In an unexpected move, David Howell, for two decades one of the most consequential behind-the-scenes players in U.S. battery development, will retire next week from the Department of Energy, where he helps to lead the multibillion-dollar U.S. challenge to China’s dominance of the battery industry. One of Howell’s most lasting legacies is conceiving, almost on a lark, the industry’s widely used measure of how much a battery needed to cost to make electric vehicles competitive with combustion vehicles—$100 per kilowatt-hour.