If the battery is weak and putting extra load on the alternator, I could see how it could make the idle slightly rougher, kind of the same way that the A/C compressor make the idle slightly choppy on a 4 banger.

I tend to average 5 years out of my batteries here. I consider myself lucky, most people locally only seem to get 3-4 years with the same brand and class batteries I use. The SDL has a Duralast Gold in it, so far 2 years old and still starts on the 1st or 2nd compression hit.

__________________
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]