when the key is in the ignition the car remains ready to be started, so all the electrical c/u's in the car remain energized. The draw on the battery can be as much as a couple of amps, which will kill it fairly quickly. The car draws the least power when it is locked, and the alarm is armed. With those conditions being met, the draw on the battery (with no electrical issues present) is less than 30mA, which should keep your battery alive for weeks if not months.