Form what read. Most alarm system take about 2 months to drain a battery. So maybe your car battery was just on the edge of going bad and the alarm system was the straw that broke the camel's back. Also get the senitivity turned down. If the alarm is going off when the car is not running, that will drain an old battery dead. And lastly your alarm system could have a bad ground connection. The installer should check this for you for free. If not they souldn't be in business.