I have been using these maintainers made by black and decker for a few years now. They are under $20 and have a setting for 6 and 12 volts. We leave em on all winter as we dont get to our land much during that time. In Jan we went to pick up our snow machines and I started my AR which had been sitting untouched since mid November. It was a 20 degree day and the battery whipped the starter like it was brand new.
Hope this helps someone!