Measuring parasitic draw

Finding hidden consumers is done when there is increased consumption from the battery when the vehicle is at rest. Modern vehicles, after shutdown, still draw a small amount of current for memory maintenance (KAM) or the operation of some safety systems. For example, a vehicle’s anti-theft system draws relatively little current, as excessive draw can quickly drain the battery. Professionally, this rest consumption is also called a parasitic draw.

The additional current draw does not have to immediately drop to the lowest level when the engine is not running. Typically, when the key is off, the vehicle’s engine stops, and the vehicle’s systems begin to shut down and enter a “sleep” mode. On-board systems turn off at different rates. For example, on some vehicles, it takes several hours before the rear system goes to sleep and the additional current draw reaches its minimum value. Look in the manufacturer’s service data for the maximum allowed current draw and the time required for the module to sleep after turning off the ignition.

While the vehicle is stationary, the permitted current draw is minimal and cannot drain the battery. However, when the battery discharges while the vehicle is stationary, without sulfation or leakage on the battery, we have the appearance of a parasitic draw. A parasitic draw a lot of currents and can completely drain the battery while the vehicle is stationary. If we suspect that the cause of battery discharge is a parasitic draw, we proceed to check and reveal the location of the fault on the vehicle’s electrical network.

Ammeter measurement

Finding parasitic draws can be done in several ways, and one of the methods is to use an ammeter that can measure milliamps. Remove the minus terminal from the battery. The ammeter is connected in series between the minus pole of the battery and the minus terminal. The ammeter will show the value of the withdrawal current of the hidden consumer. By removing the fuse one by one, we disconnect individual electrical systems.

When, by removing one of those fuses, the current value on the ammeter drops, we have located the part of the electrical installation where the parasitic draw is located. Once again, we check by turning that fuse on and off to determine the accuracy of the search.

After we have localized part of the installation, we proceed to a detailed search of the entire network until we find the parasitic draw. The search includes all electrical devices, conductors, and connectors.

When the excessive current draw does not go away after removing all the fuses, suspect an unfused circuit such as the diodes and alternator regulator, the ignition system on some vehicles, or the control module power lines.