Parasitic Battery Drain
Parasitic Battery Drain
Making a tough job a little easier to tackle.
Michael Miller/Motor Age — Let’s face it, parasitic battery drains suck, and I’m not only referring to voltage. Locating the source of the draw also can suck up a lot of time as well. A couple of decades ago, a test light and a fuse puller were pretty much all that was required to track down a problematic circuit that was causing a continuous amperage draw on a vehicle’s battery. Now with the technological evolution of automotive electronics and its encompassed networks, the archaic methods previously used to detect parasitic draws can derail your diagnostics before you even get started. While diagnosing parasitic draws probably never will be on your list of favorite work orders, encompassing the techniques here hopefully will make narrowing them down a more efficient process.
Not too long ago when a customer had a concern about their vehicle’s battery going dead after sitting overnight or after a few days, the first step was to disconnect one of the battery cables and install a test light in series between the cable and the battery post. The glow emitted from the filament was relative to the amount of current being consumed by the vehicle’s electrical system. Common causes were glove box lights remaining on or sticking brake light switches. Tracking them down was as simple as going to the fuse box (don’t forget to trip the door ajar switch so the dome light doesn’t stay on) and pulling fuses one by one, until the test light glow diminished. Then finding out what circuits were powered by that fuse and eliminating them one by one.
Just by doing the first step, disconnecting the battery cable, on a modern vehicle can completely nullify your diagnostics. On most vehicles, removing battery power from modules on a network reboots them and that may have even fixed the problem without you knowing it, not to mention erased clues that could have helped with your diagnostics.
Start with the Source
Sometimes when looking for the source of the parasitic draw, we overlook the source of voltage itself: the battery. Uncountable hours have been wasted without first testing the battery. First, the battery needs to be adequately charged. Too low of a voltage might cause modules to flake out and your test results will be inaccurate. Also, don’t forget to measure for any drain caused by corrosion on top of the battery, referred to as case drain. Because the corrosion is conductive, it creates an electrical path for current to flow from the negative post to the positive post. To check, place the negative end of a digital multimeter (DMM) on the negative post of the battery and place the positive lead on the case of the battery. If your meter reads even half a volt, the battery needs to be cleaned. Also, while you’re at the battery, don’t forget to measure voltage drop between the battery post and its respective terminal while starting the engine. Too much resistance here can prevent a battery from charging properly, not to mention increase the amount of current needed to start the vehicle.
Source: Motor Age