I use a battery tender and whole hardily recommend them. Make sure you get one that has a good microprocessor chip and is a legitimate company. I use this one. Have one on all three of my cars. My mini is 200 miles away and I leave for for a few month (sometimes 6). Come back with a fully operational battery. Make sure you use a surge protector, and you should be fine. You should also use jack stands to lift the wheels so the tires aren't on the ground and get flattened out. Even 1/2" off the ground is fine.

Also, make sure that there is a surge protector hooked up to the trickle charger. I use this model. I use the cigarette lighter attachment and place it in the rear passenger cigarette lighter under where the arm rest is. (hard to explain). That plug seems to be always on, at least in the April 2004 build. I open the rear passenger window a crack and feed the wire out of there. I also open the sunroof a crack to let some air circulation in the car.

Battery Trickle Charger

The last thing I would recommend is make sure the battery water is fully filled. It seems that the trickle chargers take a toll on them and the water can be an issue.

If there is absolutely no power plug available, and make sure to talk with the storage people about seeing if there is a unit with one. Sometimes they are cool and have units you can pay a buck or two extra or can feed an extension cord in for you.

Else, I would suggest just disconnecting the battery, or talking with them about placing a solar trickle charger on the outside of the door facing upwards towards the sun. You can usually get them for a reasonable cost ($45 - $50).

I would suggest something like this one. Solar Trickle Charger

The last last thing I would suggest is make sure there is NOTHING in the car food or liquid wise. If the car has any spills I would recommend getting them cleaned up, if not by you the professionally. You can also put a few no fragrance dryer sheets in a small box on one of the seats or something if you're worried about the smell. I recommend only using the no-fragrance ones else the smell will percolate into the leather... Yucky

The last last last thing, would be a car cover or a few old sheets to preserve the exterior paint. A good wax job before hand is preferable. I'm not talking about the going through a car wash wax, but a real wax job. The car wash is a last resort. Remember this is going to sit for four months so you don't want bad stuff, bird droppings or something of that nature bleeding into the paint.

Good luck...