You can set it up other ways where just your stereo runs on battery #2. But, mine is as described above. Everything on the common stud. Battery 1 runs everything and 2 is isolated. If I run #1 dead I switch to #2 and I'm good to go. I charge #1 full during the week and then switch to "both" and keep charging till both are topped up (that way I don't have to take the cover off #2 to charge it). Then I switch back to #1, test fire in the driveway, and head out for the water.