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Parasitic battery drain

Parasitic battery drain— What is it?

All modern computerized vehicles draw a certain amount of power from the battery at all times. The power is needed to maintain the computer module’s memory and to stay “awake” to receive signals from the owner’s remote keyless entry key fob.

The amount of power these modules draw is much higher once you activate the door or trunk. That’s the signal for the modules to “wake up.” During “sleep mode,” the modules normally consume less than 50-milliamps. At that rate, a fully charged car battery in good condition can power the modules for about 30-days before it’s too discharged to start the vehicle.

What happens when a module doesn’t go into “sleep mode?”

Unfortunately, when a module fails to go into sleep mode, it draws its full power at all times. That can completely drain your car battery overnight. This is referred to as a parasitic battery drain because it depletes your battery.

A faulty door or hood switch, for example, can cause a body control module (BCM) to think that your door or hood is still open and that can prevent the BCM from entering sleep mode. Or, a fault on the data lines can cause a module to continually repeat a request for a module’s current state.

Each of these faults can prevent the modules from entering sleep mode and continuing to draw full power of up to 8-10-amps; enough to fully drain your battery overnight.

What is a parasitic battery drain test?

The technician measures power draws to certain module circuits when the modules are supposed to be in sleep mode to see how much power they’re drawing. If they are drawing too much power, the tech will examine each module in that circuit to find the one that’s failed.

See this post for more information on how to conduct a parasitic battery drain test.

©, 2021 Rick Muscoplat

Posted on by Rick Muscoplat


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