Have the battery charged and tested (high rate discharge/load test and hydrometer test) see if the battery is in fact good. If the battery is good the other two possibilities are the alternator is not charging the battery. At rest engine off battery voltage should be 12.6 volts with the engine running the voltage across the battery should be approximately 14.6 volts. If these are correct you need an inductive amp meter to see what parasitic draw is on the battery over night that is "drawing down" the battery. A parasitic draw could be a trunk light glove box light or some other electrical load staying on when the ignition key is turned off.