In Episode 5, the EV Answer Man addresses Aux. Batteries and D/C to D/C Converters.
Should you use an Aux. Battery?
How about only using a D/C to D/C Converter?
What is not addressed in the the video is running an alternator off of the motor aux. shaft.
Out of all of the options, it is the least desirable, for several reasons. 1) It requires a mounting setup that adds weight and a drive belt. 2) Adds a load on the motor. 3) Stops producing when you come to a stop, shifting all of the load to the aux. battery. 4) Takes up space in the engine bay.
5) Is not very attractive to look at. 6) Does not fit with EV4U's rule of Clean, Simple, and Reliable. Imagine what the Karmann Ghia engine compartment would look like with an alternator.