While in flight, try adding a substantial load to the system (such as
say the pitot heater or a landing lite) and see of the ammeter shows
the additional alternator load. If it discharges, your alternator has
definitely quit on you.

BTW a bad battery can give all sorts of weird electrical charging
system symptoms. Does your battery have a full volume of electrolyte?
It is vitally important to keep the tops of the plates covered with
electrolyte at all times. Maybe your battery has become nearly dry on
one cell?