What Is Parasitic Battery Drain?


The battery inside a vehicle provides the electricity needed to power a range of components, including the starter. It works by converting chemical energy into electrical energy needed to get a car started.

Like any other car part, batteries are something that will wear out over time — sometimes faster if you accidentally leave the lights on overnight!

In some cases, you might experience what is known as a parasitic draw, also known as parasitic battery drain, which is when a continuous and abnormal discharge of power occurs, even after the engine is shut off.

Walking out to your car ready to start it only to be greeted with a clicking noise can be frustrating. Keep reading to find out more about parasitic battery drain and how to deal with it.

Causes of Parasitic Battery Drain

When you turn your vehicle off, current generated by the car battery continues to be drawn and is used to power a range of electronics and accessories. These can range from your engine’s computer to the alarm system or internal clock.

While excessive parasitic battery drain is considered a problem, most vehicles contain a “normal” amount of battery drain. Typically, the normal amount of parasitic draw is between 50 and 85 milliamps in newer cars and less than 50 milliamps for older cars.

Several things could be causing problematic parasitic battery drain, including short circuits or electrical devices that remain energized, including:

  • Under-hood or glove-compartment lighting
  • Headlights
  • Computer module
  • Trunk
  • Relay switches

Apart from these circuits and devices remaining in the “on” position, you could simply have a faulty battery, or excess battery drain might be the result of a bad alternator diode.

The average car’s alternator has up to six diodes that electricity flows through and is converted from alternating current (AC) to direct current (DC), which is what the battery runs on. A worn-out diode can cause issues with the alternator, which can prevent your battery from charging.

Signs of a Worn Battery

Over time, the battery in your vehicle is going to wear down. It’s a part that will need replacing (usually after a few years, depending on how often you drive), but this could happen quicker as a result of a parasitic drain.

Batteries typically give some warning signs that they are on the fritz — be sure to pay attention and get yours looked at if you experience them!


As you turn your key in the ignition, your battery sends a current to your starter to get your vehicle going. If you have a hard time getting your car to start and hear clicking or excess cranking, it’s likely you have a battery issue.

Dim Headlights

You might be able to start your car when your battery is wearing down, but if you notice that your headlights are dim or weaker, this can be a sign that your battery is almost dead and unable to fully power electrical components.


Failing car batteries can sometimes cause intermittent sparks of electricity, which can ignite built-up fuel in the cylinders and cause a backfire.

Dashboard Lights, but No Starting

A sign of a weakening battery can be apparent if other electrical components are working but your car won’t start. You might notice that your dashboard lights or your radio turns on, but the battery isn’t strong enough to energize the starter.

Increased Number of Jump Starts

An obvious sign that your battery is running on its last legs is having to jump start it more and more often. It’s important that you get a replacement battery if this is happening continuously.

Parasitic Battery Drain Test

You can use a digital multimeter to determine how much draw is being placed on your battery. A multimeter is a tool that helps diagnose what the electrical draw and current is on the part.

Connecting the multimeter is something that should be done with the ignition off. It’s also important to follow safety precautions and wear protective gloves and eyewear.

While you can determine if there is too much draw coming from the battery, pinpointing where this drain is coming from is a more in-depth process that requires pulling out different fuses to see what affects the reading on the multimeter.

Taking your vehicle to a trained automotive technician is your best bet if you’ve determined you might be experiencing parasitic battery drain. They’ll be able to find the source of the issue and be able to make repairs or replacements as necessary.

Learn How to Fix Electrical Problems as an Automotive Tech at UTI

If you want to gain the skills and knowledge needed to diagnose and fix problems with your car’s battery and other parts of the electrical system, the Automotive Technology program at Universal Technical Institute (UTI) is a good place to start. The program can equip you with training and experience needed for an entry-level career as an automotive technician.1

Courses over UTI’s 51-week program cover a range of topics, including Vehicle Electronic Systems and Technology. This course can help you learn how to troubleshoot on vehicles and perform electrical tests, including ones for diagnosing parasitic battery drain.

Want to learn more? Request more information to get in touch with an Admissions Representative today.

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1 ) UTI is an educational institution and cannot guarantee employment or salary.
2 ) For program outcome information and other disclosures, visit www.uti.edu/disclosures.

Universal Technical Institute of Illinois, Inc. is approved by the Division of Private Business and Vocational Schools of the Illinois Board of Higher Education.


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