According to the YUASA technical manual, a fully charged lead acid battery should have at least 12.8V 1-2 hrs. after charging has finished.
I have not found specs for lithium batteries.
When charging at around 5000 rpm, the battery voltage should be ~ 14.4 V- 15.4V.
The rectifier/regulator is an item that wears and there is no other remedy than replacing it "regularly". It gets very hot (I measured > 100°C on the surface of my RVF400 regulator), this wears the power electronics in the reg).