From what PD and Dirk Diggler have said, it would seem sensible to use the engines ability to charge a second battery. Then there is no need to go for complicated electronic gizmos like some people have. I set this up on my Etec last year and it has worked very well, I attach a wiring diagram to show how it is set up. The right hand side shows the twin battery set up, each charger independent from the engine.
There has been a lot of bad press for the Off-1-2-Both switches, they have a tendency to fry engine EMMs/ECUs I believe, and may invalidate your warranty I think I read somewhere..