1990 kawasaki vulcan 750 wont charge battery while riding.?

I have a 750 kawasaki vulcan. Its a 1990.
I had a guy rebuild it...back yard mechanic! Got the bike back. And everytime i take it for a ride, the brand new battery dies. I have bought another new battery, thinking he put a used battery in that wasnt good. And im getting the same problem with the charging system. The battery is not charging.
Can someone tell me what i need to fix or how to check and find the problem. I will not let this guy ever work on my bike again.