If there could be a question that the battery hasn't been maintained properly it could fry in 1 season. How old is the battery? Has the electrolyte level been maintained properly? Has it been kept in a warm environment over the last winter? Has it had a proper trickle/float charger hooked up over winter and while not in use? Are the lead cells sulphated?
Gordon is on the money! Don't even attempt to diagnose a charging issue without a known good battery. New battery is about $35. You may have used all the remaining reserve in the battery when you started it. The summer heat is brutal on a marginal battery. Try a new, properly pre-charged according to instructions, battery and go from there. If it goes down too my next step would be to do what all 30+ year old SOHC4 bikes need to have done, and I know you don't really want to hear this or yet do it, but............... DISCONNECT EVERY electrical connection on the bike, clean them thoroughly, tighten them, and reassemble using dielectric grease to protect them from future corrosion. If each connection had a very small resistance caused by looseness &/or corrosion then there is no way the battery would stay up. If that doesn't do the trick then it's time to start a real diagnostic program. Most of the hardware on these bikes holds up very well and doesn't often fail.
One other thought. You haven't recently switched to high performance coils like green 3 ohm Dyna brand? Those bad boys are really great but they'll really suck the juice.
Good luck!
Electrical problems will probably be your biggest pain in the ass with these bikes if you hang onto it. Just do it right and be done with it.
