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It's not just using a handheld phone while driving that's a menace to society. It turns out that charging it in the car has consequences too.

That's because a phone drawing electricity from a USB port cuts 0.03 miles from each gallon of gasoline in a tank. Across the fleet of vehicles in the United States, that would mean about 970,000 tons of extra planet-warming carbon dioxide a year, according to calculations by Jon Bereisa, a retired General Motors engineering executive who studies vehicle power usage.

With a race under way to see how many charging ports automakers can cram into a car, the increased pollution is only going to get worse.

"Do I think we're at peak USB? No," said Mary Gustanski, vice president of engineering and program management at Delphi Automotive, which makes wiring and USB ports for vehicles. "We'll get more and more creative to not only allow you to connect with USB but also to connect wireless. Consumers want their car to be just like their home."

It's not just an environmental issue, either. The proliferation of consumer devices, the growth of dashboard touch screens and other technology, and the shrinking size of engines to meet fuel-economy mandates mean the 12-volt automobile electrical system is just about tapped out.

Some automakers are already turning to supplemental 48-volt systems in future models.

The number of vehicles sold in the U.S. with USB charge ports rose to about 14.6 million last year from about 3.3 million in 2005, the first year they were available, and is projected to climb to 16.7 million by 2022, according to a forecast from the consulting firm IHS.

Global sales of vehicles with USB ports will increase to 85 million in 2022 from about 49 million last year, IHS said.

That estimate doesn't capture how many ports are in a particular vehicle.

For example, the new Chrysler Pacifica minivan, which goes on sale later this year, will have nine USB charging points, the most of any automobile, said Bruce Velisek, director of Chrysler brand product marketing. The model it replaces has four charging points, he said.

To make his calculation, Bereisa assumed that a typical smartphone connected to WiFi or the Internet needs about 4.8 watts of energy to charge in a car.

(Delphi estimates that some less-efficient models draw twice that amount.) For a vehicle getting about 30 miles per gallon, that's a 0.03 mpg loss, he said.

Spread out across about 3 trillion road miles motorists drive each year in the U.S. — assuming an average speed of 30 mph — the estimated extra usage is 100 million gallons of gasoline, or about $200 million in costs, said Bereisa, the chief executive of Auto Electrification LLC in Sunrise Beach, Mo.

The estimated extra CO2 created by plugging in one phone in every car in the U.S. would be about the same as that produced by 185,257 passenger vehicles in one year, according to an Environmental Protection Agency website that converts greenhouse gas into real-world equivalents.

Put another way, that's the pollution created by burning 945 million pounds of coal.

By far, the cheapest way to charge a smartphone is at home, Bereisa said. With gasoline at $2 a gallon, it costs about 2 cents an hour to charge a phone in a car compared with about 0.06 cent at home, or 33 times less. Gasoline would have to fall to 6 cents a gallon to compete with home electricity, he said. It would also produce about half the carbon dioxide.

"That's why modern electricity power plants are not driven by gasoline engine generators," said Bereisa, who worked on the EV-1 and Volt electric-vehicle programs and fuel-cell models during his 35 years at GM.

"We go through life without realizing how important energy is to everything we do, and the consequences of our energy consumption. We grow up entitled to just plug it in or flip the switch or push start — with no idea of what's behind it all."