How To Find A Parasitic Drain Before It Kills Your Car Battery

If you've driven cars long enough, you can relate to our readers' stories about cars with dead batteries. Since few people get into the driver's seat without intending to drive, turning the ignition key only to hear a click, or nothing at all, is disturbing.

While there are a number of reasons why your vehicle battery can die, parasitic drain is one of the most frustrating to diagnose. Parasitic drains are typically low amperage draws within the car's electrical system that slowly deplete the battery over time while the engine is turned off, often overnight or sometimes over a few days.

The problem with parasitic battery drain is you don't usually know there is a problem until the car won't start. However, it is possible to check for any parasitic drain before it kills your car battery. To do so, shut off the car's ignition and switch off any lights or radios. Then, disconnect the negative cable from the battery and secure it so it cannot contact the battery terminal or other components. Allow about 30 minutes to pass to ensure the car's computer has shut down. Finally, take your multimeter, set it up to read at its highest amperage setting, and test for amp-draw between the negative battery terminal and the secured negative cable. Ideally, we'd like to see less than 50 milliamps, with lower readings even better.

If your amperage reading is more than that, your car battery is experiencing parasitic drain, and you'll need to find it before your battery dies. There are a couple of ways to find the drain, but one of the easiest methods involves checking individual fuses until you find the offending circuit.

There's more than one way to track down the source of the parasitic drain that's killing your car's battery, but you'll need to know how to check and replace car fuses before you start. In a YouTube video, Classic Car Maintenance suggests clamping one multi-meter lead to the negative battery terminal and the other to the negative cable and placing the meter where it can be seen from the car's fuse block, allowing you to go hands free.

Your car could have more than one fuse block, so you may want to check your owner's manual for their locations. Start with whichever fuse block you like, and pull the first fuse. If the parasitic draw remains constant, put that fuse back and pull the next. Repeat the process until you pull a fuse that lessens or eliminates the parasitic draw to identify the culprit — it could have more than one source.

Another method, shared on YouTube by Steve Rob Reviews, requires a multi-meter capable of reading millivolts, but it doesn't involve pulling every fuse in a process of elimination. Steve's method uses a multi-meter to check for minute voltage differences between the contact points visible on the exposed side of a blade fuse, instead of amperage draw between the battery's negative terminal and cable.

A reading as low as 1 millivolt across the terminals on the top of the fuse can indicate the movement of current across the fuse's contacts. Again, a fuse showing a current draw when the component should be off is a prime suspect for parasitic draw.

With either method, once you've identified the component, or components, it's just a matter of figuring out why it's drawing power when it should be off. It's often a malfunction of some sort, but it could be something disastrous, like rats chewing away your car's wiring.

If the source is found to be a malfunctioning non-essential component, like a courtesy lamp or radio, the fuse could be left out until repairs are made. In a pinch, any offending fuse could be left out when the vehicle is not in use and replaced before starting the car, or you could leave the battery disconnected to preserve its charge in some cases.

If the wires are damaged, especially by rodents, it's best to disconnect both battery cables until repairs can be made. In addition to causing thousands of dollars in damage, rodent infestations can create fire and health hazards if left unchecked. Furthermore, the wires laid bare by chewing rodents can cause electrical shorts, overheated wires, and sparks.