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In a Hurry? New Battery Recharging System Promises Quicker Turnaround for Plug-In Hybrid Vehicles

Oak Ridge National Lab researchers have come up with a fast-charging prototype drive system. But is the technology ahead of its time?
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© GENE CHUTKA, VIA ISTOCKPHOTO.COM

When plug-in hybrid electric vehicles (PHEVs) start hitting the road later this year, most drivers will plug into a normal 110-volt outlet when not driving in order to trickle charge life back into their car's battery. Whether this is a feasible model is an open question, with some consumers concerned that trickle charging may not be fast enough to sufficiently recharge batteries between trips. Without a charged battery, PHEVs rely on their internal combustion engines, something hybrid drivers aim to avoid.

To address this concern, a team of researchers at the U.S. Department of Energy's Oak Ridge National Laboratory (ORNL) in Tennessee has developed a prototype system they say could let drivers cut the time needed to fully recharge from a home electrical outlet by a factor of 10—from about eight hours to about 45 minutes. The good news is that drivers would be able to use outlets that deliver 240 or 220 volts to get this level of fast charge. This ability to accommodate a higher voltage and current means the battery can recharge at about 20 kilowatts, as opposed to the two kilowatts possible with a 110-volt outlet. The bad news is that drivers would have to upgrade their home electrical systems to accommodate 240- or 220-volt outlets, if they do not already have them to run clothes dryers or other appliances.

A plug-in hybrid electric uses a traction-drive power electronics system to propel the car forward by providing force to the car's wheels and has a charger for recharging the high-voltage battery when it is plugged into the power grid. The traction-drive system typically consists of a boost converter (which steps up voltage when the electronic circuit requires a higher operating voltage than the battery can provide alone), two inverters (which take direct current (DC) voltage and convert it to alternating current (AC)), and electric motors to provide motive power (pdf).

ORNL's prototype drive system has those same components but uses the inverters to charge the battery from an outlet, eliminating the need for a separate charger and enabling the car to recharge faster and more efficiently, says Gui-Jia Su, a senior research engineer at ORNL's Power Electronics and Electric Machinery Research Center. Although a standalone onboard battery charger in most PHEVs costs only about $300 for the slow charge rate (two kilowatts), ORNL's technology would also be able to replace fast-rate 20-kilowatt chargers, which can cost several thousand dollars, according to Su.

The new system is also designed to allow a PHEV to use its battery as an energy storage device, enabling the car to hold electrical energy in the battery while the grid has surplus power (during off-peak hours, for example) and contribute its surplus energy back to the grid when the latter needs more power to meet peak demand, Su says.

ORNL is proposing to use the inverter to charge the battery rather than having a separate onboard charger, says Andrew Frank, a University of California, Davis, professor of mechanical and aeronautical engineering. The idea is to use the inverter controllers to recharge the battery. Such a system could also be used to take energy from the car and use it to power a house or return it to the grid, he acknowledges.

But the market for ORNL's technology is a tough one to crack. AC Propulsion, a San Dimas, Calif., maker of electric vehicle drive systems, discovered this when working with Tesla Motors. The electric vehicle–maker initially licensed a drive system design and reductive charging patent from AC Propulsion but later developed its own versions of these technologies. The major automakers are likely to take the same route, developing their own technologies, Frank says.

Although ORNL's proposed system would allow PHEV-makers to leave out the onboard charger that they currently install in their cars, the first-generation PHEVs are not intended for high-speed charging, says Frank, who is also founder of Efficient Drivetrains, Inc., a U.C. Davis start-up that designs energy-management systems for electric vehicles and hybrids. "One of the issues with fast charging is that the more power you use to recharge the battery, the less efficient the charging system is, and a lot of power is wasted," he says. "For this reason, you really want to use a trickle-charge system for a PHEV, otherwise you're throwing away electricity, not to mention money."

ORNL's technology could have a much greater impact on purely electric vehicles such as the upcoming Nissan Leaf or Ford Focus RV, which do not have a combustion engine to rely on, meaning depleted batteries need to be charged quickly for the car to be practical.

The Energy Department likes the technology's potential and has invested more than $1.3 million in ORNL's project since 2008. Now the researchers are looking to license their drive system to a company that can commercialize it. Su says that several companies, including Raser Technologies (a maker of drive systems) and MBtech (which provides engineering and consulting services to the auto industry), have expressed interest in the system, although they have not discussed details as to how the technology might fit into these companies' existing offerings.

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