That Bible for smart shoppers, Consumer Reports—whose new-car reliability ratings are hugely influential—took a leap into the future in its February issue. It tested a Toyota Prius that had been converted to a plug-in hybrid, using the Hymotion L5 conversion kit sold by A123 Systems of Watertown, Massachusetts.
Did the no-nonsense, grimly methodical magazine give the plug-in a thumbs-up? As usual, the testers focused on the numbers. While Hymotion claims its kit can return up to 100 miles per gallon, CR’s Auto Test Center in East Haddam, Connecticut, logged just 67 mpg—against the 42 mpg they recorded in a stock Prius. “At almost $11,000,” the magazine noted, “the plug-in conversion clearly won’t save consumers money overall”—though they deigned to declare the plug-in technology “viable.”
The lengthy payback period is one of a handful of criticisms emerging against plug-ins, even as the 2010 production launches of the Chevrolet Volt and Toyota Prius Plug-In draw closer. While the lithium ion batteries used in that Hymotion kit hold the most energy for their size and weight, they’re also the most expensive.
Not all of the dozen or more plug-in converters use lithium cells. A conversion by 3Prong Power in Berkeley, California, for instance, uses old-fashioned lead-acid cells. They weigh more, and give an electric range of only 10 miles, but at a price of $6,700, that may be enough for local users who can recharge between short trips. As research now underway at Carnegie-Mellon University points out, correctly sizing a plug-in battery (which directly affects its cost) depends greatly on how the car will be used.
A second criticism is that in certain cases, converting a Prius to run on grid power may actually result in greater emissions of carbon dioxide (CO2) than running it exclusively on gasoline. The question was studied intensively for North America in a landmark study done by a partnership between the Electric Power Research Institute, a utility trade group, and the Natural Resources Defense Council. The study found that plug-ins emitted less CO2 on a “wells to wheels” basis than a conventional hybrid in most, but not all, cases.
Where the local power grid was fairly clean—in California, say, or hydro-heavy Washington—the plug-in clearly wins. But, in states with coal-fired grids—the two worst are North Dakota and Wyoming—the plug-in actually emits more CO2 per mile than a conventional hybrid.
A recent working paper from Duke University agrees that the local grid makeup determines whether hybrids or plug-ins have a lower carbon impact. But it also notes that plug-ins will likely cost far more than conventional hybrids for years to come—which will keep adoption rates low unless substantial tax incentives are maintained, or a carbon “price signal” is instituted. And it concludes that plug-ins become cost-effective only when gasoline hits $6 a gallon.
With credible and established conventional hybrid lines, Toyota and Honda have stressed these concerns in questioning the various benefits of plug-ins. General Motors, whose own hybrid efforts started later and sold far fewer units, has placed its green bets on the Volt, along with a Saturn Vue Plug-in Hybrid with a 10-mile electric range slated for 2011. As always, the market will be the ultimate arbiter.