Hi Earl,
A true test of any 12V battery is to test it under cranking load i.e. are you getting 12V or above when cranking or are the volts dropping. If the Volts are dropping under cranking your battery is on the way out. Conduct a dry discharge test, observe the all the battery cells during this test, if any one cell is bubbling its shot.
Never ever keep your battery permanently connected to a battery conditioner, it will overheat, sulphurise, and cause battery cell plate material to short out the plates in a battery cell as it collects in the base of the cell. This will significantly reduce the performance of your battery and give you the kind of start problems you are getting i.e. one minute its OK then another it’s not OK.
Sulphurisation i.e. sell plate material dropping to the base of the cell and shorting out the cell is compounded in the majority of batteries fitted to all ATV’s. This because the battery dimensions are 75% less than say a 12V battery on the average car. If ATV batteries are kept permanently on charge the whole process of sulphurisation is exacerbated and battery performance reduced to all time low.
Yes of course, keep your battery charged, do not permit it to drop below 12.5volts, but do this once a month. When the volts drop below 11V on a regular basis this too can cause sulphurisation and eventually destruction of your battery with the exact symptoms you are getting.