In the conversion shop

For our plug-in Prius, we chose a Hymotion L5 conversion kit sold by A123 Systems, which the company claims can yield more than 100 mpg. The work was performed at a Toyota dealership in Massachusetts, one of seven U.S. venues that do that conversion.

Though the standard Prius hybrid system, including its nickel-metal-hydride battery, remains intact, the conversion adds a large, 5-kilowatt-hour lithium-ion battery pack and a 110-volt charging port to the rear bumper. The dashboard energy monitor is also modified to show the new battery's state of charge.

On the downside, the extra battery adds 187 pounds to the car's weight, which reduces its maximum load capacity to 623 pounds. That means that four or five adult passengers could overload it. And because the new battery is placed in the plug-in Prius' spare-tire well, the spare tire is placed in a tray and strapped on top of the cargo-area floor, which reduces cargo space.

With any extensive conversion, warranty issues become a concern. A123 Systems provides a three-year warranty on the conversion components. John Hanson, a Toyota spokesman, says the company will consider warranty issues case by case. A conversion doesn't automatically void the factory warranty, he says. But a problem caused by a conversion won't be covered by Toyota. A123 says that if Toyota doesn't honor such a repair, A123 will cover it.