We second James's endorsement of the Battery Minder products. We have many battery operated and battery start things/machines/tools and all of them were on el-cheapo "maintainers" which eventually still cooked most of the batteries though better than nothing. Most of the batteries are on things with occasional, intermittent or rare use. The types run the full range from wet/gell/sealed/AGM.

Research indicated Battery Minder products were a different animal, so we bought six of them plus one of their larger charger/maintainer products. All the batteries are happy still, they even brought back four gell cells used in two electric wheel chairs that had been in a scrap yard.

 

Chrissi