Hey Grasshopper, remove the battery and take it to the nearest Parts Store and have them throw a load test on it.
You probably have an internal shorted out cell.
Then look at their batteries and ask if it is fully charged, if not say please fully charge it and I'll take it home with me.
Put it in the bike, fire it up and go for a ride.
You say you worked on bikes for 40 years, right? You never run across a internally shorted battery in all those years. You must be living right.
