Parasitic Drain (All You Need To Know)

Parasitic Drain

A parasitic drain in a car is a common problem that can cause your car's battery to drain even when the vehicle is turned off. This issue can be caused by several factors, ranging from a faulty electrical component to a short circuit.

While it can be hard to diagnose a parasitic drain without professional help, there are some steps you can take to troubleshoot the issue on your own, and that is what this article is all about.

We will explore the common causes of a parasitic drain in a car and provide tips on how to diagnose the issue and what steps you can take to fix it.

Let’s get started!

What Is Parasitic Drain?

parasitic drain or draw refers to electrical current depletion within a vehicle when the ignition is deactivated. Despite being out of operation, it garners its name because it persists in siphoning power from the vehicle's battery.

This can be engendered by several electrical components that continue to exhaust power even after the vehicle has been turned off, such as the clock, radio, or power windows.

It could also stem from a defective component or wiring issue, such as the alternator, starter motor, or ignition switch, which may still draw power when the ignition is off.

Alternatively, a short circuit can result in a parasitic draw by providing an unintended path for current to flow through.

Additionally, a stuck relay may keep drawing power even after the ignition is turned off. A parasitic discharge could damage the vehicle's battery and cause it to become non-functional if left unaddressed.

It is vital to identify and rectify the root cause of parasitic discharge to prevent harm to the battery and ensure that the vehicle starts correctly.

What Causes Parasitic Drain?

When you turn off your vehicle, the battery's current continues to power various electronics and accessories, such as the engine's computer, alarm system, or internal clock.

While excessive parasitic battery drain can be problematic, most vehicles contain a "normal" amount of battery drain, which usually ranges between 50 and 85 milliamps in newer cars and less than 50 milliamps for older ones.

Several factors could cause parasitic battery drain, including short circuits or active electrical devices, such as under-hood or glove-compartment lighting, headlights, computer modules, trunk, or relay switches.

In addition to these circuits and devices remaining in the "on" position, a faulty battery or a bad alternator diode might also be responsible for excessive battery drain.

An average car's alternator contains up to six diodes that convert electricity from alternating current (AC) to direct current (DC), which the battery runs on. A worn-out diode can cause issues with the alternator, resulting in the battery failing to charge appropriately.

How To Diagnose Parasitic Drain

A multimeter is required to identify and troubleshoot the parasitic battery drain problem. Here's a step-by-step guide on how to use it:

Firstly, ensure the vehicle is switched off and the ignition key is removed.

Next, set the multimeter to DC Amps and adjust it to the highest amp level to avoid any damage to the device.

Then, disconnect the negative battery cable from the battery terminal, which is usually black. Ensure that the wire doesn't touch any grounded surface.

After that, connect the multimeter probes to the battery cable, keeping the multimeter to the side but at the same level as the battery terminal.

Now you can observe the reading on the multimeter to determine the parasitic drain. If there's no reading, gradually decrease the setting until a reading appears.

For a newer vehicle, a typical reading ranges from 50 to 85 mA, while an ideal reading for an older car is below 50 mA. If the reading exceeds these values, an electrical issue may require a skilled mechanic's attention.

This issue could be caused by a malfunctioning circuit that powers the car's accessories, such as lights, speakers, or the radio. A mechanic can locate the source of the problem and repair the electrical component to resolve the issue.

If the parasitic drain reading falls within the normal range, it's still important to take measures to maintain the car's battery. This involves driving the vehicle regularly and occasionally taking it for a long drive to ensure the battery is fully charged.

Symptoms of Parasitic Drain

In essence, an electrical system that doesn't behave as expected could indicate the presence of a parasitic draw or load. The most noticeable symptom of a parasitic battery drain is a dead battery.

Over time, the stored energy will be depleted, and you'll need to recharge, boost, or replace the battery to get the vehicle up and running.

Additional indications of a battery drain include:

  • Interior lights that don't turn off after the ignition has been switched off. This could result from a malfunctioning door switch circuit, a shorted interior light switch, or corrosion or resistance in a module. Failure of the electronics to enter a "sleep" mode, as they're designed to do, is a clear indication of a parasitic draw.
  • Electrical systems that don't operate normally. If the radio continues to play unexpectedly, the headlights refuse to turn off, or the windshield wipers keep going even after the key has been removed from the ignition, it could be due to a component failure or a faulty relay.
  • A replacement battery or alternator didn't resolve the problem. If you've swapped out a failed battery, alternator, or starter motor, but the issue persists, it could result from a parasitic load causing the part to fail.

Symptoms of a Worn-out Battery

Over time, your vehicle's battery will undergo wear and tear, eventually requiring replacement. However, a parasitic drain can accelerate this process and lead to a quicker battery failure.

Typically, some warning signs indicate an impending battery failure, and it's essential to pay attention to them and have your battery checked if you experience them.

Excess Cranking/Clicking

As you insert your key in the ignition and try to start your vehicle, the battery sends a current to the starter. If your car has a problem starting and you hear clicking or excessive cranking, the battery is likely malfunctioning.

Dimming Headlights

Your car might start even when the battery is wearing down, but if you notice that your headlights appear dim or weaker, this could be a sign that your battery is almost dead and incapable of providing enough power to electrical components.


A failing car battery can intermittently create sparks of electricity, igniting fuel accumulation in the cylinders and causing a backfire.

Dashboard Lights, but the Car Won't Start

If other electrical components of your car are functioning correctly, but it won't start, this could indicate a weakening battery.

Your dashboard lights or radio might turn on, but the battery may not be powerful enough to energize the starter.

Frequent Jump Starts

If you need to jump-start your battery repeatedly, it's an evident sign that it is near the end of its lifespan. It's crucial to replace it if this problem persists.

How To Prevent Car Battery Drain

Proper maintenance of your car battery is crucial for ensuring optimal vehicle performance. By adhering to these simple guidelines, you can prevent the battery from draining prematurely.

Regularly remove debris or dirt from the top of the battery to avoid corrosion or accumulation. Inspecting battery cables and terminals frequently to ensure a secure connection is also essential.

Avoid using electrical devices when the engine isn't running, as this can result in unnecessary battery drainage-instead, power on the car to operate accessories like the radio.

Be sure to securely clamp down the car battery to avoid vibration, which can reduce battery lifespan. Parking in a garage, away from the harsh elements, can also help prevent battery wear.

In colder weather, use a battery blanket to maintain optimal battery temperature. Taking longer drives can also keep your car battery charged and running smoothly.

Frequently Asked Questions

What causes parasitic drain?

Parasitic drain can be caused by various factors such as a malfunctioning alternator, faulty battery, damaged wiring or switches, or an electrical device that does not turn off when the vehicle is switched off.

How do I detect parasitic drains?

The easiest way to detect parasitic drains is to use a multimeter to measure the current flow from the battery.

Disconnect the negative cable from the battery and connect the multimeter between the negative terminal and the negative cable. If the reading shows above 50mA, you may have a parasitic drain.

How can I prevent parasitic drain?

You can prevent parasitic drains by ensuring that all electrical devices are switched off before turning off the vehicle, driving the car regularly to keep the battery charged, and parking in a garage or shaded area to prevent extreme temperatures from affecting the battery.

Can a dead battery be recharged?

Yes, a dead battery can be recharged using a charger or by jump-starting the vehicle.

However, if the battery is severely damaged or has reached the end of its useful life, it may need to be replaced.

How do I fix the parasitic drain?

To fix the parasitic drain, you need first to identify the root cause of the problem. This may involve inspecting and repairing faulty wiring or switches, replacing damaged electrical components, or repairing the alternator or battery.

It's best to consult a professional mechanic for assistance in diagnosing and fixing the parasitic drain.