Advanced battery drain diagnostics

Nov. 1, 2019
Let’s review some time-honored methods as well as explore new methods for unmasking phantom battery drains.

A nominal amount of battery drain is present on every vehicle in the road once the vehicle is switched off and had enough time for its various modules to complete their “go to sleep” processes. The KAM (Keep Alive Memory) current draw for individual ECUs is typically around 1-3 mA per device. KAM is necessary for keeping both long term and short-term memory alive for functions such as the vehicle’s clock, DTC storage, telematics and module adapts to name just a few. The number of ECUs has increased in recent decades making the overall vehicle normal parasitic drain tally up to as much as 30 mA to 50 mA – the numbers that seem to be the unofficial max specs.  For this reason, many OEMs have moved toward greater use of EEPROM to provide more permanent memory storage and improved power management to decrease battery drain. This has helped to lower the normal parasitic on many newer vehicles to as low as 5-10 mA in some cases. Factory battery saver devices at the battery or in the BCM along with automatic module power down algorithms during extended storage times have also contributed to aiding today’s mobile electronic monstrosities in keeping their 12-volt starting batteries alive for storage periods of months at a time.

Intermittent battery drain – the phantom!

Parasitic drains on newer vintage vehicles seem to be intermittent more times than not due to the complexities of data buses and modules staying awake when they should be asleep. The most complex modern vehicles, however, can still have old school drains that can run a battery down in a few hours or few days. Even a brand-new vehicle (non HEV/EV) has an alternator that can suffer from a battery draining diode leak. Faulty door latch/door lock switches can cause drains as can glove box and trunk light switches. The old trick of opening a trunk or glove box and grabbing the bulb to see if it’s “two hours hot” or “two seconds warm” still works today just as it did 40 years ago.  Ouch: this test still burns your fingers too . . . but at least you found the drain!

The following steps will help you unmask your next phantom battery drain.

Step 1. Make sure it’s not something else. Document all DTCs in all modules. DTCs stored in modules after a battery drain are often “effect” DTCs rather than “cause” DTCs. Regardless, retrieve and document prior to clearing. Some DTCs that return may be good clues especially if they are U-codes (communications related) as we’ll see later. ALWAYS test the battery and charging system prior to performing a parasitic battery drain test. Many times, the customer complains of a battery drain or “short” when the actual problem is a faulty battery or charging system problem. A battery with an intermittent shorted cell can sometimes mask itself as a phantom battery drain. If there are any doubts on the condition of the battery — replace it! An underperforming charging system (faulty or incorrect alternator, poor connections, an undersized alternator or slipping drive pulley) can lead to false battery drain assumptions as well. If the vehicle is a HEV, PHEV or EV keep in mind there is a DC-DC converter that functions as an alternator. The same charging system tests complete with fully loaded electrical system conditions should be performed just the same as with a conventional mechanical alternator.

Step 2. False Ohms test — Prequalifying excessive parasitic current draws. DMMs (Digital Multimeters) read false resistance when you use their ohmmeter function on a live circuit.  You can use this tidbit of knowledge to quickly judge whether an above normal amount of current is flowing out of the battery.

Try this test in your shop: 

  1. On a vehicle that has been sitting for an hour or more (all modules presumably asleep) connect an ohmmeter between the battery negative post/cable and a known good ground such as one of those short ground wires to a fender or radiator support. You’ll probably see the predicted 0.2 to 0.5 ohms of resistance which is mostly the resistance of your leads and alligator clips. 
  2. Open the door to allow the BCM to wake up/dome light to come on. You almost always see a “false resistance” of several ohms on your ohmmeter. Switch on the ignition and you’ll see even more of this false resistance.  
  3.  Turn off ignition, close the door and lock the vehicle with the keyless entry fob to speed up the sleep process. It may take several minutes but you’ll see that false resistance on the known good ground go back to the original minimal resistance you had to begin with (Figure 1).
Figure 1 - False resistance on grounds – your clue that there IS an excessive battery drain present

The photo on the left shows 0.5 ohms between the battery negative cable at battery post and ground wire to fender. Meter’s leads connected (together) measure 0.4 ohms, so we have a good ground. Parasitic current draw was normal. (18 mA).

The photo on the right shows the same ground resistance measurement only with a moderate parasitic current draw of around 1.5 amps. 28 ohms of resistance? 

False reading tells you there is a drain!  it’s time to do a detailed parasitic current draw test

Now that you’ve seen for yourself how this trick works, use it on your next vehicle with a phantom drain. Simply watch the ohmmeter. When extra ohms pop up, there is an above normal drain occurring at THAT moment. Catching the phantom drain in the act BEFORE running time consuming and intrusive parasitic drain quantification and offending circuit isolation is the best way to avoid the frustration associated with unmasking the phantom.

Figure 2 - A calibrated quality inductive amp clamp with jaws large enough to incircle the battery cable(s) coming off either battery post in the hands of an above average tech is great. That holds especially true when attempting to scope the drain over time. Many of us will get in a hurry, however, and make the common mistake of not correctly converting 1 mV / 100 mV measured at the scope or DMM into actual mA.  The same holds true for those who use a shunt resistor in series with the  battery cable to measure mV of voltage drop. An extra decimal point in either direction can make or break an accurate diagnostic decision! Figure 3 - The Fluke 87 DMM ammeter wired in series across a battery disconnect switch (400 mA ammeter scale used) is displaying  08.66 mA. The most accurate inductive amp clamp / DMM I own with large enough jaws to accommodate multiple battery cables is showing 00.03 A. That’s 30 mA which is a huge difference from the actual 8.66 mA. My advice is to use a quality ammeter in series as pictured!

Step 3. Keep the battery connected to “sneak up” on the phantom! Use of an ammeter connected in the typical fashion results in the battery being disconnected. One way to avoid that is to use an inductive amp clamp that converts the current draw to mV for conversion on a voltmeter or an inductive amp clamp self-contained DMM. There are a lot of inductive amp clamps out there on the market. I’ve found very few that have large enough jaws to fit around several battery cables AND is accurate down to a few mA  (Figures 2, 3). So, if you really want to accurately know how many mA the vehicle is drawing, use an ammeter in series (Figure 3) with the battery cable on the lower amp scale (most are 400 mA). Powering down the 12-volt system to connect a meter or shunt resistor (for a voltage drop/amp draw conversion); however, is a Murphy’s Law guarantee to make an intermittent excessive battery drain go into remission. Remission can sometimes last as longer than yours or your customer’s patience. Why risk having the phantom drain go into remission? Avoid depowering the system by installing a 12-volt battery boost box powered memory saver at the DLC (Figure 4). This will keep the 12-volt system alive while you install a parasitic load test adapter (battery disconnect switch).

Figure 4 - Battery memory saver / DLC  adapter.  Connect to a 12-volt boost box to keep vehicle alive during ammeter connections. Old style 12-volt accessory plug style (cigar lighter) memory savers don’t work anymore as many are disconnected from B+ when the vehicle is off

Next, trip driver’s door latch switch into the door shut position and jumper around any hood or other switches that are associated with gaining access to the battery and fuse panel(s). You are going to want to cycle the ignition on and off or drive the vehicle to get the phantom to appear so make sure you remember to close the parasitic load test adapter whenever cycling the ignition. Failure to do so will blow the fuse in your ammeter.

Figure 5 - The Midtronics In-GEN IDR-10 battery drain data recorder graph displayed for this 2012 Chevy Malibu indicate each of these spikes is separated by a period of 28 minutes. This is normal - a result of the ECC pinging the IPC for data regarding outside air temperature.  The average magnitude of these spikes is – 2.03 A. After the 4th and final spike, the current draw should return to the normal 10 – 20 mA draw.

Step 4. Know what’s normal. What really happens with the vehicle’s battery drain over the course of several hours of key off time? Try this experiment. The next time you find yourself waiting in a vehicle for 30 minutes or more try turning off the key and sitting in silence. Listen to what goes on. You’ll likely hear some activity. The occasional relay click and buzzing sound of the vehicle’s final checks on air suspension trim height, an evap leak pump/vent solenoid, HVAC doors cycling or even a short series of HVAC after-blow blower runs to prevent evaporator odor. While you may not hear these actions during your normal workday in a noisy shop, those activities are going on behind the scenes. A little overall knowledge on what equipment the vehicle has is helpful when you’re trying to determine if a measured spike in current draw is a normal activity or the phantom battery drain you’ve been trying to catch. Unless you’re using a scope on a long time base or another piece of equipment such as a data recorder (Figure 5), always use your watch to check the time and jot down when a current spike occurs. If the same current spike occurs EXACTLY every 10 minutes, (Figure 6) is only an amp or so and lasts a second or two, you probably aren’t looking at the reason for the customer’s battery drain. In the 10-minute example, a likely normal occurrence would be a Telematics (OnStar, Lexus Link, etc.) module waking up temporarily to determine if any calls to the vehicle from a call center are coming in.  Other normal parasitic loads occur every few seconds such as a theft deterrent LED blinking (Figure 7).

Figure 6 - Measuring the current draw over time on a 2012 Cadillac Escalade. When the battery cable is initially connected through an ammeter the current draw is almost 5 amps as various modules that are active with the key off power up. Within 1 minute we see most modules have gone to sleep but the drain is still excessive. After 10.5 minutes the current draw falls to an acceptable -0.018 amps (18 mA).

Step 5. Isolate the sub circuit (without pulling fuses). OK – so you have caught phantom in the act. Let’s say you have 350 mA (0.350 amps) running the battery down.  Time to determine out what circuit that drain is on and then identify what sub circuit/component is the root cause. Do you pull fuses? NO! That’s old school. Pulling fuses only works if you pull the guilty fuse FIRST! Remember Murphy’s Law of battling the Phantom drain? Don’t waste time battling module power down/power up/entering sleep mode over and over.

Figure 7 - This vehicle’s battery drain (after 10 minutes) went down to just over 7 mA but popped up to 9.5 mA periodically. Every 2 seconds this vehicle security light blinks. Placing the meter next to the blinking LED showed the increased current draw occurs just slightly after the light blinked (meter update delay) allowing us to correlate a normal event via meter observation.

Step 5a. Voltage drop test each fuse. If the fuse’s metal contacts are not accessible (plastic covers) you’ll have to pull each fuse individually. If the drain doesn’t go away, you’ll then need to reinstall that fuse and wait several minutes if the battery drain shoots back up even higher while any modules on that fuse power back up and then gradually go back to sleep. On fuses which you can access their probe test points (as you do when checking them for being blown) you are not looking for excessive voltage drop.  You are looking for ANY voltage drop. Any circuit with current flowing (even a tiny bit) will have “some” voltage drop. Take care to get a VERY good bite with your meter tips on each fuse’s set of metal probe points. Use your meter’s mV (millivolts) setting. When you’ve got a good contact on BOTH probes with the fuse the meter will settle completely on a number.  That number will either be 0.00 mV (no current flowing – Figure 8) or some very small mV reading that is rock solid and not changing.  A constantly changing mV reading is not an accurate reading of anything – it’s simply “digit rattle” which is the meter and leads catching stray EMI through the air. Take any mV readings you measure and refer to a mV to mA conversion chart. Power Probe has one of these charts on their website www.powerprobe.com/fuse-voltage-drop-charts. The chart allows you to factor the fuse’s type and amp rating along with your measured mV voltage drop reading and determine exactly how much current that sub circuit is drawing (Figure 8).

Figure 8 - Power Probe’s easy-to-use chart. 0.00 mV means zero current draw. On a 10-amp mini fuse, 2 mV of voltage drop on a fuse equals 270 mA of battery drain. More than enough to kill the battery in a few days! 0.2 mV would be fine though.

5b. Use advanced tools. A DSO (Digital Storage Oscilloscope) can not only be used to measure current draw over time (with an accurate enough low amp inductive amp clamp) it can also be employed to observe data bus activity. ANY activity on ANY data bus after 20-30 minute of ignition off (with any smart fobs OUTSIDE the vehicle) should be investigated. Going back to Step 5 (knowing what’s normal) is a big help here. Did the HS CAN bus become active for a few minutes in order to perform an EVAP test? In that case the PCM will temporarily be awake but shouldn’t run the battery down. Is there an intermittent faulty door jam switch waking up the BCM over and over making the LS CAN bus intermittently active? That will drain a battery. While the BCM is awake, plug in a scan tool and look for signs of improper door latch status if the OEM puts such a PID on the bus.  Another advanced tool to employ in your hunt for the phantom drain is a thermal imager (Figure 9). It will show relays stuck on (big battery drains) and even a power feed circuit with insulation chaffed creating a high resistance short to ground.

Figure 9 - Photo on far left and center shows a Snap-On thermal Imager’s built in library example of a known good image of a relay that has properly shut off compared to one that has stuck on. Photo at far right is a fuse panel after the vehicle has been shut off with no excessive parasitic current drain. Residual heat is shown across the entire fuse panel with the key off.  There was no difference however, in temp images of the dome light fuse – lights on or off.

Words of wisdom

My personal mentor in phantom hunting (my dad Ray Hobbs) always said the most advanced tool you can employ to unmask the phantom drain is your experience and ability to reason. Remember pulling all DTCs in all modules back in Step 1? Any module that continues to be involved in the setting U-codes even after the battery has been recharged/replaced is worthy of your scrutiny. A module what should normally be asleep with the ignition off (i.e. ABS) and continues to be implicated with U-codes may indeed be the phantom itself. Happy phantom hunting!

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