are equipped to use the regenerative braking system, and whether the topology used is parallel or serial in nature.
The main disadvantage of regenerative brakes when compared with dynamic brakes is the need to closely match the electricity generated with the supply. With DC supplies this requires the voltage to be closely controlled and it is only with the development of power electronics that it has been possible with AC supplies where the supply frequency must also be matched (this mainly applies to locomotives where an AC supply is rectified for DC motors).
It is usual (in railway use) to include a 'back-up' system such that friction braking is applied automatically if the connection to the power supply is lost. Also in a DC system or in an AC system that is not directly grid connected via simple transformers special provision also has to be made for situations where more power is being generated by braking than is being consumed by other vehicles on the system.
A small number of mountain railways have used 3-phase power supplies and 3-phase induction motors and have thus a near constant speed for all trains as the motors rotate with the supply frequency both when giving power or braking.