EASY Test Tip for Identifying Parasitic Battery Drains - With an Ohmmeter! EVERY vehicle. It works off of the principle that ohmmeters don't read properly on live circuits. Remember; ohmmeters put out around ½ of a volt and then calculate resistance by performing a sort of voltage drop. They can't do that if the circuit already has current flowing in it. Conventional tests for parasitic battery drains sound like a fair amount of diagnostics? You bet they are. The key to knowing if an intermittent excessive parasitic drain is the cause of a dead battery is to know how to quickly detect the presence of one. Messing around with an ammeter, parasitic drain switch, backup power supply (to keep system from powering) are all time consuming. Sometimes you go through all the aforementioned steps only to find a parasitic drain is not present at the moment. More road tests on bumpy roads, activating accessories, etc. are required along with the unknown variable of "how long do I wait after shutting the vehicle off before the modules all go to sleep?" Wouldn't it be great to have a simple and non-intrusive two alligator clip connection type of test to determine if the parasitic test equipment and tests methods are warranted? A sort of 'good or bad' quick test is possible and it works the same on Photo Dave Hobbs 1. Simply connecting a decent quality high impedance digital ohmmeter (analog and $10 DIY meters will not work) between the battery negative post and a nearby known good chassis ground. 2. If a normal parasitic battery drain (below 50 mA) is present, the ohmmeter will read normal resistance of 0.1 or .02 ohms (Fig. 15) as you would expect. Test for other causes for the battery running down (i.e. an intermittent alternator) and / or perform more dynamic condition road testing (bumpy roads, acces- Figures 14 and 15: The ohmmeter reading on the left indicates you forgot and left a door open / ignition on or you have an excessive parasitic battery drain. The connections are between battery negative and a known good chassis ground wire. In these photos, the chassis ground is a short wire between the battery negative connection and the strut tower. The ohmmeter reading on the right indicates there is little or no parasitic drain present. If you see up to 0.8 or 0.9 you probably still have a normal parasitic load present. Several ohms means excessive drain is most likely present. Reverse the polarity of the leads. If you see a negative ohm reading, that ground wire is fine. You've just detected a 'false positive' of excess resistance in that circuit. Time to get out the ammeter and go to work. June 2017 7 MACS Service Reports