In an petrol or diesel car, when the driver presses the brake pedal, the car’s speed is reduced and the energy that the car had, because it was moving – kinetic energy – is lost as heat through the brake discs and pads. This is a waste of the energy, which came originally from the petrol or diesel used to accelerate the car in the first place.
In an electric car, this kinetic energy, instead of being lost as heat, can be stored back into the car’s battery by the use of regenerative braking.
Anyone familiar with Formula 1 racing will know about KERS – Kinetic Energy Recovery System. This uses a similar principle to that now used in electric cars to recover some of the energy of the car under braking and return it to the battery.
The process is not 100% efficient however so the car can never recover all the energy that it has used. Because of this, it is always more energy efficient to avoid using the energy in the first place rather than to try to recover it using regenerative braking. So, if there is a choice, without inconveniencing other road users, always decelerate by coasting, in advance of your braking point, to reduce your speed earlier rather than keeping your speed up until later and then braking.
If the car’s battery is already nearly or completely fully charged, then no regenerative recovery will be possible since the system has no room to store the energy. So when driving you may be aware of a lack of regenerative braking when the battery is full.