The short answer is, yes it can drain the battery. Depending on the various car systems that are connected to the car alarm – lights, horn, etc – you can actually drain the battery in several hours. Again, it all depends on the number of car accessories that are set to go off together with the alarm and how much power these systems will be draining from the battery. Also, you have to take note that your car’s battery may not actually have a full charge by the time the car alarm is set off. Some batteries actually discharge at a monthly rate of about 30%. You need to factor these, too.