What Causes Parasitic Drain on Your Car Battery? 

If you’ve experienced a dead battery on a regular basis, it might not actually be due to a bad battery. Instead, it may be a condition known as a parasitic drain or draw where something is staying on that’s draining the battery over time. It’s frustrating to pull out the booster cables or a battery booster pack to fire up the engine, especially if you aren’t sure why it was dead in the first place.

Parasitic draws are among the trickiest to locate and fix, and first, you need to recognize the problem. What exactlyh is a parasitic draw, and what can you do about it? Here’s what you need to know.

What is Parasitic Draw? 

A parasitic draw is a type of electrical current drain that occurs in a vehicle when the ignition is turned off. It is called a parasitic draw because it continues to drain power from the vehicle’s battery, even though the vehicle is not in use. This can be caused by various electrical components that continue to draw power even when the vehicle is turned off, such as the radio, clock, or power windows. It could also be caused by:  

  • A faulty component or wiring issue – A component that is malfunctioning or has a wiring issue may continue to draw power even when the ignition is turned off. This could include the alternator, starter motor, or ignition switch. 
  • A short circuit –  A short circuit can cause a parasitic draw by allowing current to flow through an unintended path. 
  • A stuck relay – A stuck relay may continue to draw power even when the ignition is turned off. 

A parasitic draw can be harmful to the vehicle’s battery and may cause it to go dead if it is not addressed. It is important to identify and fix the cause of a parasitic draw to prevent damage to the battery and ensure the vehicle starts properly. 

How to Recognize Battery Drain 

Essentially, if one of the electrical systems isn’t working like you expect it should, it could be an indication of a parasitic draw or load. The most common symptom you’d expect to come across is a dead battery. Obviously, a parasitic battery drain will eventually run down the stored energy and you’ll need to recharge, boost, or replace it to get the car going again.  

Other symptoms of a battery drain include:  

  • Interior lights don’t turn off when the ignition has been turned off. It could be due to any number of conditions including a door switch circuit that’s failed, an interior light switch that’s shorted, or corrosion or resistance in a module. If the interior electronics don’t go to ‘sleep’ like they’re designed to do, it’s a sign that there’s a parasitic draw. 
  • Electrical systems that aren’t working normally. If the radio keeps playing unexpectedly, the headlights don’t turn off, or the wipers continue going after the key is out of the ignition, it could be due to a component failure or a bad relay. 
  • A replacement battery or alternator didn’t fix the problem. If you’ve replaced a failed battery, alternator, or starter motor but the problem re-occurs, it could be due to a parasitic load that’s killing the part. 

How to Test for Battery Drain 

There are several steps you can follow to test for a parasitic load (also known as a parasitic draw) in a vehicle: 

1. Start with a fully charged battery

You could get inconsistent or inaccurate results if the battery isn’t charged.

2. Set up your equipment

Connect the reds lead of the multimeter to the negative cable end of the battery, and the black lead to the battery terminal, then disconnect the cable without breaking this bridge’s connection.

3. Turn off all electrical components

Turn off all electrical components, including the radio, headlights, and interior lights. Then, wait for the vehicle’s computer modules to go to ‘sleep’, which normally takes 10 to 15 minutes but could take upward of an hour.

4. Measure the current draw

Look at the multimeter and note the current draw.

For newer cars, a reading between 50 milliamps (mA) and 85 mA is considered normal. This amount of draw won’t kill a battery over a week, let alone overnight. If you have a higher draw than 85 mA, particularly if it’s over 100 mA, you should try to track down the source of the draw and fix it. 

How to Correct the Issue 

Testing for a parasitic draw can be tricky, and locating the problem can take time since the parasitic draw might not always be present. If the multimeter indicates excessive draw, begin to track down the problem by pulling fuses and removing relays one at a time until the draw goes away. Once the draw disappears, you’ve likely found where the issue is.  

Now comes the hardest part: tracing the circuit for the short or identifying if the part has an internal problem. Often, you can start by replacing the part with a known good one before cutting any wiring looms open. 

If you decide that it’s too big a job to tackle on your own, let AutoZone help you find qualified professional mechanics through our Shop Referral Program.  

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FAQ/People Also Ask 

How do you fix a parasitic battery drain?

Begin by isolating the affected circuit by pulling fuses and relays individually. Then, when you’ve found the right circuit, trace the wiring and check the associated parts for problems.

What could be draining my car battery while it’s off?

The draw could be from virtually any circuit. It could be lighting, the audio system, the alternator, the starter, or an aftermarket accessory you’ve installed, for example.

What often causes high parasitic drain?

Common causes of high parasitic drain include a circuit that’s shorted to ground or a computer module that isn’t going to sleep.

Can an alternator cause a parasitic drain on a battery?

Alternator diodes can cause a parasitic drain. It creates a closed circuit that will deplete the battery’s energy overnight with the ignition off.

How do you find the source of a parasitic drain?

You need to use a multimeter to trace the source of a parasitic load. It can be time-consuming to locate, even for a trained mechanic.

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