I'd suspect a bad battery. Since the voltage is staying up above that of a fully charged battery at idle, the alternator is at least trying. Since your battery voltage drops a full volt just sitting there for 15 minutes, either something is drawing a MASSIVE amount of power or the battery is toast.

A bad battery can draw down a good alternator and make you think that alternator is the issue. Leave it there long enough and the alternator WILL become the issue.

Given your charging voltage, the battery should at least be attempting to charge. Since it won't even turn the car over it's junk.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]