Cooper doesn’t want to start anymore

Hello, with my problem had recently fetched a new battery because the old Platt was. So the little one also ran well now he has stood now about 3 weeks and did not jump any more only cracked but you have seen the still juice on it (armatures have lit) So now I have the battery charged (according to multimeter is this full) have it built again, you hear that what does, but no life sign when you turn the key around (no lamps at nothing?) When I briefly measure the battery is to be recognized that the voltage collapses as long as it is installed in the vehicle. I build this battery again from full 12volt. I ask for help, am right now really councilless greetings