I would not scrap the Auto idea.
You could add 1 switch and make it all work.
The switch could be a test switch that just kills the relay, and makes for a good monthly test for your battery system.
Don't give up yet.
The Electronics forum, may be a good place for some Safe Low Voltage design ideas.
Fuses, Fuses, Fuses. don't let the smoke out of your design.
If you don't get help on this Forum you can get help on http://www.winforce.net/forum/electronic-talk