Parasitic drain
by Steve Smith - Pico Technology

Customer complaints surrounding flat batteries are common throughout the year with peaks during the winter season. Those who comment that their vehicle fails to start after standing for a couple of days must set alarm bells ringing with reference to parasitic drain.

Battery evaluation is normally taken care of using automated devices or better still, the PicoDiagnostics battery test procedure where we test not only the battery, but the starting and charging system in one hit. Once the above are confirmed we must then focus our attention on battery drain and parasitic current draw.

This test is often overlooked yet should conclude any customer complaints surrounding non-starts as a result of a discharged battery. We can traditionally evaluate both a battery starting and charging system very quickly indeed but parasitic drain demands a prolonged test (30 minutes is common for a modern vehicle to shut down and move to sleep mode).

For those vehicles with a history of battery discharge you may choose to monitor the parasitic drain overnight or even a number of days. This itself presents a challenge surrounding current clamps.

Given our Pico current clamps are powered by an internal battery with a continuous operational time ranging from 14 to 16 hours (new battery installed) we most certainly run the risk of erroneous results during prolonged parasitic tests (not to mention the time the customer’s car is off the road).

I think it is worth mentioning here how a current clamp operates in relation to the values we capture on our scope screen.

The current flow through a conductor has the valuable side effect (in our case) of producing a magnetic field about the conductor. By closing the jaws of our current clamp around the conductor the magnetic field can be detected and converted into a voltage directly proportional to the strength of the magnetic field. The voltage is then sent to the scope and displayed on screen as a current value. Prior to connecting the clamp around the conductor a zero operation is carried out to ensure accurate display of current values and a true zero point reference. This is achieved by pressing the zero button on the clamp, which charges a capacitor within the clamp removing any DC offset value.

The TA018 clamp in the image below has a conversion factor of 100 mV/A.

Figure 1

In order to obtain accurate readings during prolonged parasitic drain tests we must be aware of the operational characteristics of inductive-style current clamps. I am referring here to drift.

While we confidently set our current clamp to zero before taking any measurement, prolonged measurements will highlight drift in the form of our readings drifting away from the actual value.

For example zero amps measured at 20:00 may be 80 mA at 08:00 the following morning, and when measuring parasitic drain an 80 mA error could mean the difference between a pass or fail sending the technician on a wild goose chase. Any measurements taken are only as valid as the accuracy of the equipment used and so being aware of the following tips will reduce the impact on our results associated with drift.

  1. Temperature and environmental conditions are the biggest offenders when we refer to drift.  Store your current clamp in the location it will be used to carry out the measurement. Not in a warm office then used in a cold workshop (acclimatize your clamp to the test environment).
  2. Ensure the clamp is used away from sources of environmental noise.
  3. Be aware of the minimum operating temperatures of the current clamp (0° C).
  4. Ensure the clamp's internal battery is prepared for overnight testing - this may require a new battery.
  5. Switch on the current clamp and allow the device to warm up (approx. 10 minutes).
  6. The current clamp can be installed to either the battery positive or negative lead but ensure the correct orientation of the clamp (refer to arrow on clamp jaws which indicates direction of current flow).
  7. Ensure the vehicle is locked and all keys are out of the detection range of any Keyless/SMART entry systems. Customer keys within the detection range of the vehicle will prevent total shutdown of the vehicle network.
  8. Using the zero function of the clamp is essential for accurate measurements; however, for overnight testing you may opt for the technique below:

The principle of zeroing the current clamp relies upon the charging of a capacitor within the device to remove any DC offset value. The capacitor has the potential to discharge overnight (varies between clamps) so resulting in drift away from the actual measurement value.  An option here with PicoScope is to switch on your current clamp and wait for the 10 minute internal warm up to elapse (do not zero the current clamp).

Connect the clamp to your PicoScope and select the relevant probe setting (60 A Current clamp [20 A Mode]) from the Channel options menu. Select the relevant input range which will depend on the style of vehicle.  ±1 A will cover you for the majority of measurements once the vehicle has completed the door locking/security phase.

With the waveform on screen you will notice the current value is not at zero amps as we have not zeroed the clamp (by choice). Here we can now use the PicoScope software to carry out a zero offset procedure by clicking on the Channel options button and selecting Zero Offset. PicoScope will now ask you to short the BNC terminals of your chosen channel together (this would normally be a x1 test lead).

At this point click OK inside the Zero Offset dialog box and the scope will now adjust the output value of your current clamp to zero. Here we used an alternative to using the zero function of the current clamp so bypassing the capacitor discharge issue inside the clamp when overnight testing.

Remember, after completion of your overnight testing, select the Channel options button and click the Clear button from the Zero Offset menu. This will restore your scope channel to factory setting with reference to zero offset.

Figure 2

Figure 3

With your clamp prepared you can now connect around either battery lead to commence overnight monitoring of the vehicle parasitic drain. Below we can see the shutdown period of the vehicle where the parasitic drain was out of specification for over 23 minutes (typical for modern vehicles).

The final parasitic drain confirmed the vehicle to be in specification sleep mode for over 8 hours at 47 mA (80 mA being the arguable recommended maximum value).

Figure 4

When monitoring parasitic drain we are only looking for the average of very low current values (below 80 mA). Such low values are always susceptible to noise and so you will find that low pass filtering will be essential to measure the average value of concern. Periodic spikes visible during parasitic drain measurements are often attributed to security LED operation or keyless entry systems looking for the vehicle keyset.

Whilst they are visible, they tend to pulse at very rapid intervals (high frequencies) and when averaging out the peak values of these pulsations, they have a negligible contribution to parasitic drain. Such pulsations will eventually halt as part of deep sleep mode built in by manufacturers to protect the vehicle battery during prolonged periods of vehicle inactivity (airport parking springs to mind here for instance).

To capture such high frequency spikes you will be required to set a high sample rate over long time frames resulting in very large file sizes.

As a general rule of thumb: Sample Rate is equal to the Number of samples across the screen/ Time span across the screen:

Figure 5

In order to view the sample rate and other properties surrounding your scope/capture, select Views and View Properties. The Properties box will now be displayed on the right side of the screen, highlighting the relevant information about your scope settings. Adjusting your number of samples and timebase will reveal a sample rate in the Properties panel. This allows you to ensure a sample rate is set appropriate for your capture ensuring you miss nothing, while providing you with a manageable file size containing the data of interest.

To conclude: Parasitic drain testing cannot be ignored and must conclude our test procedure when investigating complaints surrounding flat batteries. Whilst the procedure will extend the diagnostic time involved it will most certainly pay dividends to the customer and or workshop and avoid embarrassment from returning customers who may have been sold a battery on a prayer.


Add comment

March 31 2017 - 11:02:01

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September 29 2016 - 10:12:01

I have a Fiat with fuses with no small test points.
What test can be used in place of the voltage drop test other than pulling individual fuses.

Harold Rice
September 29 2016 - 3:13:31

I have a Fiat Panda 1.2 Dynamic Euro 4 2006.
There are 2no. fuse boxes, one in the engine compartment and one inside the car. All the fuses do not include the small access test points as described in the article.Therefore the non-intrusive fuse voltage drop test procedure cannot be adopted.
What is the alternative procedure for testing in this situation ?

Andy Rogerson
August 26 2016 - 4:21:49

A clear to understand article giving a good practical way of using the current measuring capabilities of the Picoscope.
The calculation in the example needs the units correcting.  Its not 53.9%, rather 53.9 Ah.

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