Check this site. I have the Battery MINDer and am very satisfied with it. It can be wired permanently, has a connecter for quick disconnect, is small and will handle up to 4 batteries at a time. I use one of mine to keep two watercraft batteries charged throughout the winter, connected with stereo speaker wire. I own, and have used, other maintenance chargers but the Battery MINDer is the best of the lot.
http://www.vdcelectronics.com/automotive.htm

I bought mine from Battery Mart - very happy with the service and the price was right.
http://www.batterymart.com/batteryminder.php

Jim W.