(This is the full-length version of an earlier video we shared. This version shows additional methods, all of which can be used together to make the job of isolating the cause of key-off drain much simpler.) Your customer has their car towed to your shop after it wouldn't start that morning. You check the battery and see it's pretty low, and you do what's correct. You charge the battery and test its condition. It passes with flying colors. You reinstall the battery in the car and the car starts immediately. You check the starting and charging systems and still no culprit is uncovered. Being the professional auto service technician you are, you know there is only one other logical explanation. Something on the car is not turned off when it should be and it's draining the battery down to a point there isn't enough left to spin the engine. But finding the parasite that is causing that drain can pose a few challenges. Increasingly common are electronic control modules that insist on staying on, or "awake" when they should time out or "sleep". The traditional method of monitoring current while removing fuses one at a time can often cause the offending module to reset, and the problem to temporarily disappear. One method you can use to find the cause of a key-off battery drain without intruding on the circuits is to measure for the presence of voltage drop across the vehicle's fuses. The fuse is a resistance, isn't it? And according to the laws of electricity, voltage will be used to overcome that resistance resulting in a drop across the fuse we can measure with our meter. And that's only one way you can catch the parasite without having to open the circuit. In the March 2013 edition of The Trainer, we'll show you how to test for a "key off" battery drain using a few different methods; first, using the ammeter and voltmeter of your digital multimeter (DMM) and second, using an amp clamp and a digital storage oscilloscope (DSO). Knowing a few different ways to test for these parasites should make those challenges a bit easier to handle! Click this link to subscribe to Motor Age! http://bit.ly/MA_YT_freesub