Tommy's right. You're pretty low. 12.40-12.60 is normal on a charged battery no load.
The Alt should be approx 13-14.7 running and loaded (lights and fans)above 13.
You could have a blown rectifier or bad voltage regulator. (Some easy tests linked below.)
Least likely: The battery could be getting old and not up to cap, Best have it load tested (free at Autoparts Stores) since a meter really won't tell you the internal health of connections.
Alternator & Charging System Checks (Alternator Testing)
[ame]https://www.youtube.com/watch?v=11jbHS4XXXY[/ame]
Doubtful this is the issue (full diode pack blown) since you'd have the Red Alt charging light on in the Dash.
[ame]https://www.youtube.com/watch?v=xgikeXt91vM[/ame]