What Is Parasitic Battery Drain and How To Test Parasitic Draw

Key Points

• The battery in a vehicle provides the electricity needed to power various components, including the starter.
• Batteries can wear out over time, and leaving lights on overnight can accelerate this process.
• Parasitic battery drain occurs when power continues to be discharged even after the engine is shut off.
• Common causes of parasitic battery drain include short circuits, electrical devices that remain energized and a faulty battery or alternator diode.
• Signs of a worn battery include difficulty starting the car, dim headlights, backfiring, dashboard lights but no starting and the need for frequent jump starts.

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.

Common Causes of Parasitic Battery Drain

When you turn your vehicle off, the 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 experience 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, such as:

• Under-hood or glove-compartment lighting.
• Headlights.
• Computer modules.
• Trunks.
• 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 Parasitic Battery Drain  

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 because 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!

Cranking/Clicking

As you turn your key in the ignition or press the start button on the dashboard or key fob, 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, you likely 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.

Backfiring

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 are 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.

How To Find Parasitic Battery Drain

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 which affects the reading on the multimeter. Here's how it's done: 

1. Remove the key from your ignition to shut off the engine and turn off all electronics in your car. Close anything that could draw power from the battery, including the doors, glovebox and trunk.
2. Pop the hood and locate the battery. Charge it with another vehicle or battery charging pack. This will provide you with the most accurate results.  
3. Use your digital multimeter to check your battery voltage. Most cars read between 12.2 and 12.6 volts when fully charged.
4. Place your multimeter’s red lead in the amps port, usually labeled "A". Although we read battery amperage in milliamps (1,000 mAs = 1 amp), it’s better to measure at a higher scale because if the current draining your battery is stronger than expected, it could damage your meter.
5. Place the black lead in the COM port.
6. Turn your multimeter's dial to DC voltage amps. You can identify DC voltage by a solid line above a dotted line. Amps are normally labeled as "A" here, too.
7. Disconnect your car’s negative battery cable. It’s typically black. If you’re unsure, look for the “-“ symbol. Measuring on the negative side is best practice because if you short-ground, you won’t ground your whole system.
8. Place a multimeter lead on the negative cable.
9. Touch the other lead to the negative battery terminal.
10. Remember, 1000 mAs are equal to 1 amp, and a normal battery draw is 50 mA. Therefore, if parasitic battery draw isn’t present, your meter shouldn’t read anything because you’re measuring above scale. But, if battery drain is an issue, your multimeter will show a measure of amps.
11. If there's an amp draw, clamp the leads in place.
12. Locate your vehicle’s fuse boxes. Your owner’s manual can help you find them. You’ll want to start at the main fuse box, which is typically under the hood.
13. Open the fuse box. Pull out the fuses, one by one, keeping an eye on your multimeter. If amps drop significantly when you pull out a specific fuse, you’ve found the culprit.
14. Read the schematic (usually located on the fuse box) to determine which vehicle component is causing the draw. Schematics include labels like “Starter Relay” “A/C Relay” or “Radio/Lights.”
15. If you have to check a fuse box inside the car, unclamp your meter before opening the door. If you don’t, it will place a heavy load on the meter and potentially damage it. When you open the door, shut off all electronics (light and radio), and do not place the key in the ignition or start the vehicle. Disable or depress the door switch to prevent it from drawing power and turning on the door-ajar light.
16. Once you’ve done this, re-clamp your meter and place it on the windshield so you can see it, or recruit a helper.
17. Repeat steps 13 and 14 until you find the source of the parasitic draw.

How To Fix Parasitic Battery Drain

If you’ve completed the above steps and determined where the parasitic draw is coming from, you can check this region of the car to see if something’s off. 

Sometimes, diagnosis requires reading a wiring diagram, which can be difficult without the help of a trained automotive technician. Taking your car to an automotive repair center and relaying your findings is your best bet at this point. They’ll be able to find the source of the issue and make repairs or replacements as necessary.

Parasitic Battery Drain FAQs

Can a parasitic battery drain be fixed without professional help?

Finding the cause of parasitic battery drain without professional help is possible, but resolving the issue might require a trained professional who understands wire diagrams and automotive electronic systems. 

How can I prevent parasitic battery drain?

To prevent parasitic battery drain, turn off all electrical components before leaving the vehicle. This includes unplugging chargers and switching off lights.

Do I need special tools for a parasitic draw test?

A digital multimeter is the primary tool needed for a battery draw test. This tool will help you measure the current draw from the battery. Additional tools like a test light or a clamp meter can also be helpful but aren't 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 the training and hands-on practice needed for an entry-level job as an automotive technician.^1,18

Courses over UTI’s 51-week program cover a range of topics, including vehicle electronic systems and technology.⁷ These courses can help you learn how to troubleshoot 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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