OP, not sure you are still reading this, but IMO, there is some misinformation on this thread. I never felt comfortable leaving my normal charger plugged in for months as it gets hot. I would prefer to plug it when needed after inspecting that the area and connection are clean and safe. This is why a battery minder or other product like it is preferred.
Here is a nice link on batteries..
Deep Cycle Battery FAQ
A relevant snippet (bold added by Alan) from that link is here:
Battery Voltages
All Lead-Acid batteries supply about 2.14 volts per cell (12.6 to 12.8 for a 12 volt battery) when fully charged. Batteries that are stored for long periods will eventually lose all their charge.
This "leakage" or self discharge varies considerably with battery type, age, & temperature. It can range from about 1% to 15% per month. Generally, new AGM batteries have the lowest, and old industrial (Lead-Antimony plates) are the highest. In systems that are continually connected to some type charging source, whether it is solar, wind, or an AC powered charger this is seldom a problem.
However, one of the biggest killers of batteries is sitting stored in a partly discharged state for a few months.
A "float" trickle charge should be maintained on the batteries even if they are not used (or, especially if they are not used). Even most "dry charged" batteries (those sold without electrolyte so they can be shipped more easily, with acid added later) will deteriorate over time. Max storage life on those is about 18 to 30 months.
Batteries self-discharge faster at higher temperatures. Lifespan can also be seriously reduced at higher temperatures - most manufacturers state this as a 50% loss in life for every 15 degrees F over a 77 degree cell temperature. Lifespan is increased at the same rate if below 77 degrees, but capacity is reduced. This tends to even out in most systems - they will spend part of their life at higher temperatures, and part at lower.
Typical self discharge rates for flooded are 5% to 15% per month.