New battery drained within a few months

with the car off and a full charge is on the battery from a battery charger,it should be right at 12.6 DC volts,checking with a volt meter.with the car running,the battery voltage should be 13-15 volts DC.if so the alternator is working(Output)if not,replace the alternator!.if it is draining over night or during the day,then copy and paste the link,watch the video.//////..................... ............................................... [VIDEO]How to Test and Find a Parasitic Battery Drain … www.youtube.com/watch?v=zdIKNnwEjIs