Battery Tender is the way to go. Have three of them. Plug it in and forget about it.
Warning don't try to get the cheapest one on the market..they usually are very low amp and don't work well in extremely cold weather. Been there done that.
The Battery Tender has worked for me in all temps. Have one on my generator and it has been connected for six years now through the winter and summer and still working.
http://www.corvettemuseum.com/cgi-bin/store/shop.cgi?ACTION=ENTER&thispage=new.html&ORDER_ID=! ORDERID!
Copy and paste this address.
Go to bottom left and search "Battery Tender"
The best one on the market..
