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Discussion Starter · #1 · (Edited)
Problems with parasitic drain (standby current draw) keep popping up, particularly when aftermarket radios or non-genuine Gateway modules are fitted. This post is a generic 'How To' and it can be applied to most vehicles except EV. The EOS V6 is a special case, I don't own one but I'll include the method in theory at least.

Current drain either parasitic when the ignition is off or when items are switched on can be measured with a DVM multimeter in a number of ways:

1. By measuring the small voltage drop across individual fuses
2. By disconnecting a battery terminal and insering an ammeter.
3. My measuring voltage drop across a battery lead.

Method 1 is quick and often used by garage techs but has disadvantages - The d.c voltage across a fuse will be very small (mV) and cannot be related directly to current draw. The voltage across different value fuses will change according to their size and load and can vary if fuse blade contacts are dirty. However, with a good DVM this method can tell you there is a positive current draw but not how much.

Method 2 gives the best accuracy which I've used in the past, but needs care because you are trying to measure a small standby current in the tens of mA but higher current consumers can turn on whilst the battery connection is being made through the meter. Some DVMs can be damaged and their internal fuses are expensive. In addition, disconnecting and reconnecting a battery can bring on fault codes. Not a problem for me with diagnostics, but can be a headache for others.

Method 3 is the method I will explain here. You need to put in some effort and patience! It's non-invasive and doesn't require the battery to be disconnected or fuses to be pulled. The method can be applied to most cars, your EOS and MY07 Tdi. When a d.c current passes down a battery connection lead, a small voltage will be produced across that section of the wire. In the ideal world with no wire losses, the battery connection wire sizes would be very large gauge, but in practice their size is just fit for purposes. This voltage will be in the region of tens of mV.

Open the hood, lift the battery cover and the main fusebox cover. From now on there are exposed high current terminal points so keep metal tools away!! Look at the routes taken by the battery -ve wire (black) and the battery +ve wire (red). Note the V.W convention to connect the starter on a separate (black) wire to the battery +ve terminal). MY07 EOS is Low Line with fuses (features!) missing, others will have more fuses or a different box.

You can use voltage drop to measure current on either the black -ve wire or the red +ve wire. The Tdi negative wire is very short and bolts to a chassis ground terminal close to the battery. The voltage drop across this cable is very small. The red +ve cable loops down entering the main fusebox from underneath and attaches to a terminal in the top row. This is a longer cable and will produce larger voltage drops more easily measured.

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We will use this red +ve battery cable as a 'current shunt', but first we must calibrate it to relate current draw to the d.c voltage drop across the wire. This calibration only needs to be done once or when a battery terminal is disturbed to fit a new battery. The calibration method should be done carefully several times with a good fully charged battery.

You will need a brain, able to use a calculator, do sums in your head and the following:

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The test lead measuring mV connected to the test meter can be very long (4-5M), it won't affect accuracy and length is useful to reach the dash fuseboard or trunk. I used 6A mains flex. The DVM test meter must be a good one able to measure down to a few mV. Mine is a Brymen BM867s but there are cheaper for around $20 with the following spec.:
4000-6000 counts, > 4 display digits, d.c voltage range 200mV resolving to <1mV, 0.5% accuracy. e.g Auto Range Digital Multimeter 6000 Counts DC AC Voltage Current Meter Tester UK | eBay To calibrate the battery lead, a second DVM measuring current would be useful, but you can measure current first then quickly switch to measuring mV drop across the battery lead with the lamp lit.


The first test we do is measure the voltage drop due to parasitic current and check that it does drop after 5 min when the CANbus sleeps. This small background voltage translated to current will be deducted from later tests where we are measuring connected device load currents. This correction can be ignored for load currents >500mA.

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This was my voltage drop reading after the interior lights had shut down.

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.......and this was my reading after 5 min when CAN had gone to sleep. But we can't yet convert this to a meaningful current value.

With the DVM mV meter connected as above, we now add a test load with a second DVM in series between the fusebox +ve terminal and the battery -ve terminal post. NB: Use the battery post NOT some ground metal nearby. This is my setup, the H7 test lamp is hidden and I'm showing a second DVM used to measure the lamp current and mV drop simultaneously.

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Then we can do the sums: Subtract 0.24mV from the 11.78mV reading = 11.54 mV. i.e that is the voltage drop produced by our lamp current draw of 3.86 A. Using Ohms law V= IR we can calculate the cable resistance and make a table for several values, or even make an Excel sheet. In my case, The cable resistance is 11.54/3860 = 0.0029896 ohms. Repeat 3 or 4 times for consistent results then calculate the average resistance measurement.

The following table is calculated from Current draw I (mA) X Resistance (ohms) = Voltage drop (mV)

Current draw (mA) Voltage drop (mV)

I think you should get the idea, but your calibration results and table calculation will be different to mine! Once the battery cable is calibrated you can look up intermediate voltage drops (mV) to measure the current. There will be errors mainly due changes in terminal resistance at the battery clamps and the fusebox terminal. But if you clean them before calibration, future accuracy should be stable and good enough for fault finding. Parasitic draw conclusion: MY07 is good, shutting down to just over 50mA after 5 minutes. I think that's O.K for the alarm and CAN modules sleeping?

So far calibration and tests have been done without the engine running and with key off, which is fine for measuring parasitic drain. The alternator is connected on the terminal block and it's parasitic drain if faulty will be included in the measurement. For key on engine off, devices and controllers (ECU) will power up. To fault find suspect loads you can sequentially remove fuses until the mV reading drops. To measure a switch load you would have to subtract the mV reading with the load switched on from the standing load mV when the load was off.

Once you start the engine, everything changes because the alternator should be charging (?) and the charge current will be measured by the meter. The mV reading will change polarity. When near zero, the charge is exactly balancing the load. You can try this out by turning on headlights to find out what engine rpm is required to balance the load, then increase rpm to put the battery into positive charge - well it should with a good alternator.

The V6 has trunk mounted batteries and long cables extending batteries to the engine bay. There may be some complications due to their safety switch device, but basically the method is the same but with the advantage of a larger voltage drop. It may be necessary to use the battery negative to detect voltage drop if their safety device isolates the main battery +ve cable, whilst still powering potential parasitic consumers? The meter test lead just becomes longer to reach both ends of the battery cable. With such a long test cable that could get accientall crunched against body work you can solder a 100 ohm resistor to the far end test wire. It won't affect the measurement but will protect the long test wire.

voxmagna 02/05/2022
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