The first thing to do is check the battery voltage with the engine off and then with the engine on. If you do a search on this forum, you'll find the acceptable values, particularly for when the alternator is charging the battery.
It sounds to me like your alternator is not fully recharging your battery. If so, it might just need new brushes.
1988 560SL Black/Palomino
1988 300SEL Black Pearl/Burgandy
1984 500SEC Anthracite Grey/Palomino