bad ground could cause this but I doubt it is the alternator belt tight. If you can get the alternator tested in situ as 8V is low.
2005 320 CDI
2006.5 VW Jetta TDI
1991 560SEL (179000 Sold)
1972 280SEL 4.5 ('The Lead Sled' 320000 miles when sold.)
1972 220D (225000 when sold)