The gift of energy

How does recuperation work in electric cars?

(Bild: Stephan Schätzl)

Instead of converting kinetic energy into heat when braking, it can also be recovered in the form of electricity. This is known as recuperation. This is how it works!

Since cars with hybrid drive systems with more or less large electric motors and purely electrically powered cars have been around, there has also been recuperation. It is an important factor in achieving significant increases in efficiency. In essence, this is regenerative braking, in which electric and hybrid vehicles convert kinetic energy back into electrical energy.

Unlike cars powered purely by an internal combustion engine, which release a large proportion of kinetic energy into the environment unused in the form of heat via the conventional friction brake, this makes it possible to convert kinetic energy into traction current. Hybrid vehicles therefore need up to 20 percent less fuel, and electric vehicles can increase their range. For electric vehicles with a medium-sized battery (50 to 60 kWh), recuperation can increase the range by 50 to 100 kilometers.

Energy is collected downhill
When a hybrid or electric car drives down a hill (except in coasting mode) or the driver actively presses the brake pedal (at least in most cases), the electric motor is automatically used as a generator. The drive unit stops driving the wheels. Instead, these now transfer the kinetic energy via the drivetrain to the electric motor, which works in a similar way to the dynamo on a bicycle: It brakes the car by absorbing kinetic energy and converting it into electric current. The kinetic energy recovered during this process is fed into the high-voltage traction battery. When moving off and accelerating, it can then flow from the battery back into the electric motor that drives the vehicle.

In electric cars in particular, the principle of recuperation also enables so-called one-pedal driving, in which the right foot controls the speed primarily or exclusively via the accelerator pedal. In this mode, which can be activated at the touch of a button in some electric vehicles, the brake pedal is rarely used, which also has a positive effect on brake wear and produces less particulate matter.

Keyword torque blending
However, if a car has to be decelerated heavily, more braking power is often required than can be generated solely by an electric motor with its own strong recuperation. In this case, even with strong motor braking, the conventional wheel brake also intervenes. The friction brake torque adds up with the deceleration power of the generator to the braking torque actually required - the process is called torque blending.

Whether a vehicle is decelerated by the electric motor, by the friction brake or by both at the same time, the driver ideally notices little or nothing of this, depending on the make and technology. The driver presses the brake pedal and the control software does the rest. The aim is to make the best possible use of the regenerative braking torque so that as much energy as possible can be recovered.

The braking potential of the electric motor depends on its size, the driving speed and the engine speed. The braking torque and energy recovery are greatest at low speeds. The state of charge of the battery also plays a role. Only if it is not fully charged can the electric motor provide any braking torque and recover energy.

An electric vehicle with a 100 kW electric motor generates up to 0.03 kWh per second through regenerative braking, for example. During emergency braking or in the event of an unstable driving situation, however, the car is usually decelerated almost exclusively via the friction brake, as wheel-specific interventions via the conventional brake system are necessary. This is not the case with the new PPE electric platform from Audi/Porsche: here, recuperation works right up to the ABS control range.