Proven Techniques for Battery Drain Diagnosis – MOTOR

Proven Techniques for Battery Drain Diagnosis – MOTOR

Dream Cheeky will help you know How To Fix A Parasitic Draw 2022: Must Read

Video How To Fix A Parasitic Draw

Thanks to modern technology, battery drain diagnosis is more difficult than it used to be, and I could devote page upon page to the topic. But here I’ll concentrate on areas of the topic that seem to be totally overlooked or else badly misunderstood. My field experience has been that some technicians are both well-read and well-equipped for these tasks. But others struggle because they’re steeped in outdated methods and/or careless shortcuts. For instance, many guys disconnect batteries without considering the consequences. Others still try to gauge key-off drain with test lights. I frequently meet techs 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 you may already know; and some steps apply to all vehicles. But you must evaluate each vehicle individually to judge the most efficient way to measure key-off battery drain. One single technique isn’t fast and easy on every single car. Component access may weigh heavily in your decision. The bitter fact is that component access on many vehicles falls somewhere between horrible and terrible.

Take Control of the Job

When I was a teenager, older mechanics taught me a basic routine that diagnosed 99% of the battery drain complaints. I checked the car’s dome light first. If it was off, then I looked for courtesy lights inside the glove box, console, trunk and engine compartment. If a bulb was hot the moment after I opened the glove box, trunk, etc., then the switch controlling that bulb was stuck closed (shorted). Fortunately, these same checks are still as useful now as they 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 diagnosis is considerably more than a trunk switch R&R. That’s why I urge you to take control of the job and maintain control until you fix the vehicle. This testing takes time and knowledge; not every motorist is willing to pay for that time and knowledge-period!

The first step to controlling the job is establishing an initial diagnostic fee. You don’t have to do this, but experience shows that battery drain jobs can just devour time. Anyway, where necessary, explain the critical steps you must take first-for example, thoroughly testing the battery and charging system. Next, you must wait for the on-board electronics to go to sleep and then measure key-off current. What’s more, you may need to research the vehicle and its systems. You easily could burn 60 minutes or more researching a battery drain issue.

Meanwhile, reassure the customer that you’ll update him on the job as it progresses. If pinpointing the problem is easy, you have the option of reducing the diagnostic fee. Intermittent or elusive battery drains require more test time, and that means more money. Some battery drains can be maddeningly random.

Vehicle History Is Vital

For now, suppose that you secured a diagnostic fee. The next step is gathering the vehicle’s history. Some people think this is a nuisance. But the sharper the information the customer provides, the greater the chances of your diagnosing the problem quickly.

Okay, the driver shuts off the ignition switch and then the battery discharges. When did this symptom first appear? Did it occur only after someone worked on the vehicle? If so, then who worked on it and what did he do? Based on the work that was done, do any potential battery drain issues jump out at you?

Did someone replace the battery? If so, is it a high-quality battery with the proper reserve capacity for the application? The time it takes the battery to discharge may be a useful clue as well. For example, a battery that dies within a few hours suggests a relatively high-current drain. But suppose the battery dies only when the vehicle sits undriven for a week, in an airport parking garage, for instance. In that case, the battery itself may be the culprit. The battery could be failing or it may be a replacement item with inadequate reserve capacity. (I’ll return to batteries momentarily.)

Did someone install accessories? To me, it doesn’t matter whether a car dealer tech, an independent tech or do-it-yourselfer installed accessories. If the only change to the vehicle was that installation, then I’m suspicious of the product itself as well as the details of the electrical hookup. For all you know, a recently installed car alarm, sound system or remote-start setup could be a key-off power hog. Or, the product itself may be fine but the installer bungled the electrical hookup. (Readers, can you say hot all the time?)

The point is that the diagnosis may amount to inspecting the last place human hands touched the vehicle. Also, listen closely to the customer. Don’t disregard or underestimate things people tell you about their cars and driving experiences. Instead, try to correlate the discharged battery to the driver’s usage and habits.

For example, parking on a hill could be relevant. The mercury switch used to be a common method of operating a trunk light. Sometimes a failing mercury switch would short out only when the vehicle was parked on a hill at just the proper angle.

Changing drivers or jobs may be clues. For example, suppose a vehicle’s battery already was in marginal condition. Or someone installed a replacement battery with insufficient reserve capacity. Neither of these conditions may have been an issue until the owner started a new job and began leaving the car at an airport for a week at a time, or one of his kids borrowed it and then parked it for a week.

Watch for OEM features such as remote entry and remote start. These systems, which affect battery drain, have become topics unto themselves. You’ll find a neat summation of them in Karl Seyfert’s Trouble Shooter columns of January and February 2011. Ultimately, you may have to verify that these systems are working normally and that the drivers are using these features correctly.

Finally, check all your sources for relevant service bulletins. Some service managers and technicians also routinely visit OEM websites in case there’s recent news that hasn’t reached aftermarket channels yet.

A Fault and Battery?

Battery maintenance and capacity are other details you must not underestimate. First, batteries don’t require as much maintenance as they did years ago. Second, a damp, dirty battery case ultimately may cause a battery drain. Third, electrical specialists have warned me about this issue; they claim that batteries still don’t get the maintenance they deserve. Fourth, many techs totally ignore battery maintenance.

Obviously, underhood components such as batteries eventually get dirty and/or wet. Eventually, dirt on top of the battery may conduct current, discharging the battery. If you doubt this, get a digital multimeter with a milliamp (mA) scale. Hold one of its test leads on the positive battery terminal, then touch the other lead to various points on a damp, dirty battery case. If the meter shows that current is flowing, then that battery is discharging while you’re watching it! Clean it and dry that battery.

Be sure the battery has the correct reserve capacity (RC). Certainly, this isn’t a potential problem in every instance of a dead battery. However, RC has become more and more important on modern vehicles. Furthermore, countless techs I’ve met don’t know what RC is or why it may affect battery drain diagnosis.

By definition, reserve capacity is the length of time in minutes a battery can supply 25A while maintaining a voltage of 10.50V. Remember that RC is measured on a new, fully charged battery at 80*F. (Think about that: The battery is pumping out a steady 25A until its voltage drops to 10.50V!) Engineers boost RC by putting thicker plates in the battery.

To grossly simplify, RC does two things: First, it powers the ignition system and vehicle lighting if the alternator fails. Second, it powers electronic components that draw current after the driver shuts off the ignition switch. Generally speaking, the greater the number of these components, the greater the RC rating is likely to be and the more important the rating becomes. What’s more, the longer the customer parks his vehicle, the more important RC becomes. Arguably, the rating is less critical on a vehicle that’s driven daily.

You can connive, compute and contrive ways to circumvent or shortchange an automaker’s RC rating. Remember, shortcuts are never a problem until they cause a problem. Cover your rear end-not to mention the shop’s reputation-by installing a replacement battery with the proper, specified reserve capacity.

The typical RC ratings on automotive batteries range from 70 to 150 minutes. Usually, the label on an OEM battery shows the specified RC. What’s more, RC should be listed in the electrical section of a good shop manual or other service literature. However, the situation is more complicated on replacement batteries. For example, the RC rating may appear on a battery manufacturer’s replacement products. But when that battery is private-branded, the label may not show the RC rating. Similarly, some battery catalogs list RC ratings and others don’t. So in some instances, you’re at the mercy of the battery vendor.

Planning Your Work

Carefully planning the procedure is another aspect of controlling the job. Failing to do so can cost you a bundle! Your initial goals here are twofold: First, allow the on-board electronics to time out. Depending on the vehicle and its systems, waiting for sleep mode could take anywhere from 35 minutes to several hours.

Second, measure key-off current after all the electronic components are asleep. Many techs call key-off current the parasitic drain. Some check parasitic drain directly at the battery with a digital ammeter and a test switch. Others do it by clipping an inductive current clamp around a battery cable. If the drain is excessive, the most common way of isolating the cause is removing one fuse at a time. If the current drops to normal when you remove a fuse, then that fuse identifies the offending circuit.

If you’re using these methods, then hopefully the battery is under the hood and easy to reach. You may have to remove ductwork and/or brackets to access the battery. Occasionally you have to remove the hold-down and carefully slide the battery a little bit one way or the other to create some working room.

Of course, the battery may be under the rear seat or rear floor. It also could be hidden in the trunk or cargo area. Sensible service managers make customers remove their personal junk from the vehicle beforehand so techs don’t waste time removing and storing it. Plus, you may have to find a safe, clean place to stow such items as a rear seat cushion, an interior panel or interior trim pieces.

Yes, these two initial steps require a secure place where the customer’s vehicle can sit undisturbed for up to several hours-with the hood, trunk, rear hatch and/or a door wide open. At some shops, bays are kept open specifically for these kinds of tasks. But at other garages, space is at a premium. In those cases, scheduling wisely and working smart are doubly important.

Smart technicians also anticipate the chores of finding fuses and closing door switches. Typically, you’ll be touching the fuses at some point during battery drain diagnosis. Pay attention because many vehicles have more than one fuse panel. Also, the fuse panel(s) may or may not be readily accessible. And the fuse you’re seeking may not be located where you assumed it would. Finally, the fuses may not be technician-friendly, another detail I’ll cover later.

Dealing with door, trunk or hatch switches is vital to your battery drain diagnosis. Obviously, you have to open the vehicle in order to reach the battery and/or fuse panel(s). The moment you do, one of the above-mentioned switches turns on the interior lighting and wakes up vehicle networks. This causes an erroneous parasitic drain.

For years, I’ve used Thexton’s tools ( to close door switches. Tool No. 442 handles door switches on the A-pillar and No. 445 handles switches on the B-pillar. When the door switch is hidden inside the door latch, carefully inserting a tool in the latch should close the switch. Several common things may work here-a stubby screwdriver, a short punch, etc. Just take care to remove the tool and operate the door handle before trying to close the door again.

Forewarned is forearmed: There could be cases where you’ll have to access a switch and figure out how to close it or perhaps bypass it with a jumper wire.

Test Precautions

Let me stress three important points, all borne of experience. First and foremost, recognize normal parasitic drain values. Sometimes these aren’t published or they’re very difficult to find. Key-off current on a healthy vehicle is usually-typically-within the range of 30 to 50mA. That’s equal to .030 to .050A. Try building your own personal database by faithfully recording both known-good and known-bad parasitic drain numbers. What’s more, maintain solid working relationships with competent dealer techs who are making the same measurements.

Second, keep your perspective on battery drains. If a vehicle has a legitimate problem, you could encounter parasitic drains in the 90- to 100mA range, for instance. However, the vast majority of parasitic drains that cause customer complaints are substantially greater than those. Usually the difference between a normal parasitic drain and one that’s killing the battery is significant-not just a difference of 40 to 50mA.

Third, avoid disconnecting the battery. Obviously, doing this shuts down everything in the vehicle. Experience repeatedly has shown that the cause of an excessive drain may not come back when you reconnect the battery. Instead, it may take weeks or months of driving for the condition to reappear! What’s more, this isn’t a problem associated with just the newest vehicles. To the contrary; I recall field service engineers warning me about “battery disconnect” as far back as 1990.

Okay, what if you do have to disconnect a battery? (A common reason is to install a test switch.) Then road-test the vehicle after you reconnect the battery, operating as many electrical accessories as practically possible. With a little luck, the cause of the excessive drain usually returns.

And there’s a fourth point: Don’t disturb a vehicle or its systems in any way when waiting for sleep mode. In particular, keep all remote-entry fobs far away from the vehicle.

Direct Current Measurement

Using a test switch and digital ammeter is the simplest, most accurate way to measure parasitic drain directly. A test switch goes between the negative battery terminal and negative battery cable. The photo on page XX shows the most common style of test switch. Tightening the green knob closes the switch; loosening the knob opens it. Tool suppliers such as OTC (, Thexton and others offer test switches. You’ll also find them on the internet.

The plan here is to install the test switch, close it and then restart the engine. Road-test the vehicle and operate as many accessories as practically possible. Then park it in the bay assigned to this job. Create access to the areas of the vehicle where you’ll need to work. Then do what’s necessary to close the door or rear hatch switches. Next, wait for the vehicle’s 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 professional-grade multimeters feature a fused 10A as well as milliamp scales.) Select the 10A scale or the highest current scale the meter has. Connect the meter across the test switch and then open the switch. Now whatever key-off current is present has to flow through the meter. This format yields the most accurate parasitic drain measurement. Yes indeed, squeezing a test switch into a cramped engine compartment can be a hassle-but usually worth it. Sometimes you may have to improvise good connections when installing a test switch.

If your meter has a milliamp scale, use it whenever possible because it’s a more sensitive scale. Imagine that the initial ammeter reading, captured on the 10A scale, was .365A (365mA). If the meter’s milliamp scale goes up to 500mA, then you can safely measure 365mA with it. So, close the test switch at the battery, select your meter’s milliamp scale and then open the test switch again.

Remember that a proper test switch can handle starter current. So it enables you to restart the engine. Restarting the engine and operating accessories has been vital to recreating the battery drain conditions that we’re trying to diagnose. You can’t restart the engine when using some battery drain test methods, including a test light.

Indirect Current Measurements

There are two ways to measure parasitic drain indirectly. The first is the inductive current clamp method; the other is checking voltage drop across each fuse. Let’s look at the inductive clamps first. Over the years, I’ve been using a variety of inductive current clamps (sometimes called probes) for assorted electrical tests. I’m convinced these clamps have great value. But you have to choose the product very carefully-one size doesn’t fit all. Wherever possible, try out the clamp before buying it.

The challenge here is finding an inductive tool that meets two criteria. First, its clamp has to be large enough to fit around common battery cables. Second, it has to be sensitive enough to measure relatively low milliamp values. Maybe you can find an inductive clamp with these features that connects to your existing multimeter.

Another option is a clamp meter, with is basically a small multimeter with an attached inductive clamp. Electronics Specialties ( offers two clamp meters that are better suited to parasitic drain testing than many previous models. Tool No. 687, which I’ve cited in my Foreign Service column, has a relatively large clamp opening. I’ve used this one on a wide range of battery cables. The tool features a low-current scale (0 to 4A) as well as push-button zero capability. Recently, Electronics Specialties released clamp meter No. 688, which has a larger clamp opening than the previous model, a milliamp scale and the push-button zero feature. I highly recommend the push-button zero capability on any inductive current clamp, including those used for parasitic drain testing.

Understandably, speed and convenience are the key advantages of the inductive current technique. Because you’re not disconnecting any wiring, you’re not disturbing the electrical system-a valuable feature when you’re waiting for its components to go into sleep mode. Some techs use a clamp meter to complement-not replace-the test switch. First, they use it for quick preliminary parasitic drain readings. For example, with the vehicle in its “virgin” condition, does the parasitic drain appear to relatively low or high?

Second, suppose you’ve installed a test switch on the vehicle and you’re waiting for components to time out. Meantime, you’re working on another vehicle. Periodically check parasitic drain on the first vehicle with the clamp meter. Suppose you check it several times over the course of an hour. Let’s say the parasitic drain has settled down to a consistent, 300mA reading. That strongly suggests that the components have gone to sleep. Third, the clamp meter has been helpful for those really tough diagnoses where you want to check parasitic drain at the customer’s home or office, for instance.

There’s a down side to this approach. The lower end of the parasitic drain range is down at the lower end of the typical inductive clamp’s sensitivity.

A Fuse’s Voltage Drop

Measuring voltage drop across each fuse is the other way to check parasitic drain indirectly. The folks at CARQUEST Technical Institute ( presented this technique in their training seminars, and Karl Seyfert discussed it in his December 2010 and January 2011 Trouble Shooter columns. This method is based on a simple electrical principle: If no current flows through a connection, there’s no voltage drop across the connection. But when current does flow, there’s a measurable voltage drop across that connection.

To use this technique, grab a multimeter with a millivolt (mV) scale, as well as test leads equipped with pointed probe tips. After you prep the vehicle for parasitic drain testing, per the directions I gave earlier, locate the fuse panel. Set the meter to the millivolt scale and connect its leads across a fuse. To do this, probe the tiny holes in the top of the fuse. Connected this way, the meter is measuring the voltage drop across the fuse.

A meter reading of 0 means no voltage drop, no current flowing-the circuit is off. In the case where the circuit powers a component that normally draws current when the ignition switch is off, the meter will show some sort of voltage drop across the fuse. The greater the voltage drop, the greater the parasitic drain. Of course, the parasitic drain should be within spec or within the normal range discussed earlier.

Therefore, this approach offers a major advantage: It pinpoints the exact circuit that’s causing the excessive parasitic drain without installing a test switch or pulling fuses. The less fuse R&R you do, the less risk of mistakes such as shutting off the wrong circuit, accidentally waking up a “sleeping” computer, etc.

But as I noted earlier, there’s no perfect test format for diagnosing battery drain. For example, some fuse panels are in awkward locations. So performing the voltage drop could put you in a painful, contorted position-probably on your back. Also, you can’t perform the voltage drop on some fuses because there’s no access. You see, the shell or body of some fuse designs completely covers the fuse’s terminals. Obviously, you can’t probe covered terminals.

The fuses in late-model Ford products are an interesting example. Some of the fuses have accessible terminals, others do not. What’s more, other Ford fuse designs appear technician-friendly because they have removable, clear covers on them. The trouble is that these transparent covers seem to break very easily during removal. Worse yet, the cover’s little retaining tabs on the main body of the fuse break readily, too.

Many late-model Toyota fuse panels hit us with a diagnostic double-whammy. First, some fuses have covered terminals that prevent us from probing them. Furthermore, these covered fuses often are grouped into one, much larger fuse assembly. This is a long row of fuses molded into a single housing. So removing this fuse assembly would kill several circuits simultaneously-whether you wanted that result or not.

Network Nuisances

Wow, I’ve kept you for a while here. Hopefully, you’ve gleaned helpful information from this feature. Here are some closing thoughts on the topic.

Don’t overlook network activity. For instance, suppose you pull the fuse for the radio and the excessive parasitic drain disappears. Your first instinct may be to blame the radio itself. But suppose a node or module on the network is awake when it should be asleep. That troublesome node could be talking to the radio; hence, the drain caused by the active radio is only the symptom, not the root cause of the excessive drain.

When in doubt, scan the vehicle’s network to see if any modules are awake when they should be sleeping. The challenge here is that capabilities vary from one scanner to another. Hopefully, your scanner can communicate well enough with a network to confirm which module is sleeping and which is talking, for example.

Remember, depending on the remote entry system a vehicle has, someone walking by with a remote fob from an entirely different vehicle could accidentally wake it up. Once the system wakes up, your meter would show an additional, erroneous drain that would disappear when the entry system goes to sleep again.

When researching a parasitic drain symptom, watch out for those less obvious possibilities. For example, if the vehicle has a natural vacuum evap system, the ECM could remain awake and energize the evap system’s vent solenoid valve for some period of time.

Usually, a professional-grade multimeter is equipped with a Min/Max capture capability. The Max capture feature may help you catch the additional parasitic drain a module creates when it wakes up. Until next time, good luck and keep smiling.

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