How to Conduct a Parasitic Draw Test Plus Causes and Prevention Tips

It’s frustrating when your car battery fails out of nowhere, especially when it worked just fine yesterday. If you don’t spot any issues that could have caused it, there might be something in your car that’s wasting electricity when you’re not driving.

Here are some of the signs that point to a car battery that suffers from a parasitic draw:

• Some electronic devices in the vehicle don’t function properly.
• Lights, the radio, and other devices don’t switch off even when the ignition is off.
• Electronic devices don’t enter sleep mode or disable themselves by default.

Testing for a parasitic draw is the best way to determine whether something is siphoning power from your battery.

How to Test for a Parasitic Draw

To test your vehicle for a parasitic draw, you’ll need the following tools:

• Multimeter (inductive or standard DVOM)
• Zip-ties
• Fuse puller
• Wrench appropriate for your vehicle for battery terminal removal
• Infrared camera if available

Once you have everything, follow these steps to determine whether a parasitic draw is affecting your vehicle.

Disable All Electrical Components and Turn off the Ignition

The first step is to ensure nothing is drawing power from the battery. Unplug or deactivate your vehicle’s electrical components. Turn off the radio, air conditioning system, heater, audio system, and other parts that get power from the car battery.

You must also park your vehicle, switch off the engine, and disable the ignition. Make sure all the doors are closed properly to disable the lights that turn on when they’re open.

Charge the Car Battery

When testing for a parasitic draw, the battery must be full to get reliable and consistent results. After all, batteries low on charge won’t give accurate results, while a full battery guarantees that you’ll get readings based on your vehicle at peak performance. Disconnect one battery cable when charging and leave it disconnected for the remainder of the test if you’re using a multimeter.

Set up the Multimeter

Once the battery is fully charged, set up the multimeter by attaching it to the battery and the cable you removed as shown in the illustration, which shows the test being done on the positive side of the battery. This test can be done on either side of the battery (use clamps or zip ties of your choice to make the connections stay in place during testing), but connect the black DVOM terminal to the negative battery post if you’re doing it this way on the negative side of the battery.

Because parts of many electrical modules on vehicles must be “charged up,” you may see more current draw initially than you will if you let the car sit with the meter connected for a full hour, which gives the modules time to charge up and “sleep.” After an hour, you’ll be able to see if you have more than 0.050 amps (50 milliamps). More than that and the battery will be dead within 2 days on most vehicles.

Because parts of many electrical modules on vehicles must be “charged up,” you may see more current draw initially than you will if you let the car sit with the meter connected for a full hour, which gives the modules time to charge up and “sleep.”

– Richard McCuistian, ASE Certified Master Automobile Technician

Find the Source

Open your fuse box, then, one by one, pull the fuses connected to the circuit and check the reading on the multimeter. Better yet, have an assistant watch it while you remove the fuses one at a time, first from the junction box under the hood and then from the other fuse panels in the vehicle.

Pro Tips are nuggets of information direct from ASE-certified automobile technicians working with CarParts.com, which may include unique, personal insights based on their years of experience working in the automotive industry. These can help you make more informed decisions about your car.

Pro Tip: You’ll need to know where all the panels are before starting this project. Some vehicles have fuse panels under the rear seat cushion or in the trunk, so be aware of where yours are.

If the current draw decreases exponentially after pulling a fuse, take note of which circuit the fuse was connected to and disconnect any components fed by that fuse to further isolate the source of the current draw. After this, reconnect the fuse and check the circuit. Be sure to take note of the different switches and devices that correspond to the circuit.

What Causes Parasitic Draw?

Parasitic draws are as inconvenient as they are frustrating, so it pays to know what causes them. It’ll help you pinpoint potential sources of parasitic draws and take action appropriately.

Aftermarket Accessories

Aftermarket accessories are notorious for causing parasitic draws. While they work just fine, there’s a chance they weren’t designed to work well with your electrical system specifically. They might continue drawing power from the battery even when not in use.

Short Circuits

Another potential cause for parasitic draws is short circuits.

Short-circuiting is when electrical currents don’t follow their set paths, taking unintended shortcuts. This can lead to circuits blowing because too much electricity runs through one route instead of being evenly distributed.

Short circuits are often caused by electrical overloads, damaged wiring, wires that haven’t been connected properly, and defects during manufacturing. It’s difficult to avoid short circuits, but proper maintenance and knowing when to replace old or damaged parts minimizes the risk tremendously.

Stuck Relays

Stuck relays can cause parasitic draw by drawing power from the car battery even when the engine has been switched off. Relays typically get stuck when dust or debris gets into them or when they get damaged.

Wiring Issues

Last but not least, one of the most prominent causes of parasitic draw is wiring issues. As mentioned previously, wires that haven’t been connected properly can cause short circuits, which leads to parasitic draws.

In other words, poorly wired devices could draw power even when the engine is turned off. Typical culprits include the radio and electric windows.

How to Prevent Parasitic Draw

Knowing how to find the source of a parasitic draw is one thing, but knowing how to prevent it is another. Here are some handy tips to help you minimize the risk of a parasitic draw.

Regular Maintenance

Keeping your vehicle well-maintained goes a long way in preventing parasitic draws. The draws usually happen when your vehicle’s electronics malfunction or are incompatible with your car. Regular upkeep will help you nip potential issues in the bud.

Take care of your vehicle’s electricity-powered tech and accessories to minimize the risk of draining your battery.

Use a Trickle Charger

Trickle chargers take longer to fully recharge a battery compared to conventional chargers because they provide a slow and steady power output. They also help extend battery life, helping you save money and allowing you to leave the car battery charging overnight.

These chargers help prevent the loss of power brought about by parasitic draw by providing your battery with constant power. There’s also no risk of overcharging and damaging a battery because the power output is so weak.

Drive Regularly

It’s a good idea to drive your vehicle around for extended periods to keep the alternator moving. This provides your car battery with a little extra juice, all while keeping the circuits flowing.

Dust also settles and builds up when your vehicle isn’t in use and this can up the chances of relays getting stuck as well as short circuits. Worst of all, inactivity can kill car batteries.