Thanks for all the advice. I think I will replace my charger when I replace the battery. Noco seems to be mentioned a lot, so maybe I will pay the extra and get one. But what brands of batteries have you had luck with?
I admit that I haven't used the ATV much since I put the new battery in, but I believe the battery was fully charged, and I put the charger on it every couple of months. I guess if my charger wasn't working properly, that could explain things. I will also try to check the charging system on the ATV itself.