What is it?
Same principle as for ordinary hybrids: There's an electric motor and a gas engine, except that the battery powering the motor would be recharged from an electrical outlet, at home or someplace else. The motor would power the car until battery power waned. Then the gas engine or another secondary power source would kick in.
What's good about it?
Powering the battery from the electrical grid could be much cheaper than gasoline. If owners charged up overnight, they'd be tapping into off-peak power that costs utilities less—so some savings could be passed on to consumers. Plug-ins could also get higher mileage than today's hybrids, with fewer emissions.
What's bad about it?
The large battery packs would take up extra space, add weight to the car, and degrade performance—and right now they're too expensive and unproven for mass production.
Where would it be most useful?
As with hybrids in general, plug-ins would be best for people who make a lot of short trips or commuters who drive frequently in traffic, since that's when the battery-powered electric motor would be doing most of the work.
How much will it cost?
Not sure, because the battery technology isn't mature. But with the right batteries, plug-ins could be cheap to fuel. At overnight, off-peak rates, the cost of electricity might be one-fourth what an equivalent amount of gasoline would cost.
When's it coming?
Unknown. There's not yet an affordable battery that can handle the deep charges and discharges that would occur under normal use in a plug-in. Prototype lithium ion batteries—like those in power tools and laptops—are promising, but it's not clear if they can be scaled up for something as big as a car.
What's taking so long?
The battery technology. The nickel-metal hydride batteries used in today's hybrids don't hold enough energy to be viable for plug-ins and can't handle the deep charges and discharges needed. Lithium-ion batteries might work, but nobody has mass-produced one that's safe, reliable, and potent enough for a car.
Who's doing it?
Toyota, Ford, and other automakers are testing prototypes. Google, some municipalities, and a few utilities are also working to convert conventional hybrids into plug-ins, along with enthusiasts retrofitting their hybrids with lithium-ion batteries. (But don't try this at home.)
Could it be a silver bullet?
Should the battery technology mature and plug-ins become viable, the math would be very compelling. Some experts think plug-ins could get the equivalent of 100 miles per gallon. Powering them from the grid would bring further savings, cutting the cost of fueling by 75 percent or more.