classic motorcycle forum

Hello. I'm at a loss with my charging system. I have an xj550rh Seca. When I hook my multimeter to the battery terminals and rev it up to 3000rpms the volt meter reads 14.5 volts. Great, right? Okay, the problem is that after 20 minutes of riding it no longer has that same reading. So, after a ride of about 20 minutes I'll go home and hook up the multimeter to the battery terminals again and rev the engine to 3000rpms and it only reads 12.5 to maybe 13 volts. Is this suppose to be happening?