Source: The ChronicleHerald

The following is an excerpt of a post Jim Kerr wrote on the evolution of regenerative braking by electric vehicles. The entire post can be read by clicking HERE.

HALIFAX, N.S. – Heat! That is what braking creates. When you step on the brake pedal, the brake pads press against the brake rotors and the friction slows the car. It also generates heat — a lot of heat! I have seen race cars where the cast iron brake rotors were so hot that you could see right through them. Brake engineers love to design big brakes so they can absorb that heat and provide better braking, but all that heat is wasted energy. We don’t use it to heat the interior, or the engine, or power the vehicle. It just dissipates to the atmosphere.

Then there are hybrid and electric vehicles. They use regenerative braking. Regenerative braking is simple in concept — use the energy of the vehicle as it slows to create electrical power that can be stored in the vehicle’s battery and then reused. Making it feel right and work smoothly is much more difficult, but regenerative braking is getting better with every generation of electric vehicle introduced. So how much do we gain with regenerative braking?

“Mileage may vary with driver” certainly applies to regenerative braking. At best the conversion of electrical energy into mechanical motion is about 80 per cent. This applies in regenerative braking as well, where converting mechanical motion into electrical energy is about 80 per cent. Mathematically, combining those would give us 80 per cent of 80 per cent which equals about 64 per cent overall efficiency, but it is not that simple. Several factors must be considered: driving terrain (hilly versus flat roads), speed of vehicle when slowing, the mass of the vehicle, the aerodynamic drag and rolling resistance of the vehicle and how the driver decelerates the vehicle.

For hybrid and electric vehicles, stepping lightly on the brake pedal doesn’t apply the mechanical brakes, it starts the regenerative braking. The energy of slowing the vehicle flows from the tires back through the drivetrain and electric motor, which now becomes a generator to put energy back into the battery. For many of these vehicles, regenerative braking takes place automatically. The more you step on the brake pedal, the more electricity is put back into the battery, and if you need to stop faster, pressing harder on the brake pedal will apply the mechanical brakes. There are systems on other vehicles that can enhance regenerative braking.