Guide to Tracking Current Leakage in Cars

How-To Guides

Current Leakage and the Hunt for the Clandestine Consumer

When a customer indicates that their battery is empty after 3 days of inactivity, the initial thought is of a bad battery. However, the battery can be easily tested. But there might also be current leakage, often caused by a clandestine consumer. If the battery is fine or if a new one has been installed and the problem persists, there must be current leakage. This article explains how to trace this illegal consumer.

When is there Current Leakage?

Current leakage is spoken of when the current consumption is higher than 35 mA, often caused by an illegal consumer. To determine if there is current leakage, you need to know the actual current. An Ampere clamp is used for this, placed around the ground cable near the battery.

Tracking Current Leakage (the old method)

You can trace the current leakage by placing a lamp between the negative pole of the battery and the ground cable. If there’s a leakage, the lamp will light up. The brighter the lamp, the more leakage there is. Then, you remove the fuses one by one until you find the one where the lamp goes off, indicating the circuit of the clandestine consumer. However, this method is not applicable to modern cars.

Modern Cars Require a Different Approach

With the current generation of cars having multiple computers, the old method is not usable. There’s a risk that removing a fuse might blow up a computer due to a voltage spike. To prevent this, computers have a safety feature that retains voltage for some time after a power interruption.

Tracking Current Leakage the Right Way (the new method)

The amount of leakage is measured with an Ampere clamp. If it’s higher than 35 mV, there might be current leakage. For further investigation, a multimeter set to the mV scale is used. Ensure you can access the fuses easily. Wait for at least 15 minutes, then measure the voltage ACROSS the fuse for all fuses.

Practical Example

For instance, if the interior lighting of a car is turned off, but the door light is still on, and you measure a voltage of 2.0 mVolt across a 7.5A fuse, it indicates a current consumption of 184 mA. As a rule of thumb, a consumption of 35 mA is acceptable. Anything above that indicates current leakage.

How long does it take for a battery to drain so much that it won’t start?

Taking a 45 Ah battery as an example, a car won’t start if the battery has only 60% capacity left. With a leakage of 0.184 Ampere per hour, the car won’t start after about 4 days.

Tip: A Bluetooth module is a recognized illegal consumer and thus a suspect for current leakage.

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