Battery long term / trickle charging shouldn't damage your battery unless enough of the water component of acid / water mixture evaporates to expose the tops of the plates in the battery. Some of the batteries now days still have caps (cap bar) that allows monthly inspection of the fluid levels in each cell. Those that have caps also provide a split two sided sleeve as you look down into the cell, ideally the fluid level should be at the bottom of this sleeve to maintain proper acid to water ratio (specific gravity). Some auto store still carry a battery water jug that prevents over filling the cells. It has a spring loaded spout that cuts off the air flow into the jug and allows only the correct amount of water into a given cell. In the old days the thought was to only use distilled water to limit the mineral content in the acid/water mixture, I feel that unless you live in an area with very hard water you don't need to worry; regular tap water will do fine. One aspect of the maintenance free batteries is that you don't need to check cell fluid levels; My thought is this a major factor in why the batteries to day fail to hold a charge after a few years of service, especially in parts of the county with very hot summers. Granted other factors like the amount of active plate material and how deep the wells below the bottom plate support ribs. This the opinion of an old ex-navy aircraft electrican.
A secondary thought regarding having your battery disconnected and out of service during the winter time; batteries that becomed dis-charged are more likely to freeze, resulting in a cracked battery case and having to replace a ptherwise good battery, not to mention the electrolite leaking onto you garage floor.
