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Advanced battery drain diagnostics

Friday, November 1, 2019 - 06:00
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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.

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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.

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