One of the auto industry's most closely guarded secrets, the enormous cost of batteries for electric cars, has spilled out. Speaking at a forum on green technology, Ford Motor Co. CEO Alan Mulally indicated battery packs for the company's Focus electric car costs between $12,000 and $15,000 apiece. "When you move into an all-electric vehicle, the battery size moves up to around 23 kilowatt hours, [and] it weighs around 600 to 700 pounds," Mulally said at Fortune magazine's Brainstorm Green conference in California. "They're around $12,000 to $15,000 [a battery]" for a type of car that normally sells for about...