New York Times: ELECTRIC-CAR doubters often say that a vehicle like the 2011 Chevrolet Volt, intended to drive most of its miles on batteries charged from the power grid, is effectively powered by coal. While that is a vast oversimplification, it contains a germ of truth: half of the electricity generated in the United States comes from coal-fired power plants. That is not to say the Volt and a slew of coming plug-in hybrids proposed by General Motors and other automakers will be belching smoke like …