The A123 conversion makes barely any changes to the car. Electrically speaking, it sits between the original battery of the Prius and the car’s computer, serving as a buffer for the factory-installed battery. Mostly what it does is tell the Prius that there is still lots of charge remaining, and thus no need to start the car’s engine to recharge the battery.

Driving around Washington last week, the Prius engine started up as normal whenever the combination of the accelerator pedal position and the grade of the road demanded more torque than the electric motor could deliver. Stomp on the gas, so to speak, and the Prius drew energy from both the internal combustion engine and the combined battery pack.

Maximizing the value of the extra watt-hours requires the same expert touch that driving a stock Prius does. So I recruited a self-described Prius geek, Charlie Richman, who lives in Bethesda, Md., and drives to his job in the District of Columbia municipal planning department in a Prius that is still equipped as Toyota built it.

“Very cool,” said Mr. Richman, test-driving the battery-equipped 2007 Prius.

Mr. Richman said that the car “handles just like a Prius.” But there is a difference. In his own car, when he accelerates gently and drives for extended periods at just below the level that causes the gasoline engine to kick in (though eventually it will to re-charge the battery). With the A123 pack installed, the gas engine never had to do that, at least not in the 10 miles or so that he cruised along North Capitol Street and then New Hampshire Avenue N.W., four or six-lane city streets with a few straightaways and frequent traffic lights.

The Prius comes with an instantaneous fuel economy gauge that runs up to 99.9 miles per gallon, but A123 installs another with an extra digit. After I drove the Prius for a distance the gauge said my mileage was in the 80’s, but Mr. Richman quickly moved the average back up over 100. (The test car was covered with decals proclaiming 120 miles per gallon.)

A123 uses a battery technology it calls lithium ion nanophosphate, developed at M.I.T. It stores 5,000 watt-hours in a 140-pound module. In comparison, Toyota’s nickel metal-hydride battery weighs about 100 pounds and holds 1,300 watt-hours.