Is your car still under the original 4 year/50,000 mile warranty? If so, call the Mercedes toll free road side assitance number. The battery should be replaced for free since that is covered under the factory warranty with no pro-ration.
Last November my 1998 E320 wouldn't start after I made a quick trip into a grocery store. It had almost a year left on the warranty and the road side assitance MB tech brought a new battery along. It was replaced on the spot with no charge. I was pleasantly surprised.