Mild hybrids are essentially conventional vehicles with oversized starter motors, allowing the engine to be turned off whenever the car is coasting, braking, or stopped, yet restart quickly and cleanly. Accessories can continue to run on electrical power while the engine is off, and as in other hybrid designs, the motor may be used for regenerative braking to recover energy. The larger motor is used to spin the engine up to operating rpm speeds before injecting any fuel.
Many people do not consider these to be hybrids at all since they do not have hybrid drivetrains (there is no electric motor to drive the vehicle), and these vehicles do not achieve the fuel economy of full hybrid models. A major example is the 2005 Chevrolet Silverado Hybrid, a full-size pickup truck. Chevrolet was able to get a 10% improvement on the Silverado's fuel efficiency by shutting down and restarting the engine on demand. Mild hybrids often use 48 volt systems to supply the power needed for the startup motor, as well as to compensate for the increasing number of electronic accessories on modern vehicles.
Compared to a "full" hybrid vehicle, however, mild hybrids may provide many of the benefits of the application of hybrid technologies, with less of the cost/weight penalty that is incurred by installing the parallel (electric) hybrid drivetrain. Fuel savings would generally be lower than expected with use of a full hybrid design.
General Motors followed the pickup truck hybrid with their Belt alternator starter (BAS) hybrid system, used in the 2006 Saturn VUE Green Line. It operates in much the same manner as the "start-stop" system in the Silverado, but the electric motor can also provide modest assist under acceleration.