Understanding the Causes of Parasitic Battery Drain

Diagnosing and Fixing the Causes of Parasitic Battery Drain

Parasitic battery drain can leave you with a dead battery without warning, even when the car is not in use. This phenomenon occurs when electrical components or systems draw power from the battery, even when the vehicle is turned off. Over time, this drain can deplete the battery, leading to a no-start condition, stranded drivers, and unnecessary headaches. In this article, we’ll explore what causes parasitic battery drain.

What is Parasitic Battery Drain?

Parasitic battery drain refers to the continuous, unintended consumption of electrical power from a vehicle’s battery after the ignition has been turned off. While it’s normal for some components—like the clock, remote keyless entry system, security system, or ECU memory—to draw a small amount of power, excessive or unexpected drains can quickly deplete the battery, especially if the vehicle is not driven regularly. 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.

A healthy battery should be able to power these small drains for weeks without issue. However, if something is wrong, even a new battery can be drained overnight or within a few days.

Carmakers design their computers to go into “sleep mode” after a set period of inactivity. This is done to reduce power from the battery. If a computer module fails to enter low-power sleep mode, the parasitic battery drains and results in a dead battery. This can happen within a matter of hours or overnight.

What causes a module to remain “awake?”

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 repeat a request for a module’s current state continually.

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.

Common Causes of Parasitic Battery Drain

• Door or hood switch stuck in the open position— In a vehicle with a body control module, a faulty door switch can prevent the module from entering sleep mode.
• Relays Stuck in the Closed Position— If a relay fails in the closed position due to pitting/sticking electrical contacts, it can allow current to flow even when the vehicle is off, draining the battery.
• Shorted Alternator Diodes— A faulty alternator can cause a parasitic drain if its diodes are damaged. This allows electricity to flow through the alternator even when the engine is off.
• Defective Modules— Modern vehicles contain numerous electronic control modules (ECMs) that manage everything from engine performance to infotainment. If one of these modules fails to enter sleep mode when the vehicle is off, it can continue to draw power from the battery.
• Aftermarket Accessories— Improperly Installed Audio Systems: Aftermarket audio systems that are not properly wired can draw power continuously, especially if the head unit or amplifier remains active even when the vehicle is off.
Security Systems and Remote Starters: While these systems are designed to provide convenience and protection, they can also be a source of parasitic drain if they’re not properly installed or maintained.

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