How To Test Parasitic Draw (short) – Best Method

Have you ever returned to your parked car only to find a dead battery, leaving you stranded and frustrated? If so, you might be dealing with a parasitic draw, a common yet often overlooked issue in the automotive world. A parasitic draw, also known as a short, is an unintended electrical load that silently drains power from your vehicle’s battery even when the engine is off.

In this article, we’ll delve into what exactly a parasitic draw is, the common signs to look out for, and essential steps to diagnose and address this issue effectively and more.

parasitic draw

what is a parasitic draw?

A parasitic draw or a short, in automotive terms, refers to an unintended electrical load that drains power from your vehicle’s battery even when the engine is turned off. Essentially, it’s like a hidden power leak that can lead to a dead battery if left unchecked.

These small but continuous drains can arise from various components and systems in your car, such as electronic modules, alarms, clocks, and even malfunctioning switches. In essence, a parasitic draw can silently sap your battery’s energy over time, causing inconvenience and potential breakdowns.

Think of a parasitic draw as if you forgot to close your cars dome light. The light is taking electric energy from your car battery and will eventually “kill it”.

Addressing and diagnosing this issue is crucial for maintaining the health of your vehicle’s electrical system and ensuring reliable starts.

What to test before Replacing your battery?

So if your are reading this the chances are that your car battery is dead. Now, before changing your car battery you might want to consider checking a few quick things to ensure your new battery go bad as well.

how to test The Battery

It is very possible that there is no problem with your car and that your battery is just old and no good anymore. Here’s how you can test a car battery:


First, you can simply start by using a multimeter to check the voltage of your battery. Set the multimeter to the DC voltage setting, connect the red (positive) probe to the positive terminal of the battery, connect the black (negative) probe to the negative terminal of the battery, and read the voltage displayed on the multimeter.

A fully charged car battery should be at about 12.4V to 12.6V anything below that indicates there is a problem. The different problems can be as follows:

  • Simply a bad battery
  • A bad alternator
  • A parasitic draw

Cold Cranking Amps (CCA) Test

The next test is a Cold Cranking Amps (CCA) Test. CCA measures a battery’s ability to start the engine in cold temperatures. To test the CCA with a multimeter, start the car with the multimeter connected to your car battery. A good rule of thumb, if the battery voltage drops below 10V when starting, then the battery might be faulty.

Load Test

A load test helps assess the battery’s ability to deliver power under a heavy load. This requires a battery load tester. Here’s how:

  • Follow the load tester manufacturer’s instructions.
  • Connect the load tester’s positive and negative leads to the corresponding battery terminals.
  • Apply the load as indicated in the tester’s instructions.
  • Observe the voltage. If it drops significantly and doesn’t recover, the battery might be weak and in need of replacement.

Battery charger

Another great way to test a battery is to charge it. Utilizing a battery charger to charge the battery, check if it is able to fully charge. You can also check if it is capable of holding a charge by testing the voltage while charging a right after disconnecting the charger.

how to test The Alternator

So if the previous test showed the battery is good, the problem may be the alternator. To test the alternator simply follow the steps below:

  • First start the car.
  • With your multimeter, locate the positive terminal of the alternator (usually at the back of it) and the negative it the alternator case itself.
  • Set the multimeter to DC volts and you want it to be around 13.6V and 14.6V

Parasitic draw

Finally, if everything check out to be good the last thing you can check is for a parasitic draw. The next section will showcase step by step how to test that.

How to diagnose a parasitic draw (step by step)

Here are the following steps to perform a parasitic draw test:

step 1: Tools required for a parasitic draw test

The first step consist of gathering the basic mechanic tools required for this job which are:

  • Digital multimeter capable of measuring current (amp)
  • Fuse puller or pliers
  • Pen and paper for notes
  • Owner’s manual or wiring diagram for your vehicle
  • Protective gloves and eye protection

step 2: Preparation

Park your vehicle in a safe and well-ventilated area, ensuring the engine is turned off and the key is removed from the ignition. Disconnect the negative terminal of the vehicle’s battery to prevent accidental short circuits during the testing process. Wait about 10-15 minutes before testing, this insure that all of the car computers and system are fully shut.

step 3: Test for a parasitic draw

Begin by setting your multimeter to the DC amperage setting. Connect the multimeter in series between the disconnected negative battery cable and the negative battery post. This will allow you to measure the current flowing from the battery.

If the reading exceeds 50 milliamps (0.05 amps), there might be a parasitic draw that needs to be addressed. This number can vary slightly from car to car.

step 4: locate the fuse box

Locate the two fuse boxes of your vehicle, which are usually found in the engine bay and under the dashboard. Consult your vehicle’s manual if you’re having trouble locating it.

step 5: remove fuses

Carefully remove fuses one by one from the fuse box. Use the insulated pliers to avoid any accidental contact. As you remove each fuse, monitor the multimeter’s reading. If the reading drops significantly after removing a certain fuse, you’ve likely identified the circuit causing the parasitic draw. After removing each fuse and looking at the result you can put it back in its place before pulling an other one.

step 6: Identify the Problematic Circuit

The fuse that caused the drop in current is connected to the circuit responsible for the parasitic draw. Refer to your vehicle’s manual to determine what components are associated with that circuit.

step 7: Inspect and Troubleshoot

With the problematic circuit identified, inspect the corresponding components. Look for any signs of damage, corrosion, or frayed wires. Additionally, check if any electrical components associated with the circuit are malfunctioning, as they might be drawing excessive power.

Step 8: Repair or Replace

Depending on your findings, you may need to repair or replace the faulty components. This could involve fixing damaged wires, replacing malfunctioning switches, or addressing any other issues you’ve identified. Once the necessary repairs are made, reassemble the fuses and reconnect the negative battery terminal.

What are parasitic draw common causes

A short is simply an electrical problem with your car. While not every electrical issue results in a parasitic draw, anything in a vehicle that uses electricity has the potential to cause such a draw if it’s not functioning properly.

This includes lights, switches, sensors, control modules, and other components. If any of these electrical components malfunction or stay active when they should be off, they can lead to excessive power consumption, which drains the vehicle’s battery over time.

Here’s a few common problems that can cause a short:

  • Faulty Relays: Relays that are stuck in the closed position can cause continuous electrical flow even when the vehicle is off.
  • Short Circuits: Wiring issues, such as frayed or damaged wires, can cause unintended electrical connections and create parasitic draw.
  • Aftermarket Accessories: Poorly installed or malfunctioning aftermarket accessories, such as alarms, stereos, or GPS trackers, can contribute to parasitic draw.
  • Module or Control Unit Malfunction: Various electronic control units or modules in the vehicle can malfunction and draw excess power when they should be idle.
  • Faulty Switches: Faulty switches, such as power window or door lock switches, can remain engaged and draw power.

What are common sign your car as a parasitic draw?

A parasitic draw, also known as a parasitic battery drain, occurs when an electrical device in a vehicle continues to draw power even when the vehicle is turned off. This can lead to a dead or drained battery if left unchecked. Common signs of a parasitic draw include:

  • Dead Battery: One of the most direct signs of a parasitic draw is a dead battery.
  • Dashboard Lights, but No Starting: In some cases, when you turn the key to start your vehicle, you might see dashboard lights come on, but the engine doesn’t actually start.
  • Increased Number of Jump Starts: If you’re finding yourself needing to jump-start your vehicle more frequently than usual, it can indicate that the battery is consistently being drained due to a parasitic draw.
  • Slow Cranking/Clicking: When you attempt to start your vehicle, you might notice that the engine cranks more slowly than usual or produces repetitive clicking sounds from the starter.
  • Dim Headlights: If your headlights appear noticeably dimmer than usual when you turn them on, it could be an indicator that the battery’s voltage is lower due to a parasitic draw.

What Causes a Car Battery to Go Bad?

Car batteries can go bad for several reasons, and understanding these factors can help you take better care of your battery and extend its lifespan. Here are some common causes of car battery failure:

  • Age: Car batteries have a limited lifespan. Generally, they last around 3 to 5 years, depending on factors like usage, climate, and maintenance. As a battery ages, its capacity to hold a charge diminishes, leading to reduced performance and eventually failure.
  • Temperature Extremes: Both extreme cold and extreme heat can affect a battery’s performance and lifespan. In cold weather, a battery’s capacity to deliver power is reduced, making it harder to start the engine. High temperatures can cause the battery’s electrolyte to evaporate, which can damage the internal components.
  • Overcharging: If the vehicle’s charging system is not functioning properly, it can lead to overcharging the battery. Overcharging can cause the electrolyte to evaporate, leading to internal damage and reduced battery life.
  • Undercharging: Frequent short trips or not driving the vehicle for extended periods can result in undercharging the battery. This can lead to sulfation, where lead sulfate crystals build up on the battery plates, reducing its ability to hold a charge.
  • Parasitic Draws: As mentioned earlier, parasitic draws occur when there’s a continuous drain on the battery even when the vehicle is turned off. Components like interior lights, faulty switches, and aftermarket accessories can contribute to parasitic draws and eventually lead to battery depletion.
  • Vibration: Excessive vibrations, often experienced in rough driving conditions or due to poor battery mounting, can cause internal damage to the battery’s cells and plates, leading to reduced battery life.
  • Corrosion: Corrosion at the battery terminals can impede the flow of electricity between the battery and the vehicle’s electrical system. This can lead to poor charging and discharging cycles, eventually affecting the battery’s overall health.
  • Sulfation: Sulfation occurs when sulfate crystals build up on the battery plates, reducing the battery’s ability to hold a charge. This can happen if the battery remains in a discharged state for prolonged periods.
  • Manufacturing Defects: Occasionally, a battery might have manufacturing defects that impact its performance and longevity. This could include issues with the internal components or the way the battery was sealed and constructed.

how to clean the terminals?

Before you reconnect the cables to the new battery, thoroughly clean the terminals and cable connectors to ensure a good electrical connection. Corrosion can hinder the flow of electricity. Here’s how you can do it:

  • Use a mixture of baking soda and water to create a paste. Apply this paste to the terminals and cable ends.
  • Use a wire brush or an old toothbrush to gently scrub away any corrosion or dirt.
  • After scrubbing, rinse the terminals and cables with water and dry them off before reconnecting.
  • Lastly, if you need to you can go one step further and use your wires brush(clean) or a small piece of sand paper and scrap the terminal down to reveal clean metal.

how to buy a new car battery?

Buying a new car battery involves a few steps to ensure you get the right battery for your vehicle’s make and model. Here’s a guide on how to buy a new car battery:

  1. Check the Owner’s Manual:
    • The first and most reliable source of information about your car’s battery specifications is the owner’s manual. Look for a section that provides details about the recommended battery size, group size, Cold Cranking Amps (CCA), Reserve Capacity (RC), and other relevant specifications.
  2. Check the Current Battery:
    • If you have access to the current battery in your car, you can often find a label with the battery specifications printed on it. Look for labels on the battery case that provide information such as group size and CCA.
  3. Check Online Resources:
    • Many online resources and battery retailer websites offer lookup tools that allow you to input your car’s make, model, and year to find the appropriate battery specifications.
  4. Contact a Professional:
    • If you’re still unsure about the battery specifications, consider contacting a professional mechanic or an auto parts store. They can provide guidance based on your vehicle’s make and model.
  5. Inspect the Battery Compartment:
    • Sometimes, the battery compartment in your car may have a label or sticker that indicates the recommended battery specifications. This is less common but worth checking.
  6. Visit an Auto Parts Store:
    • Visit a local auto parts store and consult their staff. They often have databases that can help you find the correct battery specifications based on your vehicle’s information.


In conclusion, your vehicle’s battery plays a crucial role in its overall functionality, and understanding the threat of a parasitic draw empowers you to take proactive measures.

By grasping the concept of this hidden energy drain and recognizing its common causes and signs, you can intervene before being left with a lifeless car. Armed with the knowledge of how to test your battery’s health, diagnose a parasitic draw, and even assess your alternator’s performance, you can ensure that your vehicle’s electrical system remains robust and reliable.

As you journey through the intricacies of automotive maintenance, remember that vigilance and preventive actions are your allies in preserving your car’s health and avoiding unexpected breakdowns. With these insights, you’re better equipped to keep your car ready to hit the road whenever you are.