The Volt's battery pack consists of more than 220 Li-ion cells capable of storing 16 kW-h of energy. Each cell is roughly the size and shape of a thin paperback book. Compared with the cylindrical cells powering the Tesla Roadster, this prismatic configuration is advantageous for both packaging and temperature distribution. A cylindrical cell tends to be hot at its center and cool at its periphery, while a flatter cell has an even temperature distribution that's easier to maintain with a cooling system.
After initial discussions with twenty-three battery companies, GM began working in earnest last year with two cell producers - LG Chem based in Korea and A123Systems, the firm selling Li-ion batteries to Prius owners craving extra wattage. Although production contracts are pending, clues offered about the specific chemistry inside the cells suggest that LG Chem will be GM's battery supplier.
While Li-ion batteries typically last no more than a few years in laptop and cell phone applications, such short service won't cut it in the car world. Since the Volt's batteries are considered part of its emissions control system, they must be guaranteed for eight or ten years, depending on the state where the car is sold. To stretch the useful life of the Volt's batteries, GM engineers developed means of precisely monitoring their state of charge so that the pack can be maintained between 30 and 80 percent of its total capacity. A second measure is keeping the battery pack's temperature between 68 and 77 degrees Fahrenheit during driving and while plugged in for recharging. GM vice chairman Bob Lutz adds, "We're putting a very generous warranty provision into the Volt's cost structure, because the only real way to simulate ten years [of battery use] is to wait ten years."
Plugging into a 110/120-volt wall socket for eight hours or a 220/240-volt outlet for three hours charges the Volt for daily pollution-free commuting while consuming about a dollar's worth of electricity. When a trip exceeds 40 miles, a 1.4-liter four-cylinder engine powering a 53-kW AC generator automatically fires up to relieve the battery pack as the source of electrical energy. This engine, which has an aluminum head topping an iron block, will run at some (unspecified) constant speed between 2000 and 4000 rpm. To minimize noise and vibration, the Volt's propulsion motor, gasoline engine, and generator are bolted together in one assembly supported by rubber mounts. ...next page >>