the battery will get drained by a defective alternator diode. when these fail they have low resistance and then suck down the battery.
with the engine off and the battery ground removed then remove the red output alternator wire. then hook up the battery ground see if you get current flow from alternator and the red output wire . if you do this is a defective alternator diode.
load test the fully charged battery. make sure all cables are clean and are not corroded. this age the wires get high resistance if not keep from corrosive chemicals. make sure all grounds are clean and secure.