When the vehicle is started, the gasoline engine "warms up."
If necessary, the electric motor acts as a generator, converting energy from the engine into electricity and storing it in the battery.
The gasoline engine powers the vehicle at cruising speeds and, if needed, provides power to the battery for later use.
During heavy accelerating or when additional power is needed, the gasoline engine and electric motor are both used to propel the vehicle.
Additional power from the battery is used to power the electric motor as needed
Regenerative braking converts otherwise wasted energy from braking into electricity and stores it in the battery.
In regenerative braking, the electric motor is reversed so that, instead of using electricity to turn the wheels, the rotating wheels turn the motor and create electricity. Using energy from the wheels to turn the motor slows the vehicle. If additional stopping power is needed, conventional friction brakes (e.g., disc brakes) are also applied automatically.
When the vehicle is stopped, such as at a red light, the gasoline engine and electric motor shut off automatically so that energy is not wasted in idling.
The battery continues to power auxillary systems, such as the air conditioning and dashboard displays. The battery stores energy generated from the gasoline engine or during regenerative braking, from the electric motor. Since the battery helps power the vehicle, it is larger and holds much more energy than batteries used to start conventional engines. DOE’s Alternative Fuels Data Center