SARTELL, Minn. (AP) — A plaque hanging in Lee Hart’s cluttered home office is inscribed with a Chinese proverb: ‘‘Those who say it cannot be done should not interrupt those doing it.’’ In Hart’s garage, stacks of batteries capable of powering a car line the walls. A dusty gold-colored Renault is parked with the hood open, revealing battery packs. In one corner sits a cardboard car designed and built by middle-school students to run on batteries. So when Republican presidential candidate John McCain said last month that the government should pay a $300 million reward to the inventor of a battery strong enough to run an automobile, Hart wasn’t impressed. Batteries are not the problem, Hart said, and he should know. The Sartell engineer has been helping design electric cars for decades. One of his projects, the two-seat Tango, has been sold to a handful of customers, including actor George Clooney.