2000 Mercury Mountaineer battery

A parasitic drain, check this way: Get an analog voltmeterremove the negative battery cable, attach the red lead to the battery post, black lead to cable end, then pull fuses one at a time, make sure there are no accessories or lights in the on position, Key off! Close the door after each fuse and check meter, when you pull the fuse for the bad curcuit, the meter will drop to 1 volt or so. Check all fuses, even under the hood. If meter does not drop, remove the battery connection from the alternator, if it drops now, the diodes are bad in the alternator.