A car alarm can deplete the battery, but it will take a long time to drain completely. Whether your car alarm is on or off, it will draw power from the vehicle's primary battery. A car alarm in standby mode will not completely deplete a battery for several weeks because it uses little power from the battery when it is not in use. The battery can run out in one to two days when an alarm is activated, though, as it requires substantially more energy. This is because power is required for the alarm to sound and the warning lights to blink when it is triggered. However, certain car models have alarms that will automatically turn off after a predetermined period to avoid draining the car battery.