How to do a Parasitic Battery Drain Test

A car battery drain can leave you stranded anywhere.
A car battery drain can leave you stranded anywhere. | Source

A parasitic battery drain is when abnormal and continuous discharge of power occurs after having shut off the engine. Usually, this is caused by a short circuit or an electrical device that remains in the "on" position or energized, such as:

  • A trunk
  • An under-hood or a glove-compartment light
  • A switch
  • A computer module

But a battery power drain can also be the work of a bad alternator diode or even a faulty battery.

Depending on the amount of current being drained, a parasitic draw will suck all the juice from your battery in a few hours or minutes.

Here, you have four tests to help you find the culprit:

  1. The amperage battery drain test
  2. The voltage drop battery drain test
  3. The bad alternator diode test
  4. The car battery hydrometer test

The first three tests require the use of a digital multimeter. If you don't have one, you can buy a useful multimeter for around 20 dollars. And you don't need to have much experience in car repair to apply any of these tests.

Table of Contents

Here are the sections you'll find in this guide. Skip to any particular part you are interested in or read about the tests in the order presented here (recommended) to follow a more systematic approach to your repair.

  • Preparing for the Test
  • The Amperage Battery Drain Test
  • The Voltage Drop Battery Drain Test (alternative to the Amperage test)
  • Bad Alternator Diode Test
  • Car Battery Hydrometer Test

Preparing for the Test

Before you start your tests:

  • Turn off all accessories and remove the key from the ignition switch.
  • Unplug any device from the lighter socket, even if your device is turned off.
  • Unscrew the light bulb from under the hood just enough to turn it off.
  • If your vehicle has a fuse box under the dashboard—driver's side—open the driver's door and use a clamp or similar tool to press and hold the door switch. Leave the door opened. The clamp will keep the dome light off during your test.
  • If there's another fuse box on the passenger's side, follow the above procedure so that you have access to that fuse box during your tests as well.
  • To avoid confusion, locate the fuses for your dash clock, stereo, and car computer, if necessary, so that you know that those circuits will show the presence of current during your tests. To know what circuit a fuse protects, look for the description on the fuse box lid, or look up the information on your vehicle service manual.
  • If you don't have one, you can buy an aftermarket repair manual for your particular car model online or from one of your local auto-parts stores. However, your local public library may have a copy of the manual in the reference section as well.
  • The amperage battery drain test requires disconnecting one of the battery cables. If your car radio or alarm system needs a code to reactivate it after cutting battery power, make sure you know the code to turn it back on.
  • Finally, make sure the car battery you're using for this test is fully charged.

The first test—the amperage battery drain test—is best suited for older vehicle models, 1990s and older, that come equipped with one or two computer modules. With newer cars with more control modules, usually you need to wait 30 minutes or so for the modules to turn off. And even then, you may "awaken" one or more modules while conducting a battery drain test. For this type of vehicle model—and older models as well if you wish—use the voltage drop battery drain test described in the next section.

Now, start your tests.

Note: If you haven't used a digital multimeter before or have little experience using one, watch the next video for some quick tips.

Measuring Amps With a Multimeter

The Amperage Battery Drain Test

With the amperage test, you probe the different electrical circuits searching for the presence of electrical current when there should be none. For example, if you detect electric current in the accessory light bulb circuit, you'll know this isn't right since all lights should be off. Just remember that the circuits for the dashboard clock, stereo, and computer module will have current in them.

  • To begin the amperage method test, disconnect the negative battery cable from the battery terminal. This is the black cable connected to the battery post with the negative (-) sign next to it. Then, connect your digital meter's ground (black) probe to the negative battery post.
  • Connect the meter's positive (red) probe to the battery cable terminal you just disconnected.
  • Set your multimeter to the highest setting on the DC (direct current) amperage scale and turn it on.
  • Now, check your meter's display. It should read zero amps.
  • Move the meter's dial to the next lower setting on the amperage scale.
  • Keep moving to a lower setting until you detect the presence of current. On a vehicle without a parasitic drain problem, this current is within a range of about 50 milliamps (mA). On a vehicle with a parasitic drain problem, this current will be higher than the normal range.

Preliminary Results

Reading within range: If your reading is within range, you're probably dealing with a faulty charging system or battery. To do a quick alternator drain power check using your digital multimeter, go to the section Bad Alternator Diode Test below. Then, if your alternator passes the test, do a battery check using a hydrometer tool as described in the Car Battery Hydrometer Test section.
Reading above range: A higher amperage reading means you have a problem in one or more of the electrical circuits. Continue with the next steps and leave your multimeter connected to the battery. Use a pair of plastic clamps to hold the meter probes to the battery terminals, or ask an assistant for help.


Make sure no one attempts to crank the engine, turn on the air conditioning or heater, or any other accessory while your digital multimeter is connected in series (as for the amperage test) to your car's battery. This will blow out your meter's fuse or destroy your tool.

Locating the Faulty Circuit:

  • To locate the faulty circuit, start pulling out fuses, one at a time, from each of your vehicle's fuse boxes. Begin with the under-hood fuse box, if there's one. Pull one of the fuses and watch your meter's reading. Then re-install the fuse. Keep pulling fuses one at a time until you see the unusual amperage reading on your meter go down to within normal range. When this happens, the circuit that fuse protects is faulty.
  • Once you find the circuit causing the drain, check the description of the circuit under the fuse box lid. It'll say something like "Tail, parking, side marker lamps," "Radiator cooling fan," or "Blower motor 30-amp fuse."
  • Use your vehicle repair manual to check the wiring diagram for that circuit. The diagram will tell you what switches, loads, and connections are located in that particular circuit. These are potential trouble points.
  • Start with the switches, if any, to make sure they are properly operating. If it is a door switch, for example, make sure the switch is still correctly mounted, and the door presses the switch off when closing.
  • Make sure the wires in the circuit are away from hot surfaces and connectors and loads have firm and clean connections.

Also, check for a loose wire—usually at a connector or load—or a wire with burned-out insulation causing a short circuit.

A wiring diagram can help locate potential trouble points on a circuit.
A wiring diagram can help locate potential trouble points on a circuit. | Source

The Voltage-Drop Battery Drain Test

Although the amperage battery drain test has been a popular and reliable technique for some years, it is difficult to implement on many late model vehicles. Many of these models have several computer modules that remain online for 30 minutes or more after you've shut off the engine and removed the key from the ignition switch.

So you need to wait for all these systems to go into sleep mode (standby) before you start your tests. And even then, you may accidentally "awaken" one or more modules while trying to find the faulty circuit.

Also, if one of the modules is the one draining battery power, disconnecting the battery for the amperage test will turn the module off and prevent you from identifying the parasitic source, since the module will reset.

With the drop voltage test, you don't have to worry about disturbing computer modules; it is a less invasive way to locate a faulty circuit, and it works on older models just as well.

Before you begin, though, make sure you have a fully charged battery and that your meter probes have pointy ends to reach the two small test points on the back of each fuse; otherwise, you'll make inaccurate readings.

With this test, you'll check for the presence of voltage drop (lost voltage due to resistance in a circuit caused by heat from current flow).

  • To start, check your car owner's manual for the amount of time needed for the car modules to shut off or enter standby mode (usually between 30 and 60 minutes, or up to eight hours in some models). Also, check whether any of the control modules has an on/off cycle when the car is just sitting, so that you read voltage drop on these modules during their off time period.
  • After preparing your vehicle as described in the section Preparing for the Test, set your digital multimeter to the millivolts (mV) scale and turn it on.
  • *First, test for voltage drop on the fuses for the dashboard clock or stereo and computer's circuit, which have a small current going through them. Touch one test point on the back of the fuse with one of your meter's leads and the other fuse test point with the other lead (do not remove any fuses), and watch your meter's display. Since these circuits have some current, make a note of these readings and use them as reference.
  • When ready, start with the fuses in the under-hood fuse box, if equipped. Then, test the fuses under the dashboard. Circuits with no current flow will read 0.0 mV of voltage drop. If you detect any amount of voltage drop, you've identified the circuit with the parasitic draw.

Preliminary Results

Fuse with voltage drop: Look up the wiring diagram for the circuit that the fuse with the voltage drop protects. The diagram is in your vehicle repair manual. Locate switches, connectors, or loads that may be activated or faulty. Also, look for potential short circuits—loose wires behind connectors or loads, or exposed wires making contact with metal surfaces.

No fuses with voltage drop: You may be dealing with a faulty charging system or battery. To do a quick alternator drain power check using your digital multimeter, go to the next section: Bad Alternator Diode Test. If your alternator passes the test, do a battery check using a hydrometer tool. For this, go to the following section: Car Battery Hydrometer Test below.

Probing the fuses for voltage drop can help you locate a faulty circuit.
Probing the fuses for voltage drop can help you locate a faulty circuit. | Source

Bad Alternator Diode Test

Usually, a bad alternator diode will cause your headlights, instrument panel lights, or stereo display to flicker or dim and, sometimes, drain battery power overnight or much sooner.

A diode helps convert alternate current (AC) produced by the alternator into direct current (DC) by letting electricity travel in one direction—toward your battery and other electrical systems—and blocking current in the opposite direction—from your battery or other circuits to the alternator.

When one or more diodes in the alternator rectifier fail, they will prevent a continuous electrical supply to the electrical systems—dimming headlights, the radio display, dashboard lights, and other devices—and drain battery power after the engine has stopped.

  • To check for a possible bad alternator diode, switch your voltmeter to a low setting on the AC (alternating current) voltage scale.
  • With the engine running, touch the meter probes to the battery terminals—positive (red) meter probe to the battery positive terminal, and negative (black) meter probe to the battery negative terminal.
  • Your voltmeter should read 0 AC volts.

The presence of even a small amount of AC voltage will indicate a bad diode, so you'll need to replace the alternator's rectifier or the alternator.

Car Battery Hydrometer Test

The hydrometer test is a quick way to check the state of charge and health of your battery. It will let you know whether your battery is fully charged, needs a charge, or one or more cells have failed.

A hydrometer is an inexpensive tool. Buy one at your local auto parts store or online, and then check this article on troubleshooting a car battery using a hydrometer.

Basically, a reading below the 1.265 mark usually means your battery needs charging. A difference of 25 to 50 points between one or more cell readings means your battery is defective.

Testing for a Car Battery Drain

Car battery problems, specially a parasitic battery drain, are difficult to diagnose sometimes. New vehicle models make it specially difficult, time consuming, and frustrating. They come with more control modules than previous year models and hundreds more electrical circuits. But the amperage battery drain test, voltage drop battery drain test, alternator diode test, and battery hydrometer test will help you chase down the culprit much more easily, whether you have a newer or older vehicle model.

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1 comment

Warren Scott 10 months ago

Excellent way of doing the tests explained. I am doing it right now before the snow

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