Testing current flow from the battery has changed along with the automotive electronics. It used to be sufficient to place a test light between the battery post and the disconnected battery cable. If the test light came on, there was a battery drain on that vehicle. That test no longer is valid for smaller computer drains. The only correct method of testing for a battery drain is to use an ammeter connected between the battery post and the disconnected cable. The ammeter measures the actual current flowing from the battery into the electronics systems. A typical engine computer will use between five and eight milliamps (thousandths of an amp) to keep the memories working. Most vehicles should show a maximum reading of 25 to 28 milliamps or less when the test is performed, however a few luxury cars loaded with all the toys may be a little higher. In comparison, a trunk light uses about 900 milliamps or .9 amps and would drain the battery overnight. If the measured current flow is too high, then fuses are disconnected one at a time until the faulty circuit or module is identified. Some vehicles will require a waiting period of up to 1/2 hour before an accurate measurement can be taken. During this time the computers are "awake" and use more power. After sitting for several minutes without any switches being operated the computers go back into "sleep mode". A note of caution - opening a door (which operated the interior lights) or turning on the ignition switch or headlights will damage many ammeters. The testing must be done carefully to prevent high current flows from passing through the meter. Finally, other factors can cause a battery to go dead faster than normal. Warm temperatures cause the battery chemical reactions to occur faster. This causes the battery to go dead quicker. Dirt or dust on top of the battery and high humidity levels also cause a problem. The electricity leaks through the damp dirt directly from one battery post to the other causing the battery to discharge. Keeping a battery clean, cool and dry will ensure it retains its charge longer. Borrowed from canadiandriver.com
