Plug-in electric cars such as the Chevy Volt and the Nissan Leaf have only just begun to penetrate the U.S. consumer market, but already automakers are thinking ahead to the next technological advance: a car that can recharge itself anytime and (almost) anywhere.

Consumers are perfectly happy to plug in phones and game controllers, but many have not yet adapted to the idea of plugging in a car at the end of a commute.

Stationary wireless charging
"Almost universally, all the carmakers have learned...that consumers find plugging in a vehicle is inconvenient, and the carmakers have concluded they need to offer some type of wireless, hands-free charging," says David Schatz, director of business development and marketing for WiTricity Corp. in Watertown, Mass., which makes wireless chargers for phones and cars. With WiTricity's system, a user would not have to park his or her car directly on a charging mat, let alone deal with wires. As long as the car is within range of the charging station, energy begins to flow into the battery.

The process relies on a principle called magnetic resonance coupling (pdf). The charging device, made of a coiled wire with capacitance plates on either end, uses electricity to create a magnetic field that resonates at a specific frequency. Just as an opera singer can shatter a wine glass by singing the right note, the emitting coil transfers energy only to a receiving coil that resonates at the same frequency. Magnetic resonance coupling is thought to be safer than other methods of wireless charging because the intensity of the field can be increased without affecting other, non-resonant objects nearby.

The technology, invented at the Massachusetts Institute of Technology four years ago, may end up in cars soon. WiTricity last year partnered with major automobile component supplier Delphi Electronics on a demo car, and Mitsubishi Motors recently agreed to work with WiTricity on infrastructure research.

Dynamic wireless charging
Other researchers are taking wireless charging to the next step: designing cars that can charge while on the road. Dynamic charging would help to eliminate the fear that drivers might have of being stuck on the road with a dead battery, often referred to as "range anxiety."

A team at the University of Michigan–Shanghai Jiao Tong University Joint Institute in China is testing a prototype electric vehicle (EV) that would use a similar resonant coupling system to charge. Instead of setting up refueling stations inside garages, however, the researchers suggest chargers should be installed in the road itself, so cars could juice up on the go. Because electricity in this scenario would be ubiquitous, cars could eliminate heavy batteries, using supercapacitors to store energy.

Supercapacitors could be an ideal storage device for fast, frequent charging, as opposed to the slow, hours-long process used by many of today's plug-in vehicles. Chengbin Ma, an engineer at the institute, sees this setup as perfect for a fleet of electric buses, because they have guaranteed stops where the energy could be delivered for longer periods of time.

Ma and colleagues have built a coffee can–size prototype vehicle that "can really move and stop for wireless charging automatically," he says. The next steps are to finish a go-kart–size one-seater EV prototype, boost the system's power, and perfect that car's on-the-move recharging.

"Super" capacitors?
On the other hand, supercapacitors still have some technical issues that need to be ironed out. "I don't think there's an enormous expectation for them," says Brett Smith, co-director of manufacturing, engineering, and technology at the Center for Automotive Research, an Ann Arbor, Mich., nonprofit that studies the auto industry. "I think they will potentially be a lesser cost [than batteries], but they aren't robust enough. They don't do what the industry needs them to do yet." Smith says that no matter how far the new advances have taken this technology, it will be a decade before it appears in mass-market cars.

As for implanting chargers in the roadbed, that technology is "fascinating, and very expensive," Smith says. In cities the infrastructure will likely be built sooner because there's "an incredible need for traffic and pollution management," or at least more than in suburban and rural areas. Still, "people are complaining about the infrastructure cost of putting in [just] a charging station," he says. Larger infrastructure improvements will be an even tougher sell.

Other challenges
Besides cost, the big disadvantage of both Ma's and WiTricity's technology is distance. The car's receiving coil still needs to be fairly close to the charger's emitting coil. WiTricity's Schatz says an optimum distance is roughly 12 to 18 centimeters. Ma's system has similar limitations: At about 50 centimeters away from the charging station, power transfer dropped to almost nil. Supporters argue that a trade-off in efficiency is worth it for the advantage of not having to use a physical plug. "It's a small loss compared to when you plug in a vehicle, but a huge gain in ergonomics, or ease of use," Schatz adds.

Despite the challenges that wireless charging poses for both carmakers and drivers, the concept is gaining momentum. Audi introduced a wireless-charge concept car that is half race car and half coupe, dubbed simply the Audi Urban Concept, at the Frankfurt Motor Show in September. Toyota recently announced an agreement to collaborate with WiTricity in developing its resonance wireless charging technology. Likewise, BMW and Siemens are partnering to develop inductive charging stations for eventual use by taxis. If such development continues at this pace, a future with cordless electric cars may not be too far down the road.