Keep Your Boat, Lawn Tractor and Motorcycle Batteries Charged All Winter for $19
I put fuel stabilizer in my small engines so I'm not worried about the gas going bad over the winter and I've been starting my lawn tractor every few weeks just to keep the battery charged. Even doing that is not enough...it died.
But for $19 I found this sweet Black & Decker battery maintainer on Amazon. I'm sure you can pick one up just about anywhere and I saw a different brand for 10 bucks less in my Harbor Freight catalog and their stuff had always been good to me. It's not a big, lunky thing. You can mount it on the wall!
Remember the difference between a battery charger and a battery maintainer. A charger is more expensive but will charge your battery fairly fast, then you disconnect it or you could overcharge and damage the battery. A maintainer is not a fast charge and you can leave in connected all winter. It charges, then floats, and repeats.