clearly this would also mean that the alarm has some reserve power as well. If you access the battery itself (as opposed to the disconnect terminal) you'll likely see a connection at the stem for the alarm system). I left my 740iL for a week and it was already out of power due to heavy rain & wind setting the alarm off.

PS- A suggestion: If the car will be parked outdoors, you may want to try a lighter-adapter solar charger. They are getting more popular as long-term storage solutions, since it's like a part-day trickle-charge. Think I bought mine for $15 from Costco and it works well. Just a thought!