Before you change batteries, have the alternator checked. I assume it's from 2004? It might not be putting out enough amps to properly charge it. Just checking the voltage won't tell you much.
The recommended minimum amperage is 1 CCA per cubic inch of engine, 2 CCA/inch on diesels.
Thanks - the battery rating at 500-750 CCA should be good to go. It's a 2.4L 4 cylinder engine which is 146 ci, not exactly big Detroit iron.