[Dead Parrot]

A one wire alternator might do what you want. Unless the thing is going to be parked for long periods of time, might be easier to find a spot for a deep cycle lawn tractor type battery. That way you could install a small capacity normal alternator. Some of the early 10SI versions came in 30~40 amp outputs. And you would have some power for lights if your truck ever needed repair in the dark away from home.