Each battery pack uses of lanthanum, and each Prius electric motor contains of neodymium; production of the car is described as "the biggest user of rare earths of any object in the world." The LV battery is essential to starting the car and providing initial power to the computer. The Second Generation Prius contains a 1.310 kWh battery, composed of 28 modules. Each battery module is made of 6 individual 1.2 V 6.5 Ah Prismatic NiMH cells in series forming a 7.2 V 6.5 Ah module with 46 Wh/kg energy density and 1.3 kW/kg output power density. Each module contains an integrated charge controller and relay. These modules are connected 28 in series to form a 201.6 V 6.5 Ah battery (traction battery), also known as the energy storage system. The computer-controlled charge controller and battery management computer systems keep this battery between 38% and 82% state of charge, with a tendency to keep the average state of charge around 60%. By shallow cycling the battery, only a small portion of its net available energy storage capacity is available for use (approximately 400 Wh) by the hybrid drive system, but the shallow computer-controlled cycling dramatically improves the cycle life, thermal management control, and net long term calendar life of the battery. Active cooling of this battery is achieved by a blower motor and air ducting, while passive thermal management was accomplished through the metal case design. As the Prius reached ten years of being available in the US market, in February 2011 "Consumer Reports" examined the lifetime and replacement cost of the Prius battery. The magazine tested a 2002 Toyota Prius with over 200,000 miles on it and compared the results to the nearly identical 2001 Prius with 2,000 miles tested by "Consumer Reports" 10 years before. The comparison showed little difference in performance when tested for fuel economy and acceleration. Overall fuel economy of the 2001 model was while the 2002 Prius with high mileage delivered . The magazine concluded that the effectiveness of the battery has not degraded over the long run. The cost of replacing the first generation battery varies between and from a Toyota dealer, but low-use units from salvage yards are available for around . One study indicates it may be worthwhile to rebuild batteries using good blades from defective used batteries. The HVAC system uses an AC induction motor to drive a sealed-system scroll compressor, a design principle not usually used in automotive applications.