modern atv's use Power when NOT in use, so, if your letting this atv sit for long periods, say a few months or so, its could just be killing your battery from the drain of electronic on it
its WHY folks highly recommend using a battery tender/maintainer
using one will keep the battery at full charge while it sits and make your battery last longer than NOT using one. there cheap too, wally world sells them, you just need to have power where you store your atv
YOU could also have a short some place draining power, or your charging system isn't correct
But I am guessing its just draining from time going by NOT being used by the description of your tread!
we have this issue at my hunting camp, every time we get there atv battery's were dead, some times in a few weeks?
we didn't have power there, so, we started to take battery's home or leave in the camp house on a tender!
works for us! sucks, but, thats modern atv's for you with all the fancy electronic's folks seem to love??