A single component will make or break Chevrolet's new Volt "extended-range electric vehicle"—and with it, potentially, the fate of America's largest carmaker, General Motors: its battery. It's no wonder then that some GM executives call the Volt's battery "our diva." After all, the new lithium-based pack provides the essential spark that makes the new design go in more ways than one—from its green performance and driving range, right down to its wind-cheating looks.
"Not only is the battery all-important to running the car, it is supremely sensitive to its environment and to how it is operated…almost to the point that we think of it as a living thing," says Frank Weber, Chevy Volt vehicle line executive. "Our goal is a 10-year lifetime or 150,000 miles (241,400 kilometers) from a vehicle that satisfies all the customer's expectations without sacrifices and compromises." Making that happen with technology that emerged from the lab only a few years ago is the "true challenge," he adds.
The Chevrolet Volt, which GM plans to introduce in two years (reportedly at a mid-$30,000 base price), is a new breed of gasoline-electric hybrid, cars powered by a battery-powered electric motor that can be recharged on the road with an internal combustion engine.
The Toyota Prius and other hybrids now on the road are propelled primarily by their electric motors; the gasoline engines cut in only at higher speeds. As a result, these conventional hybrids achieve higher fuel economy than cars that run solely on gasoline.
The Volt hybrid is different. It uses both power sources, but it is an electric car that uses its gasoline engine only to generate power and so extend its driving range beyond what the battery could provide on a single charge alone. GM anticipates Volt buyers will plug their cars into home electric sockets each evening and then drive more than 40 miles (65 kilometers) a day on the overnight charge. And because around 80 percent of U.S. motorists travel less than 40 miles on an average day, many should "never have to start up their gas engines," Weber says. The Volt would thus reduce our nation's petroleum consumption and, presumably, its production of greenhouse gas emissions.
The first big hurdle in GM's quest to make the Volt concept a reality was choosing the right battery chemistry and battery supplier. The lithium battery in the Volt is not the same as those in the burning laptop computers on view on YouTube. Standard lithium-ion batteries—technically, lithium–cobalt oxide—power most consumer electronics. Instead, the newer, safer lithium batteries for vehicle applications typically substitute manganese oxide or iron phosphate electrodes for the more costly and less stable cobalt oxide ones, reports professor M. Stanley Whittingham, a researcher at Binghamton University in New York State who many call the "father of the lithium-ion battery."
"Selecting the battery supplier is also important because you cannot assume that the supplier can deliver a 'drop-in' battery," Weber tells ScientificAmerican.com. Because the battery's operating characteristics are so crucial to how the car runs, "the battery must be integrated into the entire system right from the start." This means that the carmaker and supplier must work closely to fine-tune the battery as it is being developed.
"You need a nearly perfect manufacturing process that employs strict quality-assurance methods, because a single cell failure will cause the entire [250-cell] battery pack to fail," he says. "I'd rather have an 80 percent–capable cell chemistry with a perfect production process than a perfect chemistry with 80 percent–effective manufacturing process."
GM considered two battery makers for the Volt. One, A123 Systems in Watertown, Mass., is a three-year-old firm that builds batteries that use electrodes made from nanostructured iron phosphates, an innovation developed at the Massachusetts Institute of Technology (M.I.T.), according to Whittingham. (A123 is teamed with German auto components–maker, Continental, to package its cells into battery packs.) The other, LG Chem, the largest chemical company in Korea and corporate sister of giant LG Electronics Worldwide, uses manganese oxide–based electrodes.
The car company has not publicly announced the winner, but according to Reuters, it's settled on LG Chem to build the Volt's battery. If true, it is a good bet that the Korean firm's six-year effort to beef up its production of lithium ion batteries—mostly used in mobile devices—figured strongly in the decision. Some longtime observers, such as Felix Kramer, the hybrid car proponent and founder of the California Cars Initiative, wonder if GM might also contract with A123 for Volt batteries to expand the supplier base.
Keeping the Volt's "diva" happy was the next big issue that its engineers had to address. In particular, Weber says, "You have to avoid letting the battery get exposed to high temperatures, which threatens its operating life." Too much heat drives the cell's electrochemical reactions faster, leading to accelerated aging. "There's a big difference between 70 degrees Fahrenheit (21 degrees C) and 90 or 100 degrees F (32 to 38 degrees C)," he says, so the car's thermal management system has to keep the diva within her comfort zone.
The other major piece of the Volt equation is the control system and associated power electronics. The operation of every modern automobile is determined by a set of computerized control functions—a hierarchy of rules that govern when each subsystem should activate, to what degree, and for how long. Each control function "constantly talks to and affects each other," Weber says and, so, they must work together in a finely coordinated manner to ensure smooth sailing. Because the Volt constitutes a "whole new propulsion category," he notes, "hundreds of control functions that have never before been implemented must be integrated so they operate seamlessly and flawlessly." Usually this process takes several years of progressive iteration, but Weber's engineering team must rethink all these basic interactions in the relatively short time remaining until the new model is rolled out.
Not only must the engineers make all the functions operate in sync, they must ensure that the cars' systems and functions use the limited energy supply during battery-only operations extremely efficiently. So how do you reduce the energy loads of all these different systems to optimize usage?
Weber says that only becomes clear once you understand first how each system operates individually and then together. This knowledge allows the engineers to take advantage of synergies such as using the waste heat of one system to heat another. He likens this engineering process to the development of the cell phone: "First it fit in a briefcase, then it was a handheld 'brick,' now, it's palm-sized." He expects his team to make similar progress toward greater integration and efficiency.
But this all takes time and the Volt team has only two years left. The key will be to use the remaining time before introduction to implement all the necessary integrations while keeping ‘the diva’ as content as possible. Only that will make the Volt the revolutionary vehicle that GM hopes it will be.