I bought a 1.5 amp trickle charger, made for motorcycles, at Wallmart a couple of years ago, seems to work fine. I'm on the 3rd year on the same battery this year. I also use it on the lawn tractor. I think it's Schradder, or something like that, about $20.00. It also came with a pigtail that you can leave permantely attached to the battery in your ski, and just plug the charger in when you need it.
Lou