[Overdrive]

If you have a manual for the car, definitely look in there for some info along with what I'll share.

The car does stay very powered compared to more conventional cars, with a lot of systems staying active even while the car sits not running. For example, I'm sure you've found that after about 5 days your keyless entry will be deactivated and you will need to key into the car. So things like that will deactivate over time, but in the meantime they do run the battery down. Power windows stay active for up to two hours in case you have a passenger sitting in there while the car's off, as well as the seat adjustments if you have electric ones. The trunk lights should shut off after about 30 minutes, so if they're still on beyond that I'd find that a little abnormal.

Anyways, leaving the car for a month, without hooking it up to a maintainer, and locked with the alarm active will definitely run the battery down. So if you're going to leave the car for that long of a period, put it on a maintainer (not a charger) at least overnight after maybe two weeks at the most just to keep the battery strong. If the battery goes through several deep cycles of charge and discharge it really kills the lifespan, which is why those $200 Porsche batteries sometimes don't even make it a year before getting toasted when someone puts the car away for half the year and doesn't touch it again until that other half of the year comes back around.

If the car's driven regularly enough and there's no major electrical issues to speak of you should never have a problem with the battery getting run down. I just replaced my alternator and battery recently because the alternator failed and the battery had deep cycled so many times that it could no longer take a charge even with the good alternator. Now the car starts strong every time, even after sitting for a week, so maybe you do have an issue, but probably, and hopefully, not one quite as expensive as mine was.