In a vehicle, slowing or stopping is accomplished by friction braking (not counting some degree of engine braking, air resistance and so on). Friction is created between the brake pads and the brake discs coming in contact with each other as you depress the brake pedal. In the process of slowing down or stopping, the kinetic energy of the vehicle is converted to heat energy that is eventually lost. In regen braking however, some amount of this kinetic energy can be recovered by converting it to electrical energy and fed back into the battery pack.
Modern electric vehicles have one or two electric motors that drive the wheels while consuming energy from the battery pack.
The motors are mostly common ones like the AC induction motor or an AC synchronous motor. These motors are similar to the ones that drive a household fan or a leaf blower but in this case they are adapted to the characteristics and needs of an electric vehicle. Battery current which is DC, is inverted, conditioned and filtered through some kind of a controller before it's fed to the motor.
Regenerative braking is only possible in a hybrid or a pure electric vehicle. In normal operation, the battery powers the motor to drive the wheels and gradually depletes the energy stored. But when the vehicle is coasting i.e., forward motion with not enough (or no) electricity supplied to the motor and during braking, the motor in turn acts like a generator that produces electric power; essentially converting some of the kinetic energy to electrical energy. The major components of an electric vehicle involved in the regenerative mechanism are the wheels, the electric drive motor, the battery pack and the wiring between the battery pack and the electric motor.
This phenomenon can be clearly demonstrated in an electric vehicle coasting down a hill. When the car ends up at the bottom of the hill it will have more energy in the battery than when it first started its descent. Some, not all of this kinetic energy is thereby recovered.
So how does regen work?
The figures below give you an illustration of how this mechanism works. The normal forward motion of an electric vehicle is depicted in figure 1. The battery supplies the specified power to the variable speed drive controller that inverts the DC power to 3-phase AC. This power drives the motor that in turn drives the wheel all while depleting energy from the battery.
The regenerative mechanism is illustrated in figure 2. Here, the reverse torque acted upon the wheel during coasting or braking is generating current in the system and charging the battery pack replenishing some of the lost energy. The torque referred to above is the twisting force that is acting in the opposite direction of motoring mode torque. During coasting or braking this reverse (braking) torque generates current in the windings of the motor (which is a generator at the moment) and sends it to the battery pack after conditioning and rectification (conversion to DC) in the controller block. In both the induction motor and the synchronous motor when the rotor speed is greater than the synchronous speed, current is generated. In other words, if synchronous speed of the motor is reduced to less than the rotor speed and there is a braking torque the motor becomes a generator for this duration.
The amount of electrical energy recovered depends on a lot of factors including how you drive, terrain, size of the motor and how much braking was done in the course of the trip.