Gill if all you need is to charge and maintain the 12Vdc sealed battery in your flight box doe the fast finger...the one or similar like John Rist linked above is good...Walmart, auto parts stores
If you also need to charge some Nimh, Nicd, Lipo, Eneyloop etc then there are several chargers you run off a 12Vdc source (battery or power-supply) or there are AC/DC dual input versions. The AC one you just plug in amd follow the menus to charge your different battery chemistry ...They usually has a Pb setting (lead acid) for 2. 4 6 12 up to 24 Vdc charging and maintaining