How many miles on the alternator? Yes, I think you are having a little charging deficiency.
A regulator, or brushed, might do the trick and get you charging again, or it could be the belt, or it could be the alternator itself. At 179K miles it was a bad shaft bearing for me. I had the replace the alternator. I purchased mine from FastLane for about $151. A quick and easy installation.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.