I assume that the battery light is NOT illuminated. That being the case, I would think that you have a parasitic draw on the battery. Some electrical device is on when the car is off either by short circuit or mistake or the diodes in the alternator are burnt allowing the battery to drain back. The alternator is charging when the car is running because the computers have enough juice to run the car. The battery may need to be changed because of all the charge/discharge cycles the draw has caused. Good luck finding the draw.