Plug-In Prius Conversion

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

_{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.

We could usually drive an average of 35 miles before the new battery was depleted; unlike the original battery, it can't be charged by the gas engine. After that, the plug-in Prius returns to its normal operation. But the extra weight of the new battery drops its gas mileage from 42 to 40 mpg overall and from 34 to 29 mpg in the city.

The 67 overall mpg we got during the first 35 miles is a 60 percent improvement over the original Prius Touring. But that's based only on measuring gasoline consumption. A full recharge took about 6 hours and consumed about 5 kWh of electricity. That's about 55 cents at the national average of about 11 cents per kWh. With help from the Argonne National Laboratory, we calculated that by adding this energy into the equation, overall fuel-economy is equivalent to 53 mpg.

If gas prices were to hit $4 per gallon again, the cost to operate the standard Prius Touring version would be about 10 cents per mile; the converted version, about 8 cents per mile.

Your results will vary
Because the converted Prius gets its best mileage while using the lithium-ion battery, drivers who can recharge frequently — at work, for example — will be able to extend their travel distance beyond the 35-mile sweet spot while getting optimum fuel economy. But finding outlets while on the go could be a challenge.

People who use fuel-efficient driving techniques will probably get better gas mileage than the average in our tests.

Toyota's forthcoming plug-in hybrid, initially available to fleets only, will operate similarly to our converted Prius. But the designs of other plug-ins will vary. The Chevrolet Volt is expected to run solely on electric power while the batteries are charged, which could mean that drivers who make shorter trips might not use any gasoline. The Volt's gas engine will kick in only to recharge the battery pack when needed; it doesn't power the car directly.

Cars that run longer on electric power can dramatically cut tailpipe pollutants. But producing power still generates emissions somewhere. In New England, where our test track is located, about 42 percent of power comes from natural gas, which has cleaner emissions than those from gasoline-powered car engines.

Other areas depend heavily on coal to produce electricity. Still, according to a 2007 study by the Natural Resources Defense Council and the Electric Power Research Institute, deploying plug-in hybrids on a national scale would reduce CO2 emissions by about 45 percent compared with conventional vehicles and would result in a net reduction in air pollution, especially in urban areas.

Bottom line
Our Prius' conversion to plug-in power cost more than you could ever expect to recoup in gas savings. And despite claims of 100 mpg, our best real-time reading was 87 mpg on a stretch that involved rural back roads and cruising on a highway with a slight downgrade. But our plug-in Prius showed us that this type of technology can produce significant fuel-economy gains. And as a sign of things to come, we found it encouraging.