is there a reason you need a second battery?? caravan? winch?
i did melbourne to perth via a **** load of places including Inaminka, Silverton, Wiliams Creek, and **** nose where else, ave forgotten but it took me 6 months doing about 150k's a day. all this in a Holden HB panel van with fooked back supension . i did how ever put a new battery in before i set off. the only battery related trouble i has was bull dust getting into the alternator.
so if yer dunt really need one don't bother as its just more stuff to go wrong. Take more VB instead
and if yer haven't already got one get a flying doctor radio or a VHF set