2000 altima wont hold charge

car has 200k miles. mechanic has checked pretty much everything battery and alternator are ok. the alternator was loose and so was the belt replaced the negative battery terminal and fixed a led light behind the gauge cluster
Before all this happened the factory alarm would drain the battery so the mechanic removed it. Now we charge the battery and the car will start all day but as soon as it sits over night the car wont start. any kind of input would be appreciated. thanks