Wednesday, November 19, 2008

BMW's Electric Car

The new Mini borrows a drive system from a high-performance electric sports car.

By Kevin Bullis

--- from technologyreview.com

Mini electric: The electric version of the Mini looks just like the regular version, except that it has a yellow plug logo and yellow trim. Credit: BMW

On Wednesday, BMW introduced an electric version of the Mini compact car at the Los Angeles Auto Show. The first 200 of the cars have already been delivered to the United States, well ahead of a wave of new electric cars expected from other major automakers starting in 2010.

"They're the first to have a rollout," says Felix Kramer, founder of CalCars, a group that promotes the development of plug-in hybrids and electric vehicles. BMW was able to speed the introduction of the car by adapting an electric drive system from a small company in California to work in an existing vehicle. Other automakers are instead developing electric-car models from scratch. BMW, which acquired the Mini brand when it bought the Rover Group in 1994, plans to build an additional 300 electric cars by the end of the year, bringing the total to 500. The cars will be leased to select consumers for a year. The automaker's goal is to use the leased vehicles to gather information about real-world driving that will aid the engineering of future mass-produced electric vehicles. "There's no reason, after the first 500, they couldn't sell as many as they could build," Kramer says.

BMW first approached the company that developed the Mini's drive system, AC Propulsion, only a year ago, says Tom Gage, the company's CEO. GM, by contrast, announced its plan to develop an electric car nearly two years ago and says that it will deliver its first vehicles at the end of 2010. (In the 1990s, GM aborted an earlier electric-vehicle program, which had produced a car called the EV-1 in limited quantities.) GM is working with suppliers to develop new battery packs designed specifically for its car, called the Volt. With AC Propulsion, BMW got a ready-made electric drive system that could be incorporated easily into the Mini, and which has been field-tested in AC Propulsion vehicles. Some of these vehicles have well over 100,000 miles on them, Gage says.

The technology used in the Mini is based on a drive system developed for the tzero, AC Propulsion's high-performance electric sports car, which can accelerate from 0 to 60 in 3.6 seconds. The Roadster, a car produced and sold by the startup Tesla Motors, was inspired by the tzero, and the first prototype was based on AC Propulsion's technology. Adapting the tzero's system for the Mini required only minor modifications, Gage says.

The AC Propulsion design is distinctive in part because it uses a single set of hardware to both charge the battery and control the electric motor, saving space and weight. The system is also flexible. The battery can be charged at a variety of rates and voltages, from overnight charging from a standard 110-volt outlet to a 2.5-hour recharge from a special outlet that will be installed in the garages of those who lease the electric Mini.

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."