Parasitic battery drain

Jan. 1, 2020
Just by doing the first step, disconnecting the battery cable, on a modern vehicle can completely nullify your diagnostics.  

Let’s face it, parasitic battery drains suck, and I’m not only referring to voltage.  Locating the source of the draw also can suck up a lot of time as well.  A couple of decades ago, a test light and a fuse puller were pretty much all that was required to track down a problematic circuit that was causing a continuous amperage draw on a vehicle’s battery.  Now with the technological evolution of automotive electronics and its encompassed networks, the archaic methods previously used to detect parasitic draws can derail your diagnostics before you even get started.  While diagnosing parasitic draws probably never will be on your list of favorite work orders, encompassing the techniques here hopefully will make narrowing them down a more efficient process.

The top of the battery case itself can be a cause of the parasitic drain due to corrosion.  Here there was nearly 8 volts constantly draining the battery.

Not too long ago when a customer had a concern about their vehicle’s battery going dead after sitting overnight or after a few days, the first step was to disconnect one of the battery cables and install a test light in series between the cable and the battery post.  The glow emitted from the filament was relative to the amount of current being consumed by the vehicle’s electrical system.  Common causes were glove box lights remaining on or sticking brake light switches.  Tracking them down was as simple as going to the fuse box (don’t forget to trip the door ajar switch so the dome light doesn’t stay on) and pulling fuses one by one, until the test light glow diminished.  Then finding out what circuits were powered by that fuse and eliminating them one by one.

Just by doing the first step, disconnecting the battery cable, on a modern vehicle can completely nullify your diagnostics.  On most vehicles, removing battery power from modules on a network reboots them and that may have even fixed the problem without you knowing it, not to mention erased clues that could have helped with your diagnostics.

Start with the Source

One the best ways to prevent loss of power when installing an DMM in series with the battery cable.

Sometimes when looking for the source of the parasitic draw, we overlook the source of voltage itself: the battery.  Uncountable hours have been wasted without first testing the battery.  First, the battery needs to be adequately charged.  Too low of a voltage might cause modules to flake out and your test results will be inaccurate.  Also, don’t forget to measure for any drain caused by corrosion on top of the battery, referred to as case drain.  Because the corrosion is conductive, it creates an electrical path for current to flow from the negative post to the positive post.  To check, place the negative end of a digital multimeter (DMM) on the negative post of the battery and place the positive lead on the case of the battery.  If your meter reads even half a volt, the battery needs to be cleaned.  Also, while you’re at the battery, don’t forget to measure voltage drop between the battery post and its respective terminal while starting the engine.  Too much resistance here can prevent a battery from charging properly, not to mention increase the amount of current needed to start the vehicle.

Not Just for Check Engine Lights
The first step I do when checking for parasitic draws, at least if the battery has not been completely drained since it was brought in, is attach a scan tool and checking for codes.  I’m not looking for powertrain codes, but I don’t dismiss any that are present as they also might be a clue.  I am looking for Loss of Communication or UXXXX codes.  The reason is a large amount of modules have their own power and ground circuits, but rely on another module or an ignition switch input signal to wake up or power down.

By supplying power at the DLC, you avoid the acrobatics required to try and hold the jump box clamps on the battery terminal and clamp while simultaneously installing an ammeter in series between the two.

When a module stays awake and keeps talking, also known as a babbling node, it also will keep sending out state-of-health messages to other modules.  When other modules are powered down and do not respond, the babbling node module sets Loss of Communication codes for the modules it is looking to receive messages from but didn’t.  Depending on what type of network and which module is the babbling node, several modules might stay awake and create a large parasitic draw. 

When the bus is still active, there will usually be a draw of 1 amp or more.  A tip to keep in mind is that on most vehicles, modules will power down more quickly when the vehicle is put in the locked position with the key removed, rather than just turning off the ignition and closing the doors.  Also keep in mind that some modules will stay awake when the key is in the ignition, or in the case of a proximity key, even when it is near the vehicle.

The ammeter is installed in series between the negative battery post and the negative battery terminal.

Testing Without Disconnecting the Battery
There are a couple of methods described in a previous Motor Age The Trainer video (May 2013) on how to avoid losing power to a vehicle while installing a DMM in series with the negative battery cable and post.  Attaching a jump box to the corresponding battery cables and disconnecting the negative cable while installing a digital ammeter in series is one way. The only problem is you need to be careful about knocking off the jumper box clamp while removing the cable from the battery.  Some manufacturers recommend trying to hold a multimeter probe on the negative battery terminal and the other on the negative battery post while removing the clamp while maintaining continuous contact with the probes. 

I think this sounds more like a challenge on a reality game show than a diagnostic procedure as you need three sets of hands to accomplish this successfully. My favorite method is using an OBDII connector with a power point adaptor that plugs into the outlet on the jump box.  They are fairly inexpensive, most are available for less than $17, and you don’t have worry about knocking off the clamp while trying to remove the battery cable and installing the ammeter.  There are also jump boxes available designed specifically for saving computer memory with the OBD II connector made to it.  Forget about using the old 9-volt battery-style memory saver that plugs into the cigarette lighter.  It’s power is not sufficient for modern vehicles and many modules “Keep Alive” power feeds are not shared by the same circuits.

When beginning your diagnosis, a good place to start is a visual inspection for aftermarket accessory installations such as alarms and stereos. 

What About Using an Amp Clamp?
That would negate removing the battery cables at all.  In my experience, low amp probes have too small of a jaw opening to fit around a battery cable.  The amp clamps that do fit around the cable seem to be too unstable and the readings that were displayed were not accurate when compared to the same vehicle while using an ammeter in series with the negative battery cable.

Getting Started
After the computer memory saver method has been put into place, disconnect the negative battery cable and install the black lead DMM lead to the negative battery post and the red lead to the negative battery cable.  Depending on your meter, it is usually a good idea to set the readings to Amps first, and then switch to milliamps after you determine the current draw is not too large.  While it is best to check manufacturer specifications for the vehicle you are working on, most manufactures allow up to a 50mA normal draw.

These Acupuncture Probes available from AES wave (05-762) make measuring voltage drop across the top of a fuse an easy task because of their fine point.

Another item to keep in mind is the time required for modules to power down.  While some turn off with the ignition switch position signal, others remained powered up for varying lengths of time.  Some manufacturers recommend waiting 20 minutes, while another recommends waiting up to one hour.  This is another area where it is best to consult service information for appropriate times.  As a side note, if your meter has an “Auto-Off” feature that times out and shuts off the meter, make sure you disable it or you will be starting over from the beginning. Open all doors/trunks/hoods/liftgates and trip the switches so they appear closed to the module(s) in charge of monitoring them.  This makes it much easier to access multiple fuse boxes, modules and components without disturbing the sleeping bus.

During the interview with the customer, have your service consultant ask some specific questions that may help you to focus on smaller or specific areas that may be the cause of the parasitic drain. Some questions to ask are:

• How many hours does the vehicle remain parked before the battery is discharged?

• How often is the vehicle used?

• How many miles was the vehicle driven before the vehicle was shut off?

• Was the vehicle locked or unlocked when parked?

• Where any activities taking place before shutting off the vehicle, such as watching a DVD, charging a cell phone, using the auxiliary port, using a Bluetooth device or using the navigation system.

• Has there been any recent work performed or accessories installed? (Aftermarket alarms and stereos are common sources of key off battery drains)

Placing the voltmeter leads across to two exposed legs of the fuse allows you to measure the voltage drop.  A voltage reading here indicates that there is current flowing in the circuit.

Using Voltage Drop
Another way to track down the offending circuit is by performing a voltage drop test across a fuse.  Keep in mind that each type of fuse will have different resistances, so it is best to consult the fuse manufacturers’ specifications for cold resistances. A fuse’s resistance is going to be measured in milliohms (0.001 ohms).  Keep this in mind when using Ohm’s Law to determine the amperage draw on the circuit.  A set of backprobes attached to the end of a meter works best to get on to the two exposed legs of the fuse. 

On J-Case type fuses, you will need to pop of the clear protective cap to access the fuse legs for testing.  The theory is that if a fuse is showing a voltage drop, current must be flowing across it.  Therefore, that is very likely the circuit that is running the battery down.  Keep in mind when measuring voltage drop across fuses that the type of fuse and the manufacturer will change the resistance factor.  For example, a 20 Amp rated Mini Fuse has a cold resistance of 3.21 milliohms (0.00321 ohms) and a standard 20 Amp ATO Blade fuse has a cold resistance of 3.38 milliohms (0.00338 ohms).  A 20 Amp Maxi Fuse has a cold resistance of 3.10 milliohms (0.0031 ohms), while a 20 Amp J-Case fuse has a cold resistance value of 4.29 milliohms (0.00429 ohms).

Here is the reading taken on a Mini 15 amp fuse when the dome light was turned on.

Here’s an example to clarify a bit.  An ammeter is installed in series with the negative battery cable.  After waiting for the modules to power down, a reading of 10mA is observed on the ammeter.  Reaching through the window and turning on the dome light, a reading of 611 mA (0.611 Amps) is displayed.  This method involved disconnecting the battery and installing an ammeter between the battery terminal and cable clamp.  A 15 Amp Mini-fuse protects the power feed to the dome light. 

According to the chart, the cold resistance of the fuse is 4.58 milliohms.  Placing the voltmeter leads across the top of the fuse shows a voltage drop of 2.8 millivolts.  Using Ohm’s Law (volts/ohms = amps), I can fill in the fields and come up with 0.0028 volts / 0.00458 ohms = 0.611 amps which is the same amperage that was measured when disconnecting the battery cable and installing an ammeter. There will occasionally be a slight variation between the numbers on the ammeter and the results of using the voltage drop across the fuse depending on the accuracy of the DMM and the manufacturer of the fuse.  However, it should not exceed 1/100th of an amp.

The same reading in milliamps is found when using an ammeter installed in series between the negative battery terminal and clamp as when using voltage drop across the fuse and Ohm’s Law.

For simplicity’s sake, a less cluttered math equation can be used since volts, amps and ohms are generally all in “milli” or thousandths of their respective value when dealing with parasitic draws. So the same equation would look like this: 2.8mV / 4.58mohms = .611A (611mA).

Divide and Conquer
So now you have narrowed down the offending circuit, but what do you do when the fuse powers more than a single circuit or component? Divide and conquer.  Combining the method of checking voltage drop across a fuse and separating each leg of the circuit, a draw can be more precisely isolated.  Locate the splice point in the circuit powered by the fuse with the excessive voltage drop.  Cut each leg coming from the splice and install another fuse of the same amperage rating. 

I have a couple inline fuse holders available for a few dollars each at a local parts store that I install at the splice in the circuit temporarily to determine which one of the legs is staying awake and also, which component is causing the battery drain.  This method also can be used when there is an excessive load in a system causing a fuse to blow.  Instead of unplugging multiple components one at a time and test driving to see which ones are not causing the fuse to blow, the offending circuit can be narrowed down in one step. By separating each leg, and installing an individual fuse on each, only the one that has the overcurrent will blow. 

In situations like this, when two or more wires are powered by the same fuse, you need to isolate which one is the problem.

However while looking for a parasitic draw, it gives you an opportunity to narrow the possibilities and stop trying to unplug everything powered by a particular fuse. There is really no easy method to try to keep the circuit powered up when doing this, unfortunately in this case, power will need to be removed and the process of waiting for modules to time out will have to be repeated.

Sometimes, the Eengineers Already Have Diagnosed the Problem for You
Another area we cannot overlook when dealing with battery drains is Technical Service Bulletins (TSBs). Checking before delving headfirst can save you hours of frustration. One example is Chrysler TSB 08-026-12 that explains that when the audio from a Bluetooth phone mutes during a traffic announcement it may not resume after commencement.  The radio will inevitably lock up when the U-Connect call ends which will cause a battery drain. 

The repair is a simple reflash, but not knowing about this could cause you to spend a lot of time locating the problem, then possibly replacing a radio that only needed to be reprogrammed.  Toyota TSB T-SB-0103-10 lists the Remote Engine Starter (RES) ECU as a probable cause of parasitic draw.  In fact, if the initial reading is over 75mA with the module connected, and disconnecting it allows the amperage draw to drop below that number, the TSB instructs you to replace the RES ECU.  Audi TSB 96 09 15 states that on vehicles integrated with Advanced Key, pressing the start button on the vehicle when the key is not present, wakes up modules on the CAN comfort bus and will keep the system awake. 

By installing an inline fuse holder in series with each leg of the circuit, you can use the voltage drop method to determine which leg to follow.  This also works great when tracking down a short which blows a fuse as the specific leg with the defective component or shorted wire can be isolated.

When the customer tries to start the vehicle later, the battery is too discharged.  Again, a reflash is the fix for this problem.  Both Ford (TSB 07-10-13) and General Motors (PIT4599A ) have had problems with rear wipers on SUVs not parking and causing a battery drain while periodically attempting to return to the correct position. You thought Intermittent drivability problems are hard to find! This would be very elusive since the draw would only happen during a module waking up and trying the wiper. 

Hopefully some of these techniques will come in handy the next time your routed a vehicle that has a complaint about the battery going dead after sitting for a short period. The automotive technology we encounter on a daily basis has evolved drastically over the last decade, so it only stands to reason that the approaches we take to repairing them must also.

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