This usually is caused by a shorted diode in the alternator; which drains the battery when the motor is off. Unfortunately, many mechanics and parts store clerks do not know how to test for bad alternator diodes; so they will tell you the alternator is good when it is really defective.
Here's a test you can do yourself at home: The battery should be well charged before doing this; all lights and accessories should be off, and all doors closed. With the engine turned off; disconnect the battery ground cable, wait two minutes, and then briefly touch the ground cable clamp back to the negative battery post from which it was disconnected. Watch for a spark when the cable touches the battery post. If there is a spark when the ground cable clamp touches the negative battery post, do not conect the battery. Instead; disconnect the heavy power cable from the alternator, and tape up the metal terminal on the end of that cable so it cannot touch any metal objects. Then touch the battery ground cable clamp to the negative battery post again, and watch for a spark. If there continues to be a spark when the battery ground cable touches the negative terminal; there is a short somewhere else in the vehicle's electrical system; which should be repaired by a competent electrical technician.
If there is no spark this time; the alternator has an internal short, and must be replaced.
There is an epedemic of poorly repaired and improperly tested alternators at most parts stores. These alternators are worked on in Mexico by semi skilled people who do not have the necessary replacement parts or proper test equipment. As a result; they often are defective right out of the box. NAPA parts stores are the only place I have found which has properly rebuilt and thoroughly tested alternators and starters. Since I have switched to NAPA, I have not had a single defective alternator or starter.