My 123 cars charge in the 13's like you mention. My BMW has a 14.1V regulator (hella or bosch, cant recall), and is 14.1 at some temperature around freezing, goes lower at higher temps, higher at lower ones. Still at steady state under load it is in the 13's too.

My saab is consistently at 14.4-14.6, regardless of load. On long drives with no real auxiliaries and a charged battery, it may go down to 14.1.

Id sure like a higher voltage...

__________________
Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (116k)
1991 350SD (206k)
1991 300D (228k)
2008 ML320 CDI (199k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (267k)

Past Diesels:
1983 300D (228K), 1985 300D (233K), 1993 300D 2.5T (338k), 1993 300SD (291k)