Plug-in hybrid (© Jacob Gordon)Click to enlarge picture

This Toyota Prius features a Hymotion L5 conversion kit sold by A123 Systems.

One of the advantages of today's gas/electric hybrids is that they don't need to be plugged in to recharge their batteries. So why is there so much buzz about automakers over the next few years introducing plug-in hybrids, such as the Chevrolet Volt?

By using larger or additional batteries, plug-in hybrids can run on electricity more of the time than conventional hybrids can, which means they use less gasoline. How much less? We had our 42-mpg Toyota Prius Touring model converted to a plug-in version and found that its gas mileage jumped to 67 mpg overall for the first 35 miles of driving, or until the added battery depleted its charge. That means that a driver with a relatively short commute could dramatically cut gas usage. And according to the Bureau of Transportation Statistics, 78 percent of Americans drive less than 40 miles per day to work and back.

In an era of volatile gas prices, that sounds appealing. But production plug-in cars won't be available to consumers before late 2010. And while some companies are converting current hybrids now, few drivers are likely to see the $10,875 conversion cost we paid as being worth it.

Prius Fuel Economy: Standard vs. Plug-in


Standard Prius


Plug-in version,

battery charged1

Plug-in version,

battery depleted

Overall42 mpg67mpg40mpg

1- First 35 miles only.

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.

On the Road
We measured the Prius' gas mileage on our standard fuel-economy test cycle, which involves a mix of highway, rural, and city driving. While the converted Prius operates the electric motor and gas engine as needed, as in a standard Prius, it spends more time running on electricity and relies less on gas. Still, when running solely on electricity, any moderate acceleration or mild grade makes the gas engine kick in.