Buy one of these for $30 instead:
http://www.partsamerica.com/ProductD...tnumber=SE112S

Keeps 12 volt batteries charged. Fully automatic. Turns on and off as
needed. Allows safe charging of RV, deep-cycle or most batteries that
may have a long storage period. "On Board" feature allows mounting of
charger next to battery on collectibles, street rods and other
infrequently used vehicles. Hardware and mounting brackets are included.