Assuming you have a volt meter.
1. check battery voltage (after charging) static bike off should be about 12.5 volts. Then wait a couple of hours ( go have coffee or a beer) check again should still be 12.5 volts it it has dropped then bike is drawing power when off or battery is bad. Disconnect battery and repeat above test this will test just the battery.
2. With bike running check voltage at battery should read ~13 volts more as RPM increase if not check bike charging system.
This is a way of isolating the probem to either battery or bike.
I have had a PC680 in my 74 R75/6 for 4 years no problem great battery. Uses half the space as original and I can remove it without dropping subframe.
Only problem is the price of a new one I see they are about $150 now.