Definitely battery has gone bad,could be your voltage regulator,
https://www.amazon.com/GOOFIT-Voltag.../dp/B00UCXFO6A
My voltage regulator went bad and was allowing to many volts to go to the battery and cooked my battery.I'm not much on sealed lead acid batteries but I went with a Mighty Maxx battery from Amazon and have had no trouble with it.Seems to be a good battery plus not expensive.
I check my voltage going to my battery every now and then hoping to prevent that from happening again.I check it not running then I start my atv up and check the voltage at the battery,plus turn the headlights on & check the voltage again.Voltage should go down a little when you turn your headlights on but should always be 13+ volts with the headlights on or off.