I'll guess your ATV gets used periodically during the winter?
Keeping a charger on seems to help a lot. Something like the Battery Tender Jr. which can be had for $20. I bought extra battery end cables and have everything wired so that I can move the charger around from one unit to another. Or buy enough chargers to cover everything. I have the ATVs, mower, motorcycles, etc. all wired and move the charger around every week during winter.
Ken