I use a "Battery Tender" type charger for my motorcycle, and just leave it plugged in all the time when I get home. It will peak the battery then just maintain a charge in it, not over charge it.

This is the one I use in my Ofna box, it even has a two pin plug on it and one to put on your bike (I bought another and put it on my box, I dont even have to take the over off to charge it now)

Schumacher SEM_1562A