There are a variety of inexpensive wall-wart packaged float chargers for lead acid batteries around. Might be easier to just get something off the shelf. http://www.power-sonic.com/images/powersonic/chargers/AC-Series_12_Aug_15.pdf http://www.mouser.com/search/refine.aspx?Ntk=P_MarCom&Ntt=172260151