The Toyota Prius is a full series-parallel hybrid electric compact car developed and manufactured by the Toyota Motor Corporation. The second generation Prius had been completely redesigned as a compact Kammback. The XW20 series represented the second generation of the Toyota Prius, replacing its XW10 predecessor. The United States Environmental Protection Agency (EPA) and California Air Resources Board (CARB) rated the Prius as among the cleanest vehicles sold in the United States based on smog forming and toxic emissions in 2008. Toyota sold about 1,192,000 units of the second generation Prius worldwide. Development In 1999, Toyota began development on a second generation Prius under chief engineer Shigeyuki Hori. In 2001, Hiroshi Okamoto's exterior design was approved and frozen for production. In 2003, the Prius was completely redesigned as a compact liftback, with redistributed mechanical and interior space significantly increasing rear-seat legroom and luggage room. The 2004 Prius is even more environmentally-friendly than the 1997–2003 model (according to the EPA), and is longer than the previous version. Its more aerodynamic Kammback body balances length and wind resistance resulting in a drag coefficient ( Cd) of 0.26. Chief engineer Shigeyuki Hori's development effort led to 530 patents filed for the vehicle. The Prius uses an all-electric A/C compressor for cooling, an industry first, and also adds an electric power steering system to further minimize engine belt-driven engine accessories. Combined with a smaller and lighter NiMH battery, the XW20 is more powerful and more efficient than the XW11. In the U.S., the battery pack of the 2004 Prius is warranted for or 8 years. The warranty for hybrid components in California and the seven Northeastern states that have adopted the stricter California emission control standards is or 10 years. Battery The second generation Prius makes use of a 201.6-volt NiMH battery composed of 28 modules, where each module is made of six individual 1.2-volt 6.5 Ah Prismatic NiMH cells. The 7.2-volt modules each contain a charge controller and a relay. The 28 modules are connected in series to produce a total energy storage capacity of 1.310 kWh (201.6 volts × 6.5 Ah). The battery control computers keep the state of charge (SoC) between approximately 40% and 80% (shallow cycling), where the average SoC hovers around 60 percent, allowing about 400 Wh of useful energy storage to capture energy from regenerative braking and to release it back into the hybrid drive-train through Motor-generator 1