There are a multitude of references to this on the net.
It seems Mr Cessna doesnt know as much about alternators as Mr BMW, but there again Cessna dont build engines!
BMW Alternator
Your BMW needs a source of energy for it to run and that energy is being supplied by the fuel stored on the vehicle’s fuel tank. But aside from fuel, the vehicle also needs an electrical energy source for the essential electric components like the distributor, electronic fuel injector and the engine management computer to work. The battery is often the primary source of this energy. The battery, however, would only function well if it is properly charged. To keep the battery charged, your BMW is equipped with another electrical energy source—the BMW alternator.
An alternator is an electrical machine or generator that converts mechanical energy to electrical energy in the form of alternating current. Alternators have a wide area of applications, although they are more popularly used for vehicle electrical systems. The main function of the alternator in a vehicle is to charge the battery while the engine is running. As a secondary function, the alternator also serves as the primary power source for the various electrical and electronic components installed in the vehicle while it is running.