If you have tried all of those methods, and still your battery is not working, then it is time to replace your battery. Since there are a plethora of batteries available out in the market, it is going to be a bit confusing for you when you go out there to buy a new battery. Make sure that you know what your requirements are, and what the batteries you are looking at the offer. Compare them and then finally make a decision you think is the best one for you.
Once you have bought a battery, you need to install it. Check out the user manual for your ATV and find the location of the battery. Mostly, the battery is underneath the ATV's seat. Remove the seat by using the latch at the back and the prongs in the front. After removing the seat, remove the cables connected to the battery with the help of a socket. First, remove the ground cable, and then the positive cable. Unbolt and remove the retaining bar that is keeping the battery in place as well.
If there is some battery acid is present on the cables or the terminals, remove it with the help of vinegar & water solution. You can also make a solution of baking soda & water. After this, place the battery you have purchased. Connect the cables with the correct terminals. Reinstall the retaining bar and then attach the seat of the ATV. ATV's charging system will charge the battery. The battery is automatically charged when the engine is running. But this only works if you are doing some kind of work which is draining the battery at a rate greater than the charging rate. The charging system of an ATV charges the battery as well as powers the other loads such as the cooling fans, headlights, etc.