A hybrid automobile uses both an internal-combustion engine and an electric motor for propulsion, but the only fuel you’ll put in is gasoline. (Diesel hybrids do exist, but they’re mainly found in locomotives and other extreme heavy-duty applications.) Hybrids have small high-voltage batteries to power their electric motors, but you don’t plug them in.
With no external power source, electricity for the motor is scavenged under braking in a process called regenerative braking—regen for short. If you’re an F1 racing fan, you may have heard it called a Kinetic Energy Recovery System (KERS). The regen system does not replace the traditional brakes but instead works as a crucial alternative. Electrical energy collected via this process is saved in the battery for immediate reuse the next time you accelerate. When you leave a stoplight, the saved energy gets the car going again and delays the restart of the gasoline engine—in some cases until you reach 25 mph. When you stop again, the cycle repeats. This makes a hybrid’s city fuel economy much higher than a nonhybrid’s, not to mention its highway economy.
How Does Regenerative Braking Work?
Regen is a natural outgrowth of how electric motors function. When you put electricity into an electric motor, it moves (or helps move) the car. Once the car is moving, the same motor becomes a generator that can reverse the flow of electrons and harvest that motion to put electricity back into the battery. This is why hybrid electric motors are sometimes referred to as motor-generators. Crucially, this process slows the car, so regen is triggered and managed by a computer when you press the brake or descend a grade.
How small is a hybrid’s battery？
How small is a hybrid’s battery? They are usually no larger than one kilowatt-hour (kWh), which is just 1 to 2 percent the size of a full electric vehicle’s battery.
Still, hybrid regen can be strong enough for most routine slowing. A computer always monitors how hard you press the brake pedal, so it will blend in the traditional brakes—the pads and rotors—during harder stops and in emergency situations. This regen-first approach maximizes the collection of electricity for propulsion so that fuel economy soars.