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Electric-vehicle battery packs could shrink 20 to 30 percent, and make electric vehicles more affordable, if new sensors were developed to monitor the cells in a pack, according to the U.S. government’s Advanced Research Projects Agency for Energy (ARPA-E). The agency says such sensors could have an even greater effect on hybrid gas-electric vehicle batteries, causing them to shrink by half.

Better sensors could tell what’s happening inside each of the hundreds of cells that make up an electric vehicle’s battery pack, allowing automakers to safely store more energy in them. A $30 million ARPA-E program that’s been underway for about a year is seeking to develop the necessary technology.

Developing new battery chemistries can take a decade or more, so increasing the capabilities of existing ones could be a faster way to reduce the cost of batteries, one of the main things holding back the adoption of electric cars (see “How Tesla Is Driving Electric Car Innovation” and “How Improved Batteries Will Make Electric Vehicles Competitive”).

Automakers currently don’t allow their batteries to be charged all the way, which helps avoid the high voltage levels that can degrade battery materials or in some cases cause fires. They also keep some capacity in reserve in case overheating or other factors reduce the performance of the battery. They build in these safeguards because they don’t know exactly what’s going on inside each cell—the temperature, chemical composition, mechanical strain, voltages at each electrode, and so on.

“We asked, ‘what if we remove those blinders’? How much more could you get out of a battery system if you actually knew what was happening inside the cells?” says Ilan Gur, a program manager at ARPA-E.

Based on computer simulations, the agency believes that better data—combined with improved battery control software and computer models of batteries—would make it possible to safely drive cells right up to their limits. Since automakers could get more out of each battery cell, they could make their battery packs smaller without sacrificing performance.

A project at the Palo Alto Research Center, one of 14 that make up the ARPA-E program, is developing fiber-optic battery sensors that, because they’re not electrically conductive, can easily be embedded within battery cells without interfering with their performance. The sensor system could add about 5 percent to the cost of a battery pack, says Ajay Rahgavan, who leads the project. But he says the sensors would be paid for by an up to 25 percent reduction in the size and cost of the battery.

Another ARPA-E project, at Oak Ridge National Laboratory, is not only developing sensors for individual cells, but also technology that could cool and heat a cell from within, keeping the cell at the optimal temperature and extending its lifetime.

Also as part of the ARPA-E program, Ford is developing an extremely precise battery tester that can detect when small amounts of unwanted chemical reactions are taking place within a cell. The near-term goal is to use the tester to speed the development of new battery chemistries. “But one of the dreams is to have miniaturized versions on board cars,” says Alvaro Masias, a battery researcher at Ford. That could allow a vehicle to control how much energy is used depending on the battery performance and how the car is being driven. “If we could customize the way we control the battery, rather than relying on generic rules for everyone, each customer can get all they can out of the battery,” he says.

Not everyone agrees that sensors can reduce battery size as much as ARPA-E hopes it can. Bill Wallace, director of the battery systems engineering group at GM, says that developing better sensors is important, but reducing battery size by 20 to 50 percent might not be possible. “That’s a big leap,” he says. It would require achieving all the improvements that are theoretically possible, he says, something that is rarely practical. “But that’s not a reason not to be working on it,” he adds—GM, although not part of the ARPA-E project, is also working on battery sensors.

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Credit: Photograph by PARC

Tagged: Energy, batteries, electric vehicles, sensors, ARPA-E

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