I've largely blocked the memory of my E300 turbodiesel out of my mind, but I seem to recall seeing the battery charge warning as the battery was beginning to die. If you still have the original factory battery it's just about time for it to go out. I find those things remarkably consistent - I replaced the one in my (now gone) '98 model almost exactly a year ago.
At the very least, check the water level in the battery. No, it's not sealed, and the dealer probably doesn't top it up when you have the car serviced.
ps: the battery is under the back seat.