GM launched a hybrid version of the Silverado/Sierra in 2004, becoming the first ever GM hybrid passenger vehicle. Known within GM as the Parallel Hybrid Truck or PHT, it is not actually a parallel hybrid by the current definition, but a type of micro hybrid design. The electric motor housed within the transmission flywheel housing, serves only to provide engine cranking/starting, battery charging, and powering accessories. The engine automatically shuts down as the truck comes to a stop and uses 42 Volt electric power to the starter/generator unit to restart the engine as the brake pedal is released. Besides the typical 12 V automotive battery the PHT uses three additional 14 V valve regulated lead-acid (VRLA) batteries mounted under the rear seat to store and provide power. The truck uses a 5.3 L "Vortec 5300" V8 for primary propulsion power. These trucks were also purchased back from customers for more than what they were worth in the late 2000s.