I frequently end up with a flat battery in my Forester05, despite the garage test rig reporting that the alternator and battery are in perfect condition.
What am I doing wrong ? I am driving without the heater fan or sidelights on. If you run the battery low by parking with the lights on (say), there is no way the battery will ever recover unless you drive with the fan on. (I am not joking). Short journeys or long.
Charging lead-acid batteries is not rocket science, all you do is put
14.6v across the terminals until the charge current shows it to be fully charged, then maintain a 14.2v trickle charge. Try plugging a voltmeter into your cigar lighter socket and watch what happens on your lovely Subaru, then compare it to any other make.