In my experience I have had no problem with the car left for 2 weeks in summer. I believe cold weather is hard on batteries however and I have always put it on the "battery conditioner" in winter.
Our car has now done over 65,000 km and I believe the fact that I nearly always have the battery conditioner (trickle charger) plugged in has kept the now 7 year old battery in good condition.
I can't even remember what brand it is. I ordered it from the factory with the car.
There are solar battery charger kits available to keep boat batteries charged. One of these would be a good idea if no power is available in the garage. Provided of course that the garage is not underground