http://green.blogs.nytimes.com/2011/04/27/a-second-life-for-the-electric-car-battery/
The researchers will also try to determine the optimal point for replacing the battery in the car, Mr. Pesaran said. “So while there is so much focus on these batteries providing eight to 10 years of life in the car, we may discover it makes more sense to replace them every five years to provide the maximum value back to the car buyer,” he said.