Parasitic Draws and Ways to Tackle Them

Posted on Mon, Oct 21, 2013 @ 01:12 PM

By David Barden

parasitic drawWhy does my car battery die after sitting overnight in my driveway?  This is a common question heard from customers in almost every shop.  The battery could be nearing the end of its life, you left something on like your lights, or a component is causing a parasitic draw.  A parasitic draw can be defined as any electrical device that draws electric current when the ignition key is turned off.

All modern vehicles draw some power from the battery even when the vehicle is off. The vehicle needs a small amount of power to maintain memory in the PCM and to keep settings in things such as clocks, radios, etc. This low power draw is expected, and normally the amp draw is low enough that the vehicle can sit for long periods of time without problems. When other items are using more power than expected or not shutting down correctly, your battery will discharge faster than normal and in a worst case scenario; leave your vehicle with a dead battery.

A Parasitic Draw Test is used to measure how much power the vehicle is consuming when it is shut off, and determine if it's a normal amount or an excessive amount of current draw. This test is typically done by disconnecting one of the battery cables and placing a meter in series between the battery and cable to capture all current going into the vehicle.

Although you can measure the vehicle draw with an inductive amp pickup, due to the very low milliamp readings involved, the in-circuit testing method is desired (even using standard multi-meters) over using an inductive clamp due to the more accurate readings obtained. However, it can be risky connecting standard multi-meters in-line for current testing as they are usually limited to about 10Amps max and any unintentional load can blow expensive, hard-to-find meter fuses. The HOOK has the advantage of the Adjustable Circuit Breaker and up to 65 Amp current capacity so there is no danger to the circuit or the tool when using The HOOK in this case.

However, connecting the Power Probe Hook to measure parasitic draw is a little different than how you would usually connect a standard multi-meter. Since the Hook is always connected to the battery, it is just a matter of dis-connecting the negative cable, and then supplying ground through the Hook to complete the circuit.  Watch this video for proper testing and techniques for locating the specific circuit causing an excessive drain.

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Tags: Power Probe, Diagnostic Tools, Circuit Testing, Parasitic Draw, Battery Draw Test, Battery Drain, Key Off Drain, Fuse Testing, Current Draw, dead battery, fuse chart, The Hook, IOD (Ignition Off Draw), Milliamps