A feature on the upcoming 2014 Mazda3.
"A variable-voltage alternator freewheels while cruising or accelerating, and when you lift or brake, it generates way more charging energy than the battery could accept. A 25-kJ capacitor absorbs the rest and then apportions it to the battery in doses it can accept. Mazda says it might boost real-world fuel economy by around 5 percent when you're running a lot of electrical loads.