HiI´m not quite sure what you want to do but it sounds like you want connect a car battery to an Arduino. In that case just use a fuse to be save. But it´s very impracticle.
I'm rather late to the game, but I was searching for in-car solutions, and found this:http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_1953161_-1Vin 9-18V, Vout = 12V
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