Parasitic battery drain

What is Parasitic battery drain

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.

However, if all the computer modules used full power all the time, the car battery would be dead within hours after shutting off the vehicle. So engineers program software to force the modules to go into “sleep” low power mode shortly after you shut off 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. 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 are the most likely causes of parasitic battery drain?

Electrical Accessories not fully shutting off will cause battery drain

Leaving your door, trunk, or hood slightly ajar, for example, will keep the body control computer away and the interior lights on. Leaving your vehicle in the accessory mode will also cause battery drain.

Faulty door, hood, trunk switches and faulty relays will cause parasitic battery drain

A faulty door switch can make the computer think the door is still open even if it’s closed. The same applies to hood and trunk switches. Some electrical accessories, such as interior lights, power seats, power mirrors, and infotainment systems, may continue to draw power even when the vehicle is off. If these systems malfunction or do not properly shut down, they can contribute to parasitic draw.

Computer modules not entering sleep mode

When a computer module fails, it can short out the data bus in the vehicle causing the modules to stay awake and consume power.

Security Systems can cause battery drain

Security and anti-theft systems remain active even when the car is parked. These systems can contribute to parasitic draw, although their power consumption is usually designed to be minimal.

Remote Start Systems can cause battery drain

: If your vehicle has a remote start system, it needs to maintain a connection with the vehicle’s systems, which can lead to some level of parasitic draw.

In rare cases, a short-t0-ground alternator diode can cause parasitic battery drain

A diode is a one-way electrical valve. In rare cases, diodes can fail in a short-to-ground condition that can drain a car battery in less than an hour.

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