new here, with a new 850

hi there, im new to this fourm, just bought my first atv 2015 sportsman 850,
i am having an issue with the bike, picked it up last wednesday, parked it in my garage, went to start it friday and the battery was dead.
charged it friday night, road it for a few hours saterday with no issues, love the bike, go to start it last night and the battery is dead again. i pulled the battery and charged it, load tested it and it passed, never droped below 11.5v at half of CCA’s but the voltage is dropping off steadily by its self, my question is any one elce ever get a bad battery from new? the dealer is going to get me a new one but i like to do my own reasearch first. also i had the dealer put a 3500lbs winch on the bike, is there a factory upgrade for the battery?