General Motors (GM) unveiled the production design of its Chevrolet Volt electric vehicle on Tuesday, as part of its 100th-anniversary celebration. But significant hurdles remain before the car can start rolling off assembly lines, chief among them the need for continued development of the car’s main battery pack.
The Volt is an electric car that can be recharged by plugging it into a wall socket or by running a small, onboard gasoline, ethanol, or diesel generator. The 16-kilowatt-hour lithium-ion battery pack stores enough energy for 40 miles of driving–enough to cover almost 80 percent of the daily driving in the United States, the company says. On longer trips, the generator kicks in to recharge the battery, giving the Volt as much range between fill-ups as a typical gas-powered car. For more than a year, GM has been showing off the concept-car version of the Volt in ads. The new production version looks considerably different–it has a more aerodynamic shape–but it will have the same performance specifications that the automaker has been advertising.
Plug-in hybrid vehicles like the Volt began to seem feasible because of new technology that made lithium-ion batteries safer, more durable, and less costly. But while individual battery cells using the technology seem to work well, yoking nearly 300 of them into a battery pack has proved challenging. That, in turn, is forcing GM to design systems that make the vehicle more expensive. “At the cell level, things look good,” says Mark Verbrugge, the director of the materials and processes laboratory at GM’s research-and-development center. “There are still issues at the pack level that we’re trying to iron out, which gets pretty nerve-racking as we get close to production.”
A battery pack for an electric vehicle is complex. The cells have to be wired together to deliver power reliably, despite the harsh vibrations and jolts encountered on the road. (For an example of what can happen when things go wrong, see “Electric Cars 2.0.”) Even a few defective cells or connections can dramatically lower the performance of the pack. What’s more, the pack includes complex electronic controls for charging each cell, delivering power, and capturing energy from braking to improve vehicle efficiency. And maximizing the battery’s life requires a good cooling system. To make matters worse, methods for testing whether a battery pack will last for the life of the car are only now being developed.
“There’s only so much known about how to accelerate the testing of batteries,” says Greg Cesiel, GM’s program director for the E-Flex Vehicle Team, which is developing the Volt and related electric vehicles. Questions remain about how to simulate driving the car and charging the pack, and how to confirm that the pack will survive vibrations and exposure to hot and cold temperatures over the life of a vehicle.
“The big risk when it comes to putting these on the road is, we don’t have accelerated life testing,” Verbrugge says. “We have some at the cell level, which gives us enough confidence to say we’re going to do this thing. But I would contend that’s still the big risk.”