How regenerative braking works

In a conventional combustion engine, the explosion of the fuel moves the engine’s pistons to provide the mechanical energy that rotates the wheels. When the driver depresses the clutch, this releases the connection between the engine and the wheels. Subsequently applying the brakes causes the brake pads to physically impede the wheels by friction. Braking not only causes heat and wear but also wastes the energy that was moving the car forwards. This why urban driving uses more fuel than motorway driving.
 
In a battery powered car, charge from the battery powers the rotation of the motor’s armature to provide the torque that turns the wheels. However, these connections can also work in reverse as they do in an alternator or dynamo, where rotation of the wheels provides the force to turn the armature and generates electricity to charge the battery. When energy flows from the battery to the motor it becomes a force of acceleration, but when it is used to generate electricity it becomes a force for deceleration, or braking.
 
In regenerative braking this simple two-way principle is controlled as carefully as possible by the on-board computer. It uses the controller relay to close the electronic throttle that sends electricity and, providing the vehicle is still moving forwards, it begins to receive it through the charge controller instead. As a result, the kinetic energy of the car’s motion is reabsorbed, not wasted, recharging the battery and slowing the vehicle at the same time.
 
Due to the laws of physics, it is not possible to let the system reabsorb the energy entirely, so an electric brake system or hybrid brake system has to be supplemented with conventional brakes as well.

The most energy-efficient way to use any vehicle is to drive at an optimum speed and distance so that the driver never has to brake. We can come close to that ideal when we undertake a long-distance run on a motorway in perfect traffic conditions, however this opportunity rarely arises.
 
This also means that regenerative braking offers little advantage in perfect traffic conditions. However, it comes into its own if we are constantly speeding up, slowing down and braking. This is the familiar pattern in everyday urban driving with a succession of traffic lights, turns, traffic queues, pedestrian crossings and inconsiderate drivers.
 
A few other factors significantly affect its usefulness too. One of these is the terrain: when you are driving uphill the incline absorbs a lot of forward energy so there is less to return to the battery. Also, if you have to stop, you will depend on conventional brakes to avoid rolling backwards, as regenerative braking can’t help. You can recoup significant energy if you brake going downhill, but you are still wasting energy by braking at all.
 
Another major factor is the vehicle’s size and weight, because a lot more energy is invested in the momentum of a heavy vehicle. This means that although electric scooters, bicycles and skateboards could technically use RBS, the returns would not be worthwhile. Most average-size electric cars improve their energy efficiency and therefore their range by 15-20% thanks to regenerative braking. For hybrid vehicles the improvement is somewhat less, usually close to 10%. These figures assume an average level of urban motoring.

There is still controversy about the social and environmental impacts of electric cars in general because of the mining required to extract the materials needed to build their batteries and electronics. However as regenerative braking in a hybrid helps you to burn less fossil fuel, it is certainly better for the environment to have RBS than not. It is also likely that the final generation of petrol and diesel cars will also adopt RBS to improve their energy efficiency.

While not its primary purpose, regenerative braking also reduces wear to the brake pads. This benefit should not lead motorists to neglect regular checks, as brake pads are still vital in being able to perform an emergency stop. In order to improve energy efficiency, it is also important to look after your tyres, they make just as much difference as RBS. Choosing an energy efficient tyre can make a big difference, for example, Pirelli are making a new version of the very popular Pirelli P Zero optimised for electric and hybrid vehicles, called the Pirelli Elect.

Our tyre professionals recommend regular inspections to check tyre condition and inflation. You can book free brake and tyre checks at any Protyre garage using our online garage finder and contact form.