I purchased a [link=http://batterytender.com/default.php?cPath=11&osCsid=e4052b27b15ce35266cd16 79b8c532cc]Deltran Battery Tender[/link] and simply keep it hooked up to the flight box all the time. The Battery Tender (I use the Jr. version) is designed to maintain a car or motocycle lead acid or gel cell 12v battery in classic car or motocycles which don't get used during winter months.

It is truly a plug and forget system and cost me about $25. A properly maintained (fully charged) 12v battery will last much longer as well.