I just finished doing this to my 2001 Outback DD this week. I used the Blue Sea Add-a-battery kits. It works awesome. The installation instructions show a wire from the alternator to the battery, but you won't need it on our type of boats. It was kind of a pain to do, and the additional wire and connectors cost more than the switch. Mine was a pain because I didn't want to lose any more storage space with the second battery (or the first for that matter) so I got creative. I cut a large hole in the floor between the driver seat and the obeservers seat (kind of where the ski locker is on some boats) and dug out all of the foam and mounted both batteries below the floor. I wired them up to the switch and isloator behind the observers seat, and fabricated a new floor piece and carpet it so it blends in with the original floor. It does make a slightly raised area in the floor, but well worth having the batteries out of the way, and getting the weight of them right over the keel. The one additional thinkg I did, since the batteries are more difficult to get to, is I hard wired leads for a battery charger/maintainer to my deep cycle stereo battery so I can plug it in and charge it without having to open up the floor. As for the starting battery, the factory alternator does a fine job of keeping that charged. The great thing about this setup with the isloator, is that I can be anchored out playing the stereo, and I am only draining my deep cycle battery, not my starting battery. However, when the boat is running, they are combined and I am charging both batteries.