The streets outside baseball’s Comerica Park in Detroit were never so quiet — even on a cold Sunday in January — from the inside of Toyota’s early prototype plug-in hybrid (pictured left).

The extra battery in a 2007 Prius means the gasoline motor didn’t kick in until 62 mph. The single-battery Prius  on the road today switches over at roughly half the speed.

Toyota, the global leader in hybrid production, is working to advance the technology with University of California’s battery and transportation programs.

Never having to plug in the battery has been an important selling point for the hot-selling Prius, so Toyota is proceeding carefully with potential changes. But the goal now is to see if even more and sustainable efficiency gains are possible for the mass market — even if a household wall outlet becomes part of the equation.

“Our goal with these early prototype vehicles is to begin the process of understanding customer wants and needs in order to help determine the optimal balance between electric vehicle miles, charge time, battery size and cost,” said Bob Carter, a Toyota vice president.

The second nickel-metal hydride battery pack of the prototype enables the Prius to store greater levels of electricity. With more power in reserve, the vehicle is capable of operating in electric-only mode for longer periods and at much higher speeds, saving more gasoline and further reducing emissions. The second battery only runs for 7 miles between charges now — somewhat less if the radio and heater are on.

But Carter says Toyota sees promise in the plug-in hybrid. U.S. rival General Motors also is working on plug-in hybrid technology, hoping to offer its Chevrolet Volt for sale in 2010.

(Photo: Reuters)