BMW 540e plug-in hybrid rumored to launch in 2017 - BMWBLOG (blog)
The battery is located under the rear seats and while driving, the electric motor can be recharged by the alternator function, transforming the kinetic energy into electrical energ during moments of inertia or during braking. The system will also choose the best time to use the electric motor, or perform the battery charging or move in hybrid mode. Source: www.bmwblog.com
