Starbucks, the world’s largest coffee-shop operator, is teaming up with Duracell Powermat to let customers recharge mobile devices wirelessly instead of hunting for available wall outlets in stores. Customers can place their compatible devices on so-called Powermat Spots on counters and tables to recharge them, Seattle-based Starbucks said in a statement. While shops in Boston and San Jose, California, already offer the service, a national rollout in company-operated Starbucks stores and Teavana outlets begins in San Francisco today. The company, which operates more than 20,500 stores worldwide, plans pilot programs in Europe and Asia within a year. Coffee-selling chains have long experimented with ways to encourage customers to stay longer, using music, comfortable seating and earth-toned walls.