House batteries? Some of the discussion of batteries is confusing me. Why would you carry a spare in the back? Even if you didn't want to connect it I suppose you could just place it in the tray in the front. I think it is less of a hassle if you can just switch the connections on the fly and motor on if you are stuck. GMT400, GMT800 and current (GMT900) I believe they all have dual battery trays under the hood.