GM and Nissan will have to overcome many consumer concerns as they bring mass-market plug-in hybrid and electric cars to market this winter. One aspect that might make potential buyers hesitate is a fear that they'll forget to plug in their cars when they get home, and thus won't have a charged-up car in the morning.

Companies working on plug-free "proximity" chargers, which rely on magnetic induction, think they can solve this. Such chargers require no plugging--someone who owned an electric car would simply have to pull the car into a spot above a charging pad. However, this convenience comes with a cost--10 percent or more of the power can be lost during charging.

Induction chargers are already used to power electric toothbrushes, medical implants, and some portable electronics. Inductive charging also plays a major role in electrical transformers used on the grid.

Whether drivers will accept convenience over inefficiency will be put to the test this winter, when the Virginia-based Evatran rolls out its Plugless Power chargers. At the Plug In conference this week in San Jose, CA, the company displayed some of the system's components, and executives talked with Technology Review about their business plan.

The Plugless Power system has three main parts: a tower that plugs into the wall and converts electricity into the right frequency for the charger; a shoe-box sized, 25- to 30-pound adapter that has to be mounted to the front of a car chassis; and a long, flat pad that sits on the ground beneath a car.

The floor pad and the vehicle adapter both contain metal coils; when a car pulls up to the floor pad, the coils inside the pad move until they're within two to three inches of the coils in the vehicle adapter. The charger requires this alignment process, which is guided by magnetic sensors, to account for people parking slightly to the right or left of a parking space, and because the system has to be mounted in different locations depending on the car model. When the coils are aligned, electricity flowing in the charging tower creates a strong magnetic field in the pad's coils, and this magnetic field induces an electrical current to flow in the coils mounted on the car.

On Tuesday, Evatran representatives announced that the company will begin selling the charging towers equipped with a plug in December for $3,245. This is timed to coincide with the release of both the Nissan Leaf and the Chevy Volt. Customers will be able to prepay $800 for a plug-free charging add-on that will be available in April 2011. This price does not include installation. The company is talking with vehicle makers about incorporating the adapters into vehicles before they are sold.

Evatran and other companies developing wireless charging systems say the plugless chargers work just as fast as the kind that require a cord. The difference is that plugless versions need more electricity to provide the same level of charging, and some energy is lost during transmission. The Plugfree Power products will transfer about 90 percent of the electricity from the wall to the battery, whereas a cable transfers effectively all of it. According to Evatran, this translates to an additional $.005 per mile compared to plugged versions. Or, to put it another way, $5 for every 1,000 miles.

Some experts worry that this inefficiency may deter some people from switching to electric vehicles in the first place. It may not be a big deal to individual users, but if proximity chargers that lose 10 percent of the electricity that's pumped into them become commonplace, these losses will add up. "We're trying to be more energy-efficient, but this is going in the wrong direction for our convenience," says Michael Kintner-Meyer, senior research scientist at the Pacific Northwest National Laboratory in Richland, WA. "The future will show whether convenience is enough of a market driver to make up for inefficiency," he says.

WiTricity, a Watertown, MA-based company spun out of MIT in 2007, is working on a vehicle-charging system that the company says works at greater distances and with greater efficiency than Evatran's technology. WiTricity's technology has been engineered so that the magnetic resonance of the two coils is well-matched. This design enables a more efficient energy transfer over longer distances--an effect theorized by MIT physics professor and company founder Marin Soljačić. The company's vehicle-charging prototypes can transmit 95 percent of the supplied electricity at a distance of 20 centimeters and 97 percent at 10 to 12 centimeters.

WiTricity has no plans to sell a direct-to-consumer charger, but is in talks with automakers to incorporate its chargers into vehicles. David Schatz, director of business development and marketing for the company, says WiTricity will announce these partnerships in the coming months.

Plug-free charging could be especially well-suited to plug-in hybrid fleets, says Lee Slezak, manager of advanced vehicle systems simulation and evaluation in the office of vehicle technologies at the U.S. Department of Energy. DOE studies suggest that fleet drivers aren't as motivated by fuel costs as their managers are, and tend to forget to plug in. If the cars charged up automatically, that would solve this problem. And if efficiencies can be improved, plug-free charging "has the possibility to make the technology more acceptable to customers because some people might not like the cord or are worried they'll forget to plug in the car overnight." Slezak says.

Researchers from Utah State, Oak Ridge National Laboratory, and the government of New Zealand are collaborating to develop inductive chargers that can be embedded in highways to charge while driving.

Copyright Technology Review 2010.