One of the most frequently asked questions I get about hybrid cars is, "Do they have to be plugged in?" No, I say, they don't. The batteries are charged by the gas engine, not by being plugged into the wall. It's my impression a lot of people have avoided hybrids because of this misconception.

But here's a twist that may confuse even more people: The plug-in hybrids are coming. They are hybrids with bigger batteries, allowing them to be used as zero-emission electric cars for local runs and commuting. The gas engine is in reserve for longer trips.

Today's hybrids hint at such capacity. A little-known fact about the Prius is, in its European and Japanese editions, it comes with a switch that allows the vehicle to be driven in battery-only mode, albeit only for a mile or so. In my experience with the Ford Escape Hybrid, the gridlock I experienced on one 15-mile drive home meant I never topped 20 miles per hour -so I traveled in electric mode most of the way.

The plug-in hybrid's battery pack can do more than just serve as an adjunct to the gas engine. Felix Kramer, an amiable Californian I first encountered at a recent Bioneers conference, is a very effective booster for plug-in hybrid electric vehicles (PHEVs).

In fact, Kramer says a PHEV with a larger battery pack than those usually seen in hybrids can deliver more than 100 miles per gallon (plus electric costs of 1 to 2 cents per mile). The benefits of a PHEV, Kramer's California Cars Initiative says, are many. A PHEV is a recharging-optional vehicle that, if used mostly in electric mode, can visit gas stations only about once a month. It also should be cheaper to maintain than a zero-emission battery car - with a much greater range - making it practical.

The car companies aren't building PHEVs yet, though DaimlerChrysler is testing PHEV 15-passenger vans. Kramer and his cohort, Ron Gremban, turned the latter's 2004 Prius into a PHEV by stationing 18 lead-acid batteries, from an electric bicycle, in the empty well beneath the hatchback deck. The 300-pound pack, which is good for 10 miles of travel, can be recharged in three hours via a conventional 110-volt outlet. The "Prius+" stays in electric-only mode at a cost of 1.25 cents per mile until it reaches 34 mph and the gas engine kicks in.

With lighter, more efficient lithium-ion batteries, the performance should improve significantly. EDrive Systems is testing a prototype Prius+ using Valence Technology lithium-ion batteries that can do nearly 35 miles as an electric and deliver 120 to 180 mpg. The EDrive kits for the Prius reportedly will be available in 2006. The drawbacks are the system voids the warranty and costs $10,000 to $12,000 over and above the cost of the Prius.

High-mileage vehicles are climate-change fighters. "Our goal," say Kramer and Gremban, "is to persuade Toyota and other automakers to build PHEVs for a market we expect to expand as the Kyoto Protocols and parallel state and international greenhouse gas initiatives are phased in." Kramer envisions millions of plug-in hybrids getting their electricity from the off-peak grid and eventually from photovoltaic and wind power.

The visionary Paul MacCready, who designed both human- and solar-powered aircraft, as well as the prototype that became the General Motors electric car, is sanguine about plug-in hybrids "with enough electricity built in to provide all your transportation for maybe a 60- to 100-mile range. The average driver of such a car would operate exclusively on the battery for 80 to 90 percent of the time with the few trips farther out requiring use of the gasoline motor to go any distance they want." Makes sense to me.