Battery drain can be explained as one or more of the devices or systems in a car malfunctioning by pulling charge from the car battery when the engine is not running, instead of only when the engine is operating. Certain systems in a car are designed to always draw their power from the battery, such as a car stereo clock or car computer module. These systems, however, draw very little power, and will not cause a dead battery. You will want to check and test your systems to find other battery drain to prevent a dead battery and a car that won't start.
Disconnect the negative battery cable from the battery.
Insert the test light so that it is in the circuit between the negative battery cable and the negative terminal of the battery. This can be done by connecting one end of the test light to the negative terminal of the battery, and the other end of the test light to the end of the negative battery cable.
Take note of the brightness of the test light. A faint glow is normal. This is due to certain systems, such as the clock, running as described in the overview. A bright glow in the test light, however, indicates a battery drain.
Go to the fuse panel in the car and begin removing fuses one at a time, checking the test light as you have each one removed. If the light remains on while you have a fuse removed, that system is OK, and you can replace that fuse and remove the next one.
Identify a system that is causing battery drain by the fuse that is removed and causes the test light to go out.
