How to Find a Parasitic Battery Drain

Co-authored by Duston Maynes and Sophia Bell

Last Updated: September 6, 2024 Fact Checked

If your vehicle is drawing power from the battery and all of the lights and other electrical components are off, you might have a parasitic battery drain (or draw). Luckily, you can usually find the cause of a parasitic draw yourself. Start by connecting a digital multimeter to the negative battery terminal of your vehicle. Then, remove the fuses one at a time while watching for changes in the multimeter’s reading. Once the reading drops, you’ve found the culprit and can take steps to repair it.

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Part 1

Part 1 of 2:

Connecting the Digital Multimeter

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1
Unplug all devices from your vehicle and make sure no electrical components are on. Turn off the radio, heat or A/C, lights, windshield wipers, etc. and close your glovebox and lighted mirror covers. Engage the emergency brake, turn your vehicle off, and remove the key from the ignition if you haven't already. Then, close all of the doors and the trunk or back hatch so that none of the circuits are activated.^{[1]
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- Be sure to remove any cables that may be plugged into your vehicle, like a GPS unit or phone charger, as well.
Tip: In some cases, you may need to input a security code when you turn on the vehicle after disconnecting the battery, so check your owner’s manual for the code before you get started.
2
Charge your battery fully. In order to accurately detect the parasitic draw, you need to begin with a fully-charged battery. Pop the hood and locate your vehicle’s battery. Use a vehicle battery charger to charge the battery to 100%.^{[2]
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- Many car batteries are 12.6 volts. You can check the power with a multimeter to ensure the battery is fully charged.
- If your battery is old or damaged or isn’t reading 12.6 volts when fully charged, you may want to replace it before moving on.
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3
Remove the negative battery cable from the negative battery terminal. Find the negative cable, which will be marked with a minus sign (-) and may have a black cover over it. Remove the cover, if applicable, and use a wrench to unbolt the negative cable from the terminal.^{[3]
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- Be sure to use the negative, not the positive, cable to test for the draw to prevent electrical shorts!
- Generally, a 10-mm open-ended wrench is the tool you'll need to remove the cable.
4
Set up a digital multimeter. The multimeter has both a black wire and a red wire attached to it as well as several different input slots. Connect the black wire to the "com" (common ground) input and plug the red wire into the highest amp input (usually 20A). Set the dial on the multimeter to measure amps.^{[4]
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Expert Source

Duston Maynes
Automotive Repair Specialist

Expert Interview. 10 June 2021.}^{[5]
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- Choose a digital meter than can read up to 20 amps and down to 200 milliamps.^{[6]
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5
Attach the multimeter to the negative battery cable and terminal. Place the red lead through the metal circle at the end of the negative battery cable. Touch the black lead to the negative battery terminal.^{[7]
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- You can use plastic clamps to secure the leads in place so your hands are free and the meter continues to operate.
There are a few things that constantly draw power in a vehicle, such as the clock on the radio, so it’s normal to have a reading between 20 and 50 milliamps. If your reading is higher than that, it means there’s a draw and something is using too much power.^{[8]
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Part 2

Part 2 of 2:

Checking the Fuses

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1
Pull out the fuses one at a time while watching the multimeter reading. Locate the fuse box under the hood. Use a fuse puller to remove the fuses, starting with the ones with the smallest amp ratings and working toward the ones with the highest amp ratings. After you pull a fuse, check the multimeter to see if the reading changes. If it doesn’t, replace the fuse and move onto the next one.^{[9]
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- Once you’ve worked through all of the fuses in the fuse box under the hood, check the fuses in the fuse box(es) under the dash. It’s best if you have a friend help you with this so one person can pull fuses while the other person watches the reading on the multimeter. If you don’t have anyone to help, prop the meter against the windshield so you can read it from inside the vehicle.
Warning: Disconnect the multimeter before you open the door to check the fuses under the dash. Disable the door switch by clamping a piece of scrap wood over it so it remains depressed. Then, reconnect the multimeter.
Removing a fuse may cause the multimeter reading to drop a few milliamps, which is normal. What you’re looking for is a significant drop, such as if the reading goes from 3.03 amps to 0.03 amps. When this happens, you’ve found the electrical circuit that’s creating the parasitic draw!^{[10]
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3
Find out what’s running on the affected circuit. Consult the chart on the fuse box and/or the owners' manual to find out what components are powered by the fuse that’s causing the drain. You can also check the wiring diagram for the particular circuit to help you narrow down the problem.^{[11]
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- Usually, you can find digital copies of the manual and wiring diagrams online.
4
Check each device or component on that circuit. Replace the fuse and disconnect each lamp, heater, or electrical device one at a time. Flip all of the switches that correspond to the components as well. Watch for the reading on the multimeter to drop to find out which component is causing the drain.^{[12]
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- For instance, say the offending fuse controls the power antenna as well as the radio. Unplug the radio and see if the draw goes away. If it doesn’t, unplug the antenna and watch for the reading on the multimeter to drop.
5
Repair the component causing the draw, disconnect the multimeter, and reconnect the battery. The repair process will vary widely depending on what the problem is, so if you’re not sure how to fix the component, hire a certified mechanic to make the repair or replacement. If you can make the repair yourself, check it by ensuring the reading on the multimeter is less than 50 milliamps. Once you’re done, disconnect the multimeter and reconnect the battery cable.
- You may be able to simply flip a switch into the "off" position to eliminate the draw, or you might be facing a more complex problem, such as an issue with a wiring harness.

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Question

Why is the ammeter connected between the negative terminal and cable instead of the positive side?

Community Answer

The ammeter can be connected to either terminal with identical results with one exception: if you get against a metal object while using a positive terminal, there is potential for a short circuit with a shower of sparks. Use the negative terminal - that is safer.

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If a new stereo was installed and it wasn't done properly, can that drain the battery?

Community Answer

Yes, if it was connected straight to the battery rather than through the ignition. It should only draw a small amount for the clock when the ignition is off. If you can use the radio with the ignition in the off position, you have a problem.

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Does a car alarm drain the battery?

Community Answer

Yes, it can. If checking for a drain, the alarm should be the first thing to be disconnected to see if the drain stops.

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Tips

Don't forget to check inside the cigarette lighter and power sockets. Leaving a phone charger plugged in for a long time can cause a parasitic draw. Also, sometimes coins can fall in the sockets and cause shorts.

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50 milliamps is a good rule of thumb for max allowable parasitic draw. Anything over 50 milliamps would require further investigation into the exact device which is drawing power.

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Warnings

Connect the multimeter to the negative battery terminal, rather than the positive one, to prevent electrical shorts.

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Exercise caution when working with your car's electrical system. Protect your eyes and skin with goggles and gloves.

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The information in this article is intended to provide general answers for frequently asked questions about this topic, and may not be applicable to all vehicles. Please refer to your vehicle’s owner’s manual for details about maintenance intervals and other vehicle specifications. If you are unsure about your ability to perform any repair, we recommend contacting a certified automotive technician to perform the necessary work.

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Consider having this done by a professional. Disconnecting and reconnecting your battery incorrectly can cause damage to your vehicle's electrical system, or it could reset the module, causing the drain to stop, but only temporarily.^{[13]
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Expert Source

Duston Maynes
Automotive Repair Specialist

Expert Interview. 11 June 2021.}

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