The Arkpak and similar are great except for one thing. . . you have a very heavy battery (50 or 60lbs) loose in the vehicle. I would not want to be in the vehicle with a loose acid filled lead and plastic box in any kind of accident or roll over - a whack in the head would end all your earthly problems real quick. In Aus the Arkpak is about $400 and a decent AGM deep cycle battery is another $300 - $400 while a dual battery installation will drop the cost of the Arkpak and add the cost of a controller, a battery mount and some wiring especially if you DIY. Nett cost is about the same BUT. . . the dual battery set up will keep both batteries charged from the alternator whenever you drive and the second battery is safely mounted in case of accident. While the Arkpak battery gets flatter every time you use it until you can recharge.