How to perform the parasitic draw test?

How to perform the parasitic draw test?

How to perform the parasitic draw test?

A car battery, or an automotive battery, is a rechargeable power source designed for motor vehicles. It supplies electricity to the electric starting motor, activating the internal combustion engine that uses chemical energy to move the car. The battery also powers the car’s electrical systems while the engine is running, and the alternator keeps it charged as the electricity demand fluctuates.

In some cases, your car’s battery drains faster than before, and you should know what exactly is causing this problem. Performing a parasitic draw test is essential in diagnosing electrical issues in a vehicle. It involves measuring the amount of current drawn from the battery when the car is turned off.

This test is significant because it can help identify the source of a battery drain that might be causing problems, such as a dead battery or intermittent starting issues. There are multiple ways to perform this test, but I will tell you about the most efficient and safe way to test the parasitic draw. You should also know that there is a parasitic draw tester available online which you can purchase for directly testing battery drain problems.

You can watch this video here from humblemechanic for a visual demonstration of performing the parasitic draw test.

What is parasitic battery drain in a car?

Parasitic battery drain in a car is the battery’s continuous discharge when the vehicle is turned off, and all electrical systems are supposed to be in standby mode. It occurs when an electrical component or design in the vehicle draws power from the battery, even when the car is not in use. This can lead to a dead battery if left unchecked.

An average parasitic draw for newer cars is between 50 milliamps to 85 milliamps, but it could be much higher depending on how the vehicle is equipped. An excessive parasitic draw can run down the battery faster than expected, causing it to die.

Batteries have a self-discharging system. Self-discharge is a process in batteries when the battery’s stored charge is reduced without any external circuit or electrode connection. As a result, batteries have a shorter shelf life, and when used, they don’t hold a full charge. 

Causes of parasitic battery drain

1.   Battery Age and Condition

As batteries age, they become less able to maintain a charge. The battery may not be able to provide enough power to start the automobile, even if it is fully charged, according to this implication. A new battery might be required if the parasitic drain is being brought on by the battery.

2.   Faulty Alternator

Battery parasitic drain can also be brought on by a faulty alternator. The battery may discharge overnight if the alternator’s diode is broken. If the drain persists after disconnecting the alternator’s wiring to verify if it is the alternator, the alternator may need to be replaced.

3.   Alarm System

Battery parasitic drain might also be brought on by an alarm system. Even when the car is off, the alarm system may still use battery power if it isn’t working properly. If the alarm system is what’s causing the drain, it might need to be fixed or replaced.

4.   ECM and Sound System

While it is rare for the ECM (engine control module) to cause parasitic battery drain, it is typical for the sound system to cause this problem. If the good plan is the cause of the drain, the wiring from the proper system fuse can be disconnected to stop the drain.

The Best way to perform a Parasitic draw test.

You would want to find out what is causing your car’s battery to drain much faster. You might need to diagnose that to be able to fix the issue. You can use a parasitic draw tester or a multimeter to test. The best way to perform a parasitic draw test is to follow these steps:

  • To perform a parasitic draw test, ensure that everything is turned off, including all interior lights and accessories, and the key fob are far away from the vehicle. Then, let the car sit for at least 15 to 45 minutes, or however long the service manual recommends, to allow time for all the modules to enter sleep mode. .
  • Next, connect an inductive or amp clamp to one of the battery cables to measure the draw from the battery. The amp clamp can be connected to either of the battery cables, but in most cases, the negative line is easier to clamp around. The draw from the battery should be below the specifications for the vehicle, which can be found in the service manual..
  • One method for finding which circuit the parasitic draw is in is to pull the vehicle’s fuses one at a time until the amperage drops below the specifications for that vehicle. However, removing the fuses on modern cars can cause various modules or computers to wake up and cause a false reading on the metre. Instead, measure a voltage drop across the fuses until one reads other than zero. .
  • The circuits that are not drawing current will measure a zero voltage drop across the fuse, while courses that are drawing current will measure a voltage drop across the fuse, and the higher the current draw, the higher that number will be. .
  • Check the owner’s manual or the service manual for the fuse panels’ location, typically under the hood, under the dashboard, or behind the glove compartment..
  • After locating the fuse that has the voltage drop across it, pull that fuse, and the reading on the ammeter should drop down to specification. Follow the wiring diagram in the service manual for that circuit to determine which of the components or wiring connections in that circuit are causing the problem. .
  • Once the problem has been resolved, reinstall the fuse into the fuse panel and ensure there is no longer a voltage drop across the fuse.

For a multimeter parasitic draw test, you can watch this video here.

Final Thoughts

No one would like to see their car’s battery draining faster even though the engine is not running. A parasitic draw test is a crucial diagnostic tool for anyone experiencing battery drain issues in their vehicle. By following the steps outlined in this article, you can identify the source of the problem and take appropriate action to fix it.

Whether it’s a faulty alternator, alarm system, or sound system, the parasitic draw test can help you pinpoint the issue and prevent future battery drain.

Always consult your owner’s or service manual for specific instructions and safety precautions when performing any automotive repairs or diagnostics..


A: The length of a parasitic draw test can vary depending on the vehicle and the severity of the issue. Generally, a parasitic draw test can take 30 minutes to an hour or longer.

A: The cost of a parasitic draw test can vary depending on the location and the shop performing the test. Generally, you can expect to pay anywhere from $50 to $150 for a parasitic draw test.

A: A parasitic draw can damage your vehicle’s battery and electrical components if left unchecked. Identifying and fixing any parasitic draw issues as soon as possible is essential to prevent further damage.

A: A parasitic draw can cause a vehicle’s battery to become drained, which can lead to a no-start condition. By identifying and fixing the issue causing the parasitic draw, you can prevent further problems and ensure your vehicle is reliable.

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