I have been using one of these in a battery box with an old SLA battery out of a jump start pack wired in with it to float me over any power bumps. I fitted two lighter sockets to the box so I plug in to it directly. I have a multi stage charger that I use to give the SLA the occasional drink between uses.
http://www.jaycar.com.au/Power-Produ...upply/p/MP3079