Dan Ferrell writes about DIY car maintenance and repair. He has certifications in Automation and Control Technology and Technical Writing. Show
A car battery drain can leave you stranded anywhere. Photo courtesy of Longhair on Wikipedia What Is a Parasitic Battery Drain?A parasitic battery drain is when an 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:
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:
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 ContentsHere 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.
1. Preparing for the TestBefore you start your tests:
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. One more thing. If your battery case is dirty or damp, thoroughly clean the battery and platform it sits on, including terminals, if corroded. Use 8 oz of water and one table spoon of baking soda as a cleaning solution. Dirt and acid may lead to battery drain. 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. 2. The Amperage Battery Drain TestWith the amperage test, you probe the different electrical circuits searching for the presence of electrical current when there should be none. Scroll to Continue Read More from AxleAddictThe best way to conduct this test is to use an inductive ammeter capable of reading DC milliamps. That way you don't need to disconnect any battery cables; otherwise you can use the following method, but make sure not to open any doors or activate any circuits once your meter is connected or you may destroy your ammeter. Also, try to use a memory saver to preserve your car's computer and other electronic equipment settings.
Preliminary ResultsReading 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. Locating the Faulty Circuit:
Also, check for a loose wire—usually at a connector or load—or a wire with burned-out insulation causing a short circuit. 3. Components Amperage UseKnowing how much amperage different components use in your vehicle can give you an idea of the circuit causing the parasitic draw. For example, if you find a parasitic draw of about .235 amps (235 mA), you may suspect a light bulb that is on all the time. A 1 amp draw may indicate an electrical problem with the radio, and so on. The following table gives you a typical amperage use of different car components. Consult your car repair manual, if necessary.
These are just a few common electrical components that may cause trouble. There are many more. If necessary, consult your vehicle repair manual.
A wiring diagram can help locate potential trouble points on a circuit. Photo courtesy of Leonard G. on Wikipedia 4. The Voltage-Drop Battery Drain TestAlthough 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. However, you need a quality DMM (digital multimeter) for this test. Before you begin, though, make sure you have a fully charged battery and that your meter probes have clean, 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).
Preliminary ResultsFuse 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. Photo courtesy of havarhen on Wikipedia 5. Bad Alternator Diode TestUsually, 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.
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. 6. Car Battery Hydrometer TestThe 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. However, you can only do this test on batteries with removable caps. If your car uses a free-maintenance battery (no caps), or one with caps, you can visit a local auto parts store and have them test your battery for free. 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. 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. This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters. Questions & AnswersQuestion: I put the negative tester lead on the negative battery terminal and the positive tester lead on the negative battery cable (disconnected from battery). I get zero milliamps, but get 12+ volts. I pulled each fuse one by one, no change. Connected negative cable and get a sizzling snap each time. The car won’t start in 3 to 4 days. It starts with a fresh charge. What is happening? Answer: There seems to be something connected directly to power (a short somewhere) which is draining your battery and the engine won't start until you get the battery to full power. Remove the battery and connect your Ohmmeter between the negative and positive cables. If you get around 1 ohms, there's something shorted to ground. Check all engine grounds and look around for a loose wire, or a power wire touching the ground. Check the power battery cable from end to end. Shorts are sometimes difficult to find. You may try using a short finder. Question: I have a 1991 Ford E250. If I test between the negative post and the terminal, with everything shut off, my tester shows 4.5 amp draw. If I pull the small wire off the alternator and test again, it shows 0 amps. It's a new alternator. I'm baffled. Do I have a bad alternator or is the draw somewhere else? Answer: I've known this to happen with a bad voltage regulator. Have the alternator checked. You may want to try checking the circuit as well. This other post may help: https://axleaddict.com/auto-repair/Charging-System... Question: Approximately how long does a parasitic drain test take? Answer: It depends on how fast you locate the circuit causing the draw and how accessible are the fuses to the circuits. You may be looking into 10 to 20 minutes including setting up. Question: Is it possible for the tracking device to drain an iPhone's battery in one hour? Answer: A tracking device may use up to 2mA in one day whether the engine is running or not. So there should be another source for power drain. Question: I have a 1983 Ford F-150. She was working fine and then, all of a sudden while on a long drive, her turn signals front and back, and brake lights, went out. Headlights still worked. Now we recently tried to start her and nothing. When trying to jump her, the cables became hot to the touch but still nothing. I am going to follow this article in the morning; was just wondering if you had a good idea or had experienced it before? Answer: The fact that the cables became hot is an indication of high resistance or possibly a short in the circuit. There could be damaged cables, corrosion or loose wires in the ignition system, multifunction switch, starting system. These other posts may help you check the circuits: https://axleaddict.com/auto-repair/How-to-Use-Volt... https://axleaddict.com/auto-repair/Automotive-Volt... Make sure engine grounds are clean and tight. Question: I have a 1996 Chevy C1500. I replaced the tail lights and now won't start. I put a new battery in it and it doesn't show the alternator working at all. I changed the alternator for a new one but still won't show up on the dash that its working at all. What could be the problem? Answer: You may have a problem in the starting circuit. This two posts may help you here: https://axleaddict.com/auto-repair/Why-Is-My-Start... https://axleaddict.com/auto-repair/My-Engine-Crank... Question: Is it possible to test for a parasitic current drain while my car battery is on trickle charger? My battery drains quickly, its voltage level will drop before I finish the testing. Answer: It seems the battery is unable to hold the charge. You may need to test the battery first. You can do it yourself using a hydrometer. This other post may help: https://axleaddict.com/auto-repair/check-a-car-bat... Question: I want to check if both the alternator and battery are good, but they aren't showing any charge on the dash. What could be the problem? Answer: You can have your battery and alternator checked in one of the local auto parts stores. If the charge indicator on the dashboard still doesn't respond, then there's a problem in the indicator circuit. The circuit itself is not complicated but may be hard to gain access to. You may need the repair manual for your particular vehicle model to identify wires and the best way to make it to the wires. Question: I am reading 00.1mV on my brake light switch/chmsl fuse. Is this enough to be considered the leak? All other fuses read 00.0 Answer: That's too small to be a leak. Question: How do I check a JCASE box fuse using volt drop parasitic diagnostics? Answer: You can test each side of the fuse with the voltmeter. This video may help you: https://www.youtube.com/watch?v=lRcj1fQcWwU © 2014 Dan Ferrell What are the symptoms of a bad alternator?7 Signs of a Failing Alternator. Dim or Overly Bright Lights. ... . Dead Battery. ... . Slow or Malfunctioning Accessories. ... . Trouble Starting or Frequent Stalling. ... . Growling or Whining Noises. ... . Smell of Burning Rubber or Wires. ... . Battery Warning Light on Dash.. What causes an alternator to drain the battery?Defective charging system - A car battery drains quickly if something called the alternator on your charging system fails to push out enough voltage, ideally between 13.5 - 14.5 volts. In this case, the battery will continue to drain even while the car is running.
Does bad alternator drain battery?How A Bad Alternator Can Drain a Battery. Contrary to its purpose, an alternator can drain a battery. It's rare, but it's possible. A draw on the battery only occurs if there's a problem with either a component in the alternator itself or connections to it.
How do I find out what's draining my car battery?How To Diagnose the Battery Drain. Step 1: Remove Negative Battery Cable. Negative Cable Removed. ... . Step 2: Check the Draw Across the Negative Cable and Battery Post. ... . Step 3: Remove and Replace Fuses. ... . Step 4: Isolate and Fix the Issue. ... . Step 5: Replace Negative Battery Cable.. |