My '85 190E had an alternator problem. It seemed as if its output was getting weaker and weaker as time went by. I would have to guess that it started a few weeks ago, where my alternator would slowly die, draining my battery.( almost new)
I ran one particular test: I started the car up, ran it for a few minutes, and disconnected the battery. In theory, the alternator should be strong enough to power the whole car. When I disconnected the battery, it ran fine till I turned on the headlights. It felt as if someone had turned the ignition off.
I changed the alternator, and ran the same test. This time, with the battery disconnected, I turned on the head lights, all of them, hi-beam and fogs. Not a glitch!
If u plan to keep the car for a long time, changing the alternator would be best. It bets being stranded!