My car sits for long periods. When it is outside and the alarm is one, I plug in a small solar panel, and even on a cloudy day it adds a trickle charge and so the alarm will not drop the battery in say six weeks of sitting. It was a gift, but it basically just plugs into the lighter.
Otherwise my solution when the car is in the garage is just do w/o the alarm.
ditto on the solar panel. I use a 15W one which popped up at half price from Maplin.
Anything less than this rating is a bit useless as the 1W panel can only deliver a shade over 80mA at best and the solar panel isn't going to be at the best azimuth to the sun for long enough to really make it worthwhile.