call mercedes roadside assistance, 1-800-for-mercedes,

they come out and change your battery - jump start or chk it - just tell then your problem when you call- that the electrical system in your car is acting weird and you need a new battery - and they just charge you for the battery.

If you were to buy anything other then a Mercedes battery you wouldn't get the free jump start or roadside assistance diagnostic.

Figure around $125 for a new Mercedes Battery (think they are from a Spanish company Varta) WORTH IT!

A five year old battery in a C-class realy should be changed out for a new one, that would be the first thing I do.