My son bought a 1981 Suzuki GS450 last year. Garage kept, in good shape, etc. After putting a new battery in it, getting the carb tuned, and riding it for about 2,000 miles, It failed to start. We put it on a good brand trickle charger the first month he was off to college, and it started fine the first long weekend he was home. On the third day of riding, I noticed it was sounding like a low battery again when he'd try to start it. Okay, to shorten the story, now that he's almost done with the school year, we hooked it up to a new battery again. Same thing happened. After riding it for about 150 miles, it would just click. Oil level is fine, oil is new, plug is new, and Im befuddled.

 

Would think it either has a short, or the alternator is not working. You could try and put an ammeter at the battery and see if there is a current draw when the battery is off.