PHOTO: Max Whittaker/WPN

Soaring gasoline prices and concerns about global warming have made plug-in hybrid vehicles a cause célèbre. Indeed, plug-ins can go farther on batteries than conventional hybrids because their batteries are bigger—they have to be, to hold power drawn from ordinary wall outlets. Still, the design does have its drawbacks.

As IEEE Spectrum reported last year [“Take This Car and Plug It,”], a group of hybrid enthusiasts at an organization called the California Cars Initiative, or CalCars, based in Palo Alto, have converted their Toyota Priuses to plug-ins that can go 32 kilometers with only electrons as fuel.

Now CalCars is selling a do-it-yourself conversion package for just under US $5000 that will allow anyone to turn a Prius into a plug-in. CalCars says such a converted car would offer the fuel efficiency equivalent of 2.35 liters per 100 kilometers (100 miles per gallon), with zero emissions when driven fewer than 32 km between charges. Most daily commutes, they note, are much shorter.

But is it worth it? CalCars lead technical engineer, Ron Gremban, notes that “the result is likely to void part of the owner’s new-car warranty” and that drivers installing the kit shouldn’t expect a “positive financial payback.” In other words, the conversion is not likely to pay for itself in savings at the fuel pump.

What’s more, cars operating in this mode are necessarily pokey, because they are electrically limited to about 55 km/h. That’s lower than the minimum speed limit on some highways. Then, when the batteries are depleted, the cars’ engines automatically turn on and operate as in any other Prius—but one that is dragging around 135 kilograms of batteries. The extra load just murders fuel economy.

Finally, the kit is based on lead-acid batteries, which operate poorly in the cold, last for barely 400 deep discharges (two years, if you’re very lucky), and must be recharged within a day if they are to keep what little life they have left.

Get the latest on the Prius conversion kit at http://www.eaa-phev.org/wiki/PriusPlus.