BMW's Electric Car

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Batteries not included: AC Propulsion supplied the motor and power electronics that will be used in the electric Mini. The battery pack is not shown. Credit: AC Propulsion

AC Propulsion's battery pack uses off-the-shelf battery cells of the same type found in laptop-computer battery packs, which Gage says provide the best performance per dollar. A total of 5,088 of these cells have been wired together to store enough energy to power the car for 150 miles. In very rare cases, laptop batteries have been known to overheat and catch fire, a particular danger with the larger packs used in electric cars. Some engineers worry that if one defective cell among the thousands catches fire, it could cause a chain reaction that would destroy the entire pack and might cause injury. To limit that risk, some new battery packs use phosphate-based cells instead of the more common oxide-based cells. Gage would not provide details about the battery cells used in the Mini, saying only that they are "inherently safe." He did, however, say that the company's tests have shown that a chain reaction does not happen if one cell catches fire. BMW adds that the battery pack has passed crash tests.

It's not clear whether BMW will use AC Propulsion's system in future mass-produced electric vehicles. Other automakers are opting for battery cells that are larger than laptop cells, so that they can use hundreds of cells instead of thousands, cutting down on the number of connections inside the pack and improving reliability. Gage says that "companies like us can't afford the time and money for a major battery-development program."

AC Propulsion's manufacturing capacity is also limited. "When we started the discussions [with BMW], we were doing two systems a month, and we thought we could ramp up to 200 a year," Gage says. The company did much better, meeting the requirement of 500 systems by increasing its rate of production to a peak of 60 systems a week. But Gage says that that's a rate he wouldn't want to sustain with the company's current facilities.

The biggest obstacle to mass production may be cost. The electric Mini will be leased for $850 a month, and the company says it already has 9,500 people signed up. Kramer says that millions of people will be willing to spend extra for an environmentally friendly--and fun-to-drive--electric car. But, he says, "it's up to BMW to take great products from AC Propulsion and figure out how to help make them affordable."