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.”

Verbrugge says that one of the biggest challenges is ensuring that the batteries won’t fail in extreme climates, such as the deserts of Arizona. Conventional starter batteries already give automakers trouble in hot areas, he says. Today, they’re the car part that most commonly fails under warranty in the Southwest. “Batteries don’t like hot temperatures,” Verbrugge says. “But we’re not going to say to people in Arizona, ‘We’re not going to sell you our Chevy Volt. You can drive one, but we’re not going to give you a warranty.’ That’s not an option.”

To make up for uncertainties about the life of the battery packs, GM plans to coddle them, wrapping them in insulation and including heating and cooling systems to keep them at optimal temperatures. Questions remain about when these systems should operate, since they can eat into the energy savings that electric vehicles are supposed to provide. “Let’s say you’re charging,” Verbrugge says. “Do you run your cooling system now to keep your battery cool over black asphalt? Then your energy efficiency doesn’t look so hot. Do you do that only in Arizona? These become critical engineering issues.”

GM is also oversizing the packs, adding several kilowatt-hours’ worth of extra cells to make up for potential degradation over the life of the vehicle. That makes the packs, and the vehicle, much more expensive. “Cost is a major issue for us now,” Verbrugge says. “We’re not sure people are willing to pay.”

Indeed, the Volt and other proposed cars like it are expected to cost thousands of dollars more than conventional cars, which could limit their appeal, says Paul Werbos, a program director for the National Science Foundation (NSF), who has been promoting research on better, cheaper batteries. “I don’t expect most people are going to pay that,” he says.

Werbos and Verbrugge spoke last week at an NSF-sponsored workshop focused on improving batteries for the next generation of hybrid and electric vehicles. Speakers at the workshop emphasized that better tests for battery lifetime, combined with improvements to battery design to make them last longer, will allow automakers to use fewer batteries and cut costs.

In spite of the remaining challenges, Cesiel is encouraged by the progress that the company’s engineers have made so far and believes that the Volt will be ready for production on time. Based on its laboratory testing so far, he says, the company is “happy” with the capacity and performance of the batteries. GM also knows what the cooling system will look like and has physically integrated the pack into the vehicle. What’s more, the entire propulsion system, including the battery pack, the electric motor, and the generator, was incorporated into a test vehicle and delivered to the company’s Milford, MI, testing grounds at the end of August, just two days behind the schedule set last year. “I wouldn’t say that the battery is ready,” Cesiel says, “but we’re right on track.”