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Parasitic Drain on Batteries: Causes & Solutions

Mechanics and Auto Professionals  ·  February 1, 2023

Parasitic Drain on Batteries: Causes & Solutions

A problematic car battery can leave your vehicle stranded in its parking spot, unable to start the engine. One such problem is a condition called parasitic drain, where the battery loses power even when the car is off. But what causes that issue?

A parasitic drain occurs when something actively drains the electrical power from the battery, even when your car is off. One cause is human error, like forgetting to turn off an interior or exterior light. Faulty relay switches, electric accessories, and electrical faults (like short circuits) can also cause the problem.

Keep on reading to understand the causes and solutions of a parasitic drain on your car’s battery, fix the problem quickly, and prevent it from happening again.

What Is Parasitic Drain on a Car Battery?

A parasitic drain is when your car battery discharges electrical power after you’ve turned off the vehicle. It can happen when some accessories and components within the car are still actively consuming electricity after the vehicle is parked and the engine shut off.

This is a silent problem you likely won’t detect unless you perform a parasitic drain test, using a standard multimeter or parasitic drain tester.

Simply put, if there is an outgoing electrical charge despite the ignition being in the OFF position, something is acting as a ‘parasite’ and silently consuming your battery’s power.

Some people may also realise that their car battery experienced a parasitic drain condition when it wears out prematurely, but by then, it’ll be too late to save that battery.

As you can see, a parasitic drain on your car battery isn’t as terrifying as it sounds, but it’s something you shouldn’t ignore or assume won’t happen to you.

That condition will drain your battery, leaving you stranded wherever you last parked and forcing you to replace the battery sooner than expected.

Remember, the battery supplies the power to crank and start your engine. So, letting the parasitic drain continue will risk your car not starting the next time you want to drive.

Parasitic Drain Vs. Self-Discharge; What’s The Difference?

Parasitic drain has similarities with a self-discharging battery, but they’re not the same conditions. They are different battery issues, and you must look at the separately.

Self-discharge is a common condition that happens to all batteries. The chemical reactions within a battery cause it to gradually lose small amounts of electrical power over time, even when it’s not in use.

That means even the car batteries sitting on shelves at your local workshop gradually lose some amount of charge over an extended period, but in barely noticeable fractions of power.

Conversely, a parasitic drain is not a natural occurrence like self-discharging. As you read earlier, components are actively draining the battery's electrical power.

The power losses from a parasitic drain are significant enough to leave your battery dead after a few hours. 

A typical example is when someone leaves their headlights on after parking their vehicle and comes back to a battery that doesn’t have enough power to crank the engine.

Why Does Parasitic Drain Happen, and How Do You Fix It?

So, why does a parasitic drain occur? Well, it can happen due to human error or electrical faults. Both conditions will cause your vehicle to drain power from its battery even when parked without the ignition key.

Here are some probable causes of parasitic drain in batteries:

1. Interior and Exterior Lights

One of the most common reasons behind a parasitic drain is human error, such as leaving your lights on inside your car. It will drain battery power and quickly leave it too weak to start the engine.

It can happen with headlights, interior lights, and other lights (depending on your car’s design). Some models have small lights in the sun visor or glove compartment, which can act as significant parasites on your battery's power, especially over extended periods.

How to fix it: The easy fix is to be mindful of your vehicle’s lights, turning them off before you shut off your car. The thing is, not all lights will turn off automatically but require your input instead.

2. Faulty Relay Switches

Your car’s electrical system relies on several relay switches. Relays are electrically powered switches channeling electricity to where it’s needed. When the vehicle is off, the relays should also stay off and prevent anything from draining the battery.

However, a faulty relay switch will continue powering the car’s electrical and electronic components, expending electricity, and draining the battery.

How to fix it: You must replace all faulty relay switches. These parts aren't serviceable or repairable so new sets will fix the problem immediately.

3. Computer Modules and Accessories

The faulty relays above will continue to energise your car's computer modules, and other electrical accessories even though you're not using them. 

So, while the relays allow electricity to flow, the modules and accessories are the ones continuously draining the battery’s power.

These are very common with computerised modules that are discreetly hidden inside the car’s panels. The same can also occur with other electric-powered accessories like the in-car entertainment system or aftermarket devices you add to the vehicle.

How to fix it: Aside from fixing any faulty relays or switches, you should also troubleshoot the modules and accessories to clear them of any fault.

4. Short Circuits and Other Electrical Faults

Lastly, parasitic drain on car batteries can happen due to electrical faults like short circuits. 

A short circuit is when electrical current flows through an unintended path. For instance, the electrical current could flow through the vehicle’s frame while bypassing any relays or switches.

As a result, the battery’s power will continuously drain even when the vehicle is off and you’re not using anything onboard.

How to fix it: Electrical faults require more thorough troubleshooting by a qualified professional. They'll likely have to replace components like wires, switches, and fuses while ensuring no other faults occur. Find automotive electricians the easy way—check out CarpartAU’s directories to do a search.

A parasitic draw is just one of many potential problems your car’s electrical system can experience. That’s why it’s essential to keep up to date with the CarpartAU blog, where you’ll learn about other car issues and how to deal with them.


By Ray Hasbollah

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