Get yourself to Supercheapauto or other website and get a battery charger. Use overnight as required (dad's ute used to do about 500 km annually. Battery was often flat. Charge it the night before you need the car or jump start it if it's a manual.
A new battery should be able to sit at least for a couple of weeks - my car sits at the snow for 2 weeks + at a time and doesn't miss a beat. You can't get harsher conditions than consistently sub-zero (which does affect battery performance).
