Measuring Stand-by Current Drain

Overview

Measuring current drain levels while the vehicle is parked and the engine is not running can often lead to demonstrative answers to the common puzzling question; “Why is my battery always mysteriously dying”. This test is designed to be step one in a series of vehicle electrical and charging system “State of Health” diagnosis and is commonly done when the customer complains of the battery dying mysteriously overnight, or during the course of several days. This test is particularly important to perform if the vehicle has any aftermarket electrical accessories installed, as the suspected cause of the battery drain may be placed on the aftermarket products and/or their installation. Car dealers are notorious for blaming aftermarket electronics for this problem.

Procedure

You will need access to the vehicle battery and remove one of its cables for this test. Typically, the NEGATIVE cable is chosen because there are fewer cables to disconnect. You will also require a Digital Multimeter (DMM) with the ability to take an in-line current measurement. You will identify this type of meter with a probe socket that is labeled “A” or “mA”.  Disconnect any under hood (or trunk) lighting as it will skew the measurement. Additionally, allow the vehicle to come to an electrical “rest” before making the measurement (e.g. delayed dome lights or headlamps  off, cooling fans off, etc.) This will yield the most accurate measurement data.

Step 1 - Loosen the negative battery terminal, but do not disconnect it yet. Step 2 - Prepare the meter for the in-line current measurement by inserting the RED probe in the “A” jack and the BLACK probe in the COM jack. Turn the measurement selector dial to the “A” position. (Caution - do not take a voltage measurement while the meter probes are configured in this position. Doing so can damage the meter) Step 3 - Secure the RED probe on the negative battery terminal before removal so you will not need to hold the probe with your hand. Often an alligator clip or “jaw” type probe accessory is ideal for this procedure.

Step 4 - With the red probe secured on its own, hold the BLACK probe tip near the base of the battery post so it does not interfere with the terminal. The object will be to remove the terminal without having to move or reconfigure the BLACK probe. Step 5 - With your free hand, remove the negative battery terminal while holding the BLACK probe in position on the battery post  with your other hand. You should now be able to read a measurement.

Assessment of the Results

Use the following guidelines to assess and troubleshoot the vehicle’s stand-by current drain.

0-15 mA (0.000-0.015A) - This is a normal reading. If the battery is having problems, check the age and state of charge. It may be time to replace this battery simply from age or normal wear and tear.

16-24 mA (0.016-0.024A) - This is outside the normal range, but is not excessive. Check to see that all security and remote starter power and ground connections are functioning properly. Ensure the battery is charging properly and has a good state of charge.

Over 25 mA (>0.025A) - This is beyond what is considered acceptable. Start pulling fuses or disconnecting power of aftermarket accessories to see if the drain goes away. If still present after this, proceed to pulling OEM fuses. This method is the most effective way of determining the offending excess current drain. Once located, repair the problem and re-check.

Technical data and illustration provided by Ramsey Consulting Group, Inc.

www.ramseyconsultinggroup.com