Certified airplanes with alternators are supposed to be able to
handle ALL constant loads at the same time. Adding up all the radio and
light and pitot heat and whatever else current daws should come to
something less than alternator capacity. A drop from 14.5 to 13 volts
is a BIG drop and shouldn't be there, especially if the RPM is above
1000 or so. Sounds like maybe the alternator is getting weak or has a
bad diode or worn brushes, maybe.

Dan