Be aware that sudden battery failure is the normal routine in heavy service applications like your Mercedes Diesel .

Autozone sells : A. crappy batteries and B. usually the wrong one , Wall Mart OTOH has good ones cheaper .

BCI Group 49 IIRC . the wrong ones look similar but only have 850 Cranking Amperes , barely sufficient .

One of my Diesel 123's battery lasted 10 years ~ the other died suddenly on vacation in the middle of nowhere last August ~ don't be ' furious ' just start the car and go shopping for a new one , leaving SWMBO or a friend sitting in the idling car whilst you're in the store ascertaining if they actually have the correct one before you shut it off again .

When the engine is off , reach in and try to turn the alternator by it's fan ~ if it takes a serious shove and the belt squeaks , it's O.K. ~ if you can turn the alternator by it's fan easily or stiffly but silently , the belt is too loose and may be worn out .

Remember : when you start the car cold , DO NOT shut it back off until it's run a while ! .

You cannot charge one car by jumping it with another , that only serves to get you started .

A battery that drops from 12 VDC to 7.5 is either dead or bad .

__________________
-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better