From an engineering perspective, the Prius' neatest trick is its computer-orchestrated integration, the fluid interplay of the electric motor and gas engine. The Prius is a "strong" parallel hybrid, which means that both electric motor and gas engine act on the driveshaft through a power-splitting gearset. At low speed, the electric motor drives the car. When higher speeds or heavy acceleration are required, the gas engine and electric motor work together. Some of the engine's power is diverted to a generator that charges the battery. Also, like other hybrids, the Prius captures kinetic energy otherwise lost during coasting and braking and converts it into electricity, a technology called regenerative braking.
The result of all these dancing electrons is that the Prius extracts about twice the energy from a gallon of gasoline as a conventional car--a measure called tank-to-wheel efficiency. A lot of Prius owners have been disappointed that they couldn't get the mileage claimed by the Environmental Protection Agency--60 mpg city and 51 mpg highway--but that has more to do with the testing cycle of the EPA than with the Prius, which reliably returns 45 mpg in mixed city-highway driving.
In July 2004, [Felix Kramer] started the Prius+ discussion group on yahoo.com, soliciting technical advice. He teamed up with Ron Gremban, another Bay Area PHEV enthusiast and engineer who was about to take delivery of his own Prius. "As soon as the Prius came out I started wondering what you could do with it as a plug-in," Gremban says. Kramer and Gremban announced they would do a pilot project in Gremban's garage, using conventional lead-acid batteries.