BATTERY DRAIN DIAGNOSIS - Automotive Data · BATTERY DRAIN DIAGNOSIS FOR Pg_EDIT_Battery: ... sure...

Thanks to modern technology,battery drain diagnosis ismore difficult than it used tobe, and I could devote pageupon page to the topic. Buthere I’ll concentrate on areas

that seem to be totally overlooked or elsebadly misunderstood. My field experiencehas been that some technicians are bothwell-read and well-equipped for these tasks.But others struggle because they’re steepedin outdated methods and/or careless short-cuts. For instance, many guys disconnectbatteries without considering the conse-quences. Others still try to gauge key-offdrain with test lights. I frequently meettechs who have never used a door switch“closer” or a battery drain test switch.No, this is not your father’s battery drain

diagnosis. Sure, some basic steps are ones youmay already know; and some steps apply toall vehicles. But you must evaluate each vehi-cle individually to judge the most efficientway to measure key-off battery drain. Onesingle technique isn’t fast and easy on everysingle car. Component access may weighheavily in your decision. The bitter fact is thatcomponent access on many vehicles fallssomewhere between horrible and terrible.

Take Control of the JobWhen I was a teenager, older mechanicstaught me a basic routine that diagnosed99% of the battery drain complaints. Ichecked the car’s dome light first. If it wasoff, then I looked for courtesy lights insidethe glove box, console, trunk and enginecompartment. If a bulb was hot the mo-ment after I opened the glove box, trunk,etc., then the switch controlling that bulbwas stuck closed (shorted). Fortunately,these same checks are still as useful now asthey were in the late 1960s.Today, however, there are many more

possible causes of excessive battery drain.What’s more, the cost of a bungled diagno-sis is considerably more than a trunk switchR&R. That’s why I urge you to take controlof the job andmaintain control until you fixthe vehicle. This testing takes time andknowledge; not every motorist is willing topay for that time and knowledge—period!The first step to controlling the job is

establishing an initial diagnostic fee. Youdon’t have to do this, but experienceshows that battery drain jobs can just de-vour time. Anyway, where necessary, ex-plain the critical steps you must take

first—for example, thoroughly testing thebattery and charging system. Next, youmust wait for the on-board electronics togo to sleep and then measure key-off cur-rent. What’s more, you may need to re-search the vehicle and its systems. Youeasily could burn 60 minutes or more re-searching a battery drain issue.Meanwhile, reassure the customer that

you’ll update him on the job as it pro-gresses. If pinpointing the problem iseasy, you have the option of reducing thediagnostic fee. Intermittent or elusive bat-tery drains require more test time, andthat means more money. Some batterydrains can be maddeningly random.

Vehicle History Is VitalFor now, suppose that you secured a diag-nostic fee. The next step is gathering thevehicle’s history. Some people think this isa nuisance. But the sharper the informa-tion the customer provides, the greaterthe chances of your diagnosing the prob-lem quickly.

Okay, the driver shuts off the ignitionswitch and then the battery discharges.When did this symptom first appear? Didit occur only after someone worked on thevehicle? If so, then who worked on it andwhat did he do? Based on the work thatwas done, do any potential battery drainissues jump out at you?Did someone replace the battery? If so,

is it a high-quality battery with the properreserve capacity for the application? Thetime it takes the battery to discharge maybe a useful clue as well. For example, a bat-tery that dies within a few hours suggests arelatively high-current drain. But supposethe battery dies only when the vehicle sitsundriven for a week, in an airport parkinggarage, for instance. In that case, the bat-tery itself may be the culprit. The batterycould be failing or it may be a replacementitem with inadequate reserve capacity. (I’llreturn to batteriesmomentarily.)Did someone install accessories? To

me, it doesn’t matter whether a car dealertech, an independent tech or do-it-

44 September 2014

BY DAN MARINUCCI

Modern technology has complicatedbattery drain diagnosis. A careful,

methodical approach always pinpointsmore problems, more often, thanguesswork and false assumptions.

BY DAN MARINUCCI

Modern technology has complicatedbattery drain diagnosis. A careful,

methodical approach always pinpointsmore problems, more often, thanguesswork and false assumptions.

PROVENTECHNIQUES

FORBATTERY DRAINDIAGNOSIS

PROVENTECHNIQUES

FORBATTERY DRAINDIAGNOSIS

Pg_EDIT_Battery:Pg_EDIT_ 8/20/14 11:52 AM

yourselfer installed accessories. If the onlychange to the vehicle was that installation,then I’m suspicious of the product itself aswell as the details of the electrical hookup.For all you know, a recently installed caralarm, sound system or remote-start setupcould be a key-off power hog. Or, theproduct itself may be fine but the installerbungled the electrical hookup. (Readers,can you say hot all the time?)

The point is that the diagnosis mayamount to inspecting the last place humanhands touched the vehicle. Also, listenclosely to the customer. Don’t disregard orunderestimate things people tell you abouttheir vehicles and driving experiences. In-stead, try to correlate the discharged bat-tery to the driver’s usage and habits.

For example, parking on a hill could berelevant. The mercury switch used to be acommon method of operating a trunklight. Sometimes a failing mercury switchwould short out only when the vehicle wasparked on a hill at just the proper angle.

Changing drivers or jobs may be clues.

For example, suppose a vehicle’s batteryalready was in marginal condition. Orsomeone installed a replacement batterywith insufficient reserve capacity. Neitherof these conditions may have been an is-sue until the owner started a new job andbegan leaving the car at an airport for aweek at a time, or one of his kids bor-rowed it and then parked it for a week.

Watch for OEM features such as re-mote entry and remote start. These sys-tems, which affect battery drain, have be-come topics unto themselves. You’ll find aneat summation of them in Karl Seyfert’sTrouble Shooter columns of January andFebruary 2011. Ultimately, you may haveto verify that these systems are workingnormally and that the drivers are usingthese features correctly.

Finally, check all your sources for rele-vant service bulletins. Some service man-agers and technicians also routinely visitOEM websites in case there’s recentnews that hasn’t reached aftermarketchannels yet.

A Fault and Battery?Battery maintenance and capacity are oth-er details you must not underestimate.First, batteries don’t require as much main-tenance as they did years ago. Second, adamp, dirty battery case ultimately maycause a battery drain. Third, electrical spe-cialists have warned me about this issue;they claim that batteries still don’t get themaintenance they deserve. Fourth, manytechs totally ignore battery maintenance.

Obviously, underhood componentssuch as batteries eventually get dirtyand/or wet. Eventually, dirt on top of thebattery may conduct current, dischargingthe battery. If you doubt this, get a digitalmultimeter with a milliamp (mA) scale.Hold one of its test leads on the positivebattery terminal, then touch the otherlead to various points on a damp, dirtybattery case. If the meter shows that cur-rent is flowing, then that battery is dis-charging while you’re watching it! Clean itand dry that battery.

Be sure the battery has the correct re-serve capacity (RC). Certainly, this isn’t apotential problem in every instance of adead battery. However, RC has becomemore and more important on modern ve-hicles. Furthermore, countless techs I’vemet don’t know what RC is or why it mayaffect battery drain diagnosis.

By definition, reserve capacity is thelength of time in minutes a battery cansupply 25A while maintaining a voltage of10.50V. Remember that RC is measuredon a new, fully charged battery at 80°F.(Think about that: The battery is pumpingout a steady 25A until its voltage drops to10.50V!) Engineers boost RC by puttingthicker plates in the battery.

To grossly simplify, RC does two things:First, it powers the ignition system andvehicle lighting if the alternator fails. Sec-ond, it powers electronic components thatdraw current after the driver shuts off theignition switch. Generally speaking, thegreater the number of these components,the greater the RC rating is likely to beand the more important the rating be-comes. What’s more, the longer the cus-tomer parks his vehicle, the more impor-tant RC becomes. Arguably, the rating isless critical on a vehicle that’s driven daily.

You can connive, compute and contriveways to circumvent or shortchange an au-tomaker’s RC rating. Remember, short-cuts are never a problem until they cause

45September 2014

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a problem. Cover your rear end—not tomention the shop’s reputation—by in-stalling a replacement battery with theproper, specified reserve capacity.

The typical RC ratings on automotivebatteries range from 70 to 150 minutes.Usually, the label on an OEM batteryshows the specified RC. What’s more, RC

should be listed in the electrical section of agood shop manual or other service litera-ture. However, the situation is more com-plicated on replacement batteries. For ex-ample, the RC rating may appear on a bat-tery manufacturer’s replacement products.But when that battery is private-branded,the label may not show the RC rating. Sim-

ilarly, some battery catalogs list RC ratingsand others don’t. So in some instances,you’re at themercy of the battery vendor.

Planning Your WorkCarefully planning the procedure is an-other aspect of controlling the job. Failingto do so can cost you a bundle! Your initial

goals here are twofold: First, allow the on-board electronics to time out. Dependingon the vehicle and its systems, waiting forsleep mode could take anywhere from 35minutes to several hours.

Second, measure key-off current after allthe electronic components are asleep.Many techs call key-off current the parasiticdrain. Some check parasitic drain directly atthe battery with a digital ammeter and atest switch. Others do it by clipping an in-ductive current clamp around a battery ca-ble. If the drain is excessive, the most com-mon way of isolating the cause is removingone fuse at a time. If the current drops tonormal when you remove a fuse, then thatfuse identifies the offending circuit.

If you’re using these methods, thenhopefully the battery is under the hoodand easy to reach. You may have to re-move ductwork and/or brackets to accessthe battery. Occasionally you have to re-move the hold-down and carefully slidethe battery a little bit one way or the otherto create some working room.

Of course, the battery may be underthe rear seat or rear floor. It also could be

PROVEN TECHNIQUES FOR BATTERY DRAIN DIAGNOSIS

Normal parasitic battery drain usuallymeasures approximately 30 to 50mA.A milliamp scale is more sensitive thana regular amps scale. It also saves youthe trouble of figuring out the decimalpoint placement!

46 September 2014

Circle #30

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hidden in the trunk or cargo area. Sensi-ble service managers make customers re-move their personal junk from the vehiclebeforehand so techs don’t waste time re-moving and storing it. Plus, you may haveto find a safe, clean place to stow suchitems as a rear seat cushion, an interiorpanel or interior trim pieces.

Yes, these two initial steps require a se-cure place where the customer’s vehiclecan sit undisturbed for up to severalhours—with the hood, trunk, rear hatchand/or a door wide open. At some shops,bays are kept open specifically for thesekinds of tasks. But at other garages, spaceis at a premium. In those cases, schedul-

ing wisely and working smart are doublyimportant.Smart technicians also anticipate the

chores of finding fuses and closing doorswitches. Typically, you’ll be touching thefuses at some point during battery drain di-agnosis. Pay attention because many vehi-cles have more than one fuse panel. Also,

the fuse panel(s) may or may not be readilyaccessible. And the fuse you’re seekingmay not be located where you assumed itwould. Finally, the fusesmay not be techni-cian-friendly, another detail I’ll cover later.Dealing with door, trunk or hatch

switches is vital to your battery drain diag-nosis. Obviously, you have to open the ve-hicle in order to reach the battery and/orfuse panel(s). The moment you do, one ofthe above-mentioned switches turns onthe interior lighting and wakes up vehiclenetworks. This causes an erroneous para-sitic drain.For years, I’ve used Thexton’s tools

(www.thexton.com) to close door switch-es. Tool No. 442 handles door switches onthe A-pillar and No. 445 handles switcheson the B-pillar. When the door switch ishidden inside the door latch, carefully in-serting a tool in the latch should close theswitch. Several common things may workhere—a stubby screwdriver, a shortpunch, etc. Just take care to remove thetool and operate the door handle beforetrying to close the door again.Forewarned is forearmed: There could

be cases where you’ll have to access aswitch and figure out how to close it orperhaps bypass it with a jumper wire.


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Circle #32

Basically, reserve capacity (RC) is howlong (in minutes) the battery can sup-port the vehicle’s ignition and lights ifthe alternator fails. But RC is also im-portant for supporting normal key-offcurrent draw. Use of thicker batteryplates increases the RC rating.


Test PrecautionsLet me stress three important points, allborne of experience. First and foremost,recognize normal parasitic drain values.Sometimes these aren’t published orthey’re very difficult to find. Key-off cur-rent on a healthy vehicle is usually—typi-cally—within the range of 30 to 50mA.

That’s equal to .030 to .050A. Try buildingyour own personal database by faithfullyrecording both known-good and known-bad parasitic drain numbers. What’s more,maintain solid working relationships withcompetent dealer techs who are makingthe same measurements.

Second, keep your perspective on bat-

tery drains. If a vehicle has a legitimateproblem, you could encounter parasiticdrains in the 90- to 100mA range, for in-stance. However, the vast majority of par-asitic drains that cause customer com-plaints are substantially greater thanthose. Usually the difference between anormal parasitic drain and one that’skilling the battery is significant—not just adifference of 40 to 50mA.

Third, avoid disconnecting the battery.Obviously, doing this shuts down every-thing in the vehicle. Experience repeated-ly has shown that the cause of an exces-sive drain may not come back when youreconnect the battery. Instead, it may takeweeks or months of driving for the condi-tion to reappear! What’s more, this isn’t aproblem associated with just the newestvehicles. To the contrary; I recall field ser-vice engineers warning me about “batterydisconnect” as far back as 1990.

Okay, what if you do have to disconnecta battery? (A common reason is to install atest switch.) Then road-test the vehicle af-ter you reconnect the battery, operating asmany electrical accessories as practicallypossible. With a little luck, the cause ofthe excessive drain usually returns.

And there’s a fourth point: Don’t dis-turb a vehicle or its systems in any waywhen waiting for sleep mode. In particu-lar, keep all remote-entry fobs far awayfrom the vehicle.

Direct Current MeasurementUsing a test switch and digital ammeter isthe simplest, most accurate way to mea-sure parasitic drain directly. A test switchgoes between the negative battery termi-nal and negative battery cable. The photoon page 54 (top center) shows the mostcommon style of test switch. Tighteningthe green knob closes the switch; loosen-ing the knob opens it. Tool suppliers suchas OTC (www.otctools.com), Thexton andothers offer test switches. You’ll also findthem on the internet.

The plan here is to install the testswitch, close it and then restart the en-gine. Road-test the vehicle and operate asmany accessories as practically possible.Then park it in the bay assigned to thisjob. Create access to the areas of the vehi-cle where you’ll need to work. Then dowhat’s necessary to close the door or rearhatch switches. Next, wait for the vehicle’s


50 September 2014

Circle #33

Circle #35

Circle #36

Circle #34


electronics to go to sleep. Pass the time by working on other cars.Fetch a digital multimeter that measures current—preferably

one with fused amps and milliamp scales. (A number of profes-sional-grade multimeters feature a fused 10A as well as milliampscales.) Select the 10A scale or the highest current scale the me-ter has. Connect the meter across the test switch and then openthe switch. Now whatever key-off current is present has to flowthrough the meter. This format yields the most accurate parasitic

drain measurement. Yes indeed, squeezing a test switch into acramped engine compartment can be a hassle—but usuallyworth it. Sometimes you may have to improvise good connec-tions when installing a test switch.

If your meter has a milliamp scale, use it whenever possible be-cause it’s a more sensitive scale. Imagine that the initial ammeterreading, captured on the 10A scale, was .365A (365mA). If theme-ter’s milliamp scale goes up to 500mA, then you can safely mea-sure 365mA with it. So, close the test switch at the battery, selectyourmeter’s milliamp scale and then open the test switch again.

Remember that a proper test switch can handle starter cur-rent. So it enables you to restart the engine. Restarting the en-gine and operating accessories has been vital to recreating thebattery drain conditions that we’re trying to diagnose. You can’trestart the engine when using some battery drain test methods,including a test light.

Indirect Current MeasurementsThere are two ways to measure parasitic drain indirectly. The firstis the inductive current clamp method; the other is checking volt-age drop across each fuse. Let’s look at the inductive clamps first.Over the years, I’ve been using a variety of inductive currentclamps (sometimes called probes) for assorted electrical tests. I’mconvinced these clamps have great value. But you have to choosethe product very carefully—one size doesn’t fit all. Wherever pos-sible, try out the clamp before buying it.

The challenge here is finding an inductive tool thatmeets two cri-teria. First, its clamp has to be large enough to fit around commonbattery cables. Second, it has to be sensitive enough to measurerelatively low milliamp values. Maybe you can find an inductiveclampwith these features that connects to your existingmultimeter.

Another option is a clampmeter, which is basically a small mul-timeter with an attached inductive clamp. Electronics Specialties(www.esitest.com) offers two clamp meters that are better suitedto parasitic drain testing than many previous models. Tool No.687, which I’ve cited in my Foreign Service column, has a rela-tively large clamp opening. I’ve used this one on a wide range ofbattery cables. The tool features a low-current scale (0 to 4A) aswell as push-button zero capability. Recently, Electronics Special-ties released clamp meter No. 688, which has a larger clamp


Using a fused test lead is an old but cheap, effective way toadd another level of protection to that valuable multimeter.Replacing a blown standard fuse in the test lead is cheaperand quicker than replacing fuses inside the multimeter.

These long, blue fuse assemblies on late-model Toyotas arean example of a fuse that you can’t probe and voltage drop—its terminals are covered. Plus, the single assembly protectsor “fuses” several different circuits simultaneously.

52 September 2014


opening than the previous model, a mil-liamp scale and the push-button zero fea-ture. I highly recommend the push-buttonzero capability on any inductive currentclamp, including those used for parasiticdrain testing.

Understandably, speed and conve-nience are the key advantages of the in-ductive current technique. Because you’renot disconnecting any wiring, you’re notdisturbing the electrical system—a valu-able feature when you’re waiting for itscomponents to go into sleep mode. Sometechs use a clampmeter to complement—not replace—the test switch. First, theyuse it for quick preliminary parasitic drainreadings. For example, with the vehicle inits “virgin” condition, does the parasiticdrain appear to relatively low or high?

Second, suppose you’ve installed a testswitch on the vehicle and you’re waitingfor components to time out. Meantime,you’re working on another vehicle. Peri-odically check parasitic drain on the firstvehicle with the clamp meter. Supposeyou check it several times over the courseof an hour. Let’s say the parasitic drain hassettled down to a consistent, 300mA read-ing. That strongly suggests that the com-ponents have gone to sleep. Third, theclamp meter has been helpful for thosereally tough diagnoses where you want tocheck parasitic drain at the customer’shome or office, for instance.

There is a down side to this approach.

The lower end of the parasitic drain rangeis down at the lower end of the typical in-ductive clamp’s sensitivity.

A Fuse’s Voltage DropMeasuring voltage drop across each fuse isthe other way to check parasitic drain indi-rectly. The folks at CARQUEST TechnicalInstitute (www.ctionline.com) presentedthis technique in their training seminars,and Karl Seyfert discussed it in his De-cember 2010 and January 2011 TroubleShooter columns. This method is based ona simple electrical principle: If no currentflows through a connection, there’s novoltage drop across the connection. Butwhen current does flow, there’s a measur-able voltage drop across that connection.

To use this technique, grab a multime-ter with a millivolt (mV) scale, as well astest leads equipped with pointed probetips. After you prep the vehicle for para-sitic drain testing, per the directions Igave earlier, locate the fuse panel. Set themeter to the millivolt scale and connect itsleads across a fuse. To do this, probe thetiny holes in the top of the fuse. Connect-ed this way, the meter is measuring thevoltage drop across the fuse.

A meter reading of 0 means no voltagedrop, no current flowing—the circuit isoff. In the case where the circuit powers acomponent that normally draws currentwhen the ignition switch is off, the meterwill show some sort of voltage drop acrossthe fuse. The greater the voltage drop, the


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To measure voltage drop across afuse, slip test lead probes into thosetiny holes in the top of the fuse body.This gives you access to the terminalsinside the fuse body. This hookup al-lows your voltmeter to measure volt-age drop across a fuse.

This is common example of a parasiticdrain test switch. Turning the greenknob opens and closes the switch. Themultimeter leads are connectedacross the switch. Opening the switchforces battery drain current to flowthrough the multimeter.

Thexton’s tool No. 445 neatly closes atypical door switch located on the ve-hicle’s B-pillar. Of course, closing thisdoor switch prevents the interiorlights from turning on and/or a net-work from waking up while you’reworking on the vehicle.

Here are just three ex-amples of clampmeteropenings—ESImodelNos. 687 (top right) and688 (top left) andMidtronics No. PDF40.The ESImodels have low-current scales bettersuited formeasuringparasitic drain. Evenwithout a low-currentscale, the PDF40 stillperformed pretty well onlow-currentmeasure-ments. Try out a clampmeter and satisfy your-self before you buy one!


greater the parasitic drain. Of course, theparasitic drain should be within spec orwithin the normal range discussed earlier.

Therefore, this approach offers a majoradvantage: It pinpoints the exact circuitthat’s causing the excessive parasitic drainwithout your installing a test switch orpulling fuses. The less fuse R&R you do,the less risk of mistakes such as shuttingoff the wrong circuit, accidentally wakingup a “sleeping” computer, etc.

But as I noted earlier, there’s no perfecttest format for diagnosing battery drain.For example, some fuse panels are in awk-ward locations. So performing the voltagedrop could put you in a painful, contortedposition—probably on your back. Also,you can’t perform the voltage drop onsome fuses because there’s no access. Yousee, the shell or body of some fuse designscompletely covers the fuse’s terminals. Ob-viously, you can’t probe covered terminals.

The fuses in late-model Ford productsare an interesting example. Some of the fus-es have accessible terminals, others do not.What’s more, other Ford fuse designs ap-pear technician-friendly because they haveremovable, clear covers on them. The trou-ble is that these transparent covers seem tobreak very easily during removal. Worseyet, the cover’s little retaining tabs on themain body of the fuse break readily, too.

Many late-model Toyota fuse panels hitus with a diagnostic double-whammy. First,some fuses have covered terminals that pre-

vent us from probing them. Furthermore,these covered fuses often are grouped intoone, much larger fuse assembly. This is along row of fusesmolded into a single hous-ing. So removing this fuse assembly wouldkill several circuits simultaneously—whether youwanted that result or not.

Network NuisancesWow, I’ve kept you for a while here.Hopefully, you’ve gleaned helpful infor-mation from this feature. Here are someclosing thoughts on the topic.

Don’t overlook network activity. For in-stance, suppose you pull the fuse for theradio and the excessive parasitic drain dis-appears. Your first instinct may be toblame the radio itself. But suppose a nodeor module on the network is awake whenit should be asleep. That troublesomenode could be talking to the radio; hence,the drain caused by the active radio is onlythe symptom, not the root cause of the ex-cessive drain.

When in doubt, scan the vehicle’s net-work to see if anymodules are awake whenthey should be sleeping. The challengehere is that capabilities vary from one scan-ner to another. Hopefully, your scanner cancommunicate well enough with a networkto confirm which module is sleeping andwhich is talking, for example.

Remember, depending on the remoteentry system a vehicle has, someone walk-ing by with a remote fob from an entirelydifferent vehicle could accidentally wakeit up. Once the system wakes up, yourmeter would show an additional, erro-neous drain that would disappear whenthe entry system goes to sleep again.

When researching a parasitic drainsymptom, watch out for those less obviouspossibilities. For example, if the vehiclehas a natural vacuum evap system, theECM could remain awake and energizethe evap system’s vent solenoid valve forsome period of time.

Usually, a professional-grade multimeteris equipped with a Min/Max capture capa-bility. The Max capture feature may helpyou catch the additional parasitic drain amodule creates when it wakes up. Untilnext time, good luck and keep smiling.

55September 2014

This article can be found online atwww.motormagazine.com.

Circle #39

Suppose you encounter two battery ca-bles connected to the same terminal.When in doubt about where the drainis, use your inductive clamp to testboth cables at the same time. A largerinductive clamp such as Midtronics’PDF40 or UEI’s No. CA30MA (shownhere) may fit around both cables.


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