battery dies every three days
2 Answers
bad batteries usually can't hold a charge but, now, if the alternator is not charging it then, yeah. Those are the 2 possible reasons for your situation. Take the battery to get tested at an auto parts store and if ti's not the battery then, it's the alternator. You can also drive the vehicle to a shop and they can test both of them, without removing anything.
You've also posted a CLK question under ML section. This is a Universal question though.