Last week a Boulder company unveiled a modified Toyota Prius with extra batteries and a plug-in option so that the car can run on batteries alone, and then switch to the gasoline engine if and when the batteries are depleted. Other companies are exploring similar options. This report in the New York Times points out that there are significant hurdles to making the plug-in hybrid an economically viable alternative for most motorists. The problem is that when a battery is fully charged and then fully discharged its usable lifetime is shortened. Hymotion, a company that plans consumer conversion plug-in kits for the Prius and other hybrids in 2007, offers a warranty of 800 charge-discharge cycles on its battery. With daily use, that’s a little over two years. They anticipate about 30 miles on a charge, which probably means under ideal driving conditions. The conversion kit is $9500 installed, so that works out to about 40 cents/mile. The additional cost of actually charging the battery is negligible by comparison (a couple of pennies per mile). Your run-of-the-mill unconverted Prius, at 44 mpg and $3 for a gallon of gas, costs only 7 cents/mile to drive in fuel. If you pay $25,000 for the car itself and get 150,000 miles out of it, that’s an additional 17 cents/mile for the initial investment. So that plug-in conversion kit dominates the cost of owning and operating the vehicle. If gas cost $10 a gallon and the plug-in battery kit was good good for 4 years instead of two, then it would begin to make economical sense.