Shopping Cart
Your Cart is Empty
There was an error with PayPalClick here to try again
CelebrateThank you for your business!You should be receiving an order confirmation from Paypal shortly.Exit Shopping Cart

Parasitic Battery Drain Testing

By Lee Hearnden - LMH Engineering Services

This Technical Article is based upon searching for Electrical Loads' on the vehicle battery that are very small yet can be sufficient to drain the vehicle's battery over several hours or overnight.

Vehicle Electrical Systems

Before taking a look into the Parasitic Battery Drain we thought it was best to take a look at the Electrical System of a modern Vehicle.

Figure 1.0 - Vehicle Electrical System Layout

As the consumer demands more from the Automotive Car, Vehicle Manufacturers are under more pressure to give more features in order to both meet these demands, and to be more competitive against other models in the same vehicle class.

                     These can be systems such as the following:-

                    WI-FI - Internet and Hot-Spotting Services.

                   In Car Entertainment Systems - Radio, SAT NAV, Multimedia, RSE, USB, DVD, TV.

                  GPS - Tracker and Location Systems, SAT NAV.

                  GSM - In Car Phone Systems.

                 Telematics Systems - Combined GPS, GSM, WIFI, USB Systems

                 Powertrain Systems - Emission Control, Power Control, Safety Control.

                 Comfort Systems - Heated Seats, Lumbar Massage, Pneumatic Systems.

                 Safety Systems - Airbags, Seat Belt Pre-tensioners, Impact Detection

To be able to include these systems comes a superior and complex vehicle wiring system that has to be able to deliver the strategy of communication throughout the whole vehicle ensuring that all the systems can function to the best performance achievable.

In recent years there has been a huge increase in the number of components required to perform even a basic function which cant be seen by the consumer. For example to simply control a window involves a complex system of Switches, ECU's and Motor Assemblies incorporating position sensors and control so that the operator can control not just the local window but all the windows in the car and on the same switch, able to control window blinds.

In order for the ECU's to process this information and communicate effectively a high volume of complex cabling would be needed which is where the use of Networked Systems became a far better solution.

                    This has included systems such as the following:-

                   CAN Bus - Used for most ECU to ECU Communications throughout the Vehicle

                            - High Speed CAN

                            - Medium Speed CAN

                            - Low Speed CAN

                                       Example CAN Systems

                                           - Comfort System CAN

                                           - Drive System CAN

                                           - Entertainment CAN

                                           - Safety CAN

Flex Ray - Extremely Fast Protocol used more for Chassis and Suspension Systems

K-Line - Diagnostic Communication often gateway to the ECU Fault Code EEPROM

LIN Bus - Used for most Comfort Control Systems such as Seats.

MOST Bus - Used for In-Car Entertainment and Multimedia systems.

On most modern vehicles all these Communication networks will all be able to interact with each other through the use of a 'Gateway' device which allows better diagnostic capabilities but also better control of the vehicle using different systems to interact with each other.

Example of 2 Vehicle Systems networked together to form an Enhanced Control System

Figure 2.0 - ESP System Overview.

Communication networks work on a very small network of cables reducing the need for larger harness's therefore contributing to reduced weight of the vehicle and allowing for better packaging inside the vehicle structure. By further multiplexing these systems together there can be more communicative control through the vehicle enhancing the occupants experience.

Example Layout of Multiplexed Systems

Figure 3.0 - Typical Multiplexed Network Layout

By having so many Electrical Components on the Vehicle, this increases the Electrical Load on the Vehicles' Battery and the Charging System therefore increasing the risk of Component and/or system failure.

By having so many systems multiplexed together this changes the way in which modern cars can be diagnosed. Fault finding is often done with the use of a Scan Tool and now we are seeing more increased use of a Vehicle Oscilloscope due to the way in which Vehicle Electronics/Systems operate.

Its also just as important to understand that with all these ECU's come a lot of calibration data therefore there will be a small amount of permanent current to power the 'KAM' (Keep Alive Memory).

This is important when discussing Parasitic Current Drains.

What is a Parasitic Battery Drain?

A Parasitic Battery Drain is the term given to the Vehicle's Battery often going flat overnight or within hours/days of the Vehicle being left with the Ignition fully switched off. This can just be down to a battery that's starting to lose its internal capacity and is not capable of holding sufficient charge.

Other causes that are not controlled and causes of a Parasitic Drain are as follows:-

  • Relay Contacts can fail causing a permanent power drain.
  • Alternator Shorted Diodes can cause a permanent power drain.
  • ECU's not powering down properly (sleeping) can cause a permanent power drain.
  • Electrical components keeping ECU's awake.

Battery Testing

Before searching the Electrical System for an 'Electrical Drain' its best practise to test the vehicles battery first to be sure that the Battery has reduced internal capacity (cells damaged or faulty), or not adequate of holding enough charge to power the Vehicles Starting System after a couple of days.

Pico Diagnostics Software

This can be done really quickly and effectively using Pico Diagnostics and selecting the 'Battery Test' function.

LMH Engineering Services Customers.

For LMH Engineering Services customers, there is an enhanced section in the Training Booklet provided that explains how to carry out the Battery Test and a Battery Guide to select the correct battery information.

Common methods of searching for Parasitic Drains

Up to now there has been many different ways of searching for Parasitic Drains, for example connecting a multimeter in-line on the battery ground, between the battery negative terminal and the cable, and then one by one, pulling a fuse till the current dropped out indicating the circuit causing the drain has been shut down.

In Line Fuse Testing

Another method is to use an in-line fuse breakout device and a current clamp and test each individual circuit for current draw until the circuit showing the excessive drain can be found.

Fuse Pulling Method

Similar to the In-line fuse testing, Another method is to place an 'Amp Clamp' around the ground cable of the vehicles' battery and either connected to a multimeter or oscilloscope, set up so that the Technican can view the current and again, one by one, pulling a fuse till the current dropped out indicating the circuit causing the drain has been shut down.

Then using wiring diagrams and OEM Information the Technican would test all components on that circuit to trace the source of the Drain.

Modern Electrical Systems

On modern vehicles though these are probably the worst way to search for a parasitic drain.

Most modern vehicles are designed to have an 'Electrical shut down period' in which all the ECU's within the Electrical system all have a time to systematically shut down. On some vehicles this can be up to 25 minutes and in some cases even longer. When you remove a fuse from the fuse box and then re-install it (with or without an in-line testing device), you risk waking the system up by powering up a particular ECU that then sends a message out over the CAN Network as described above, thus waking the whole electrical system up.

The Technician then has to wait till the system fully shuts down again before removing the next fuse.

You can imagine this could take a long time and effectively waste a huge amount of time and no diagnostic information is yet been obtained. The other problem is by removing fuses and powering down circuits you also risk losing vital ECU data in which a re-calibration could be required or the vehicle would needed to be driven to perform a re-learn and adaption sequence.

Other consequence are radio codes being needed and stations need to be re-stored and clocks and information displays being reset as the KAM (Keep Alive Memory) will be cleared. By this time you

really could be spending a lot of time just putting the customers car back to the condition it arrived in and even now you may not have removed the fuse that's powering the draining circuit.

So what's a better way with today's vehicles?

A different and more adopted method of testing Parasitic battery Drains is to use the Oscilloscope as a Voltage Measuring device and checking the voltage across every fuse. If there is a current passing through the fuse, this will be seen by a measured voltage.

By testing the circuits this way there is no need to remove fuses from the fusebox therefore no risk of waking the electrical system back up from sleep mode. This will ensure ECU data is not lost nor radio codes and other stored information.

Current Readings

At LMH Engineering Services we have developed the following chart that will allow you to identify the voltage reading and look up the current value for the fused circuit you are testing.

Simply click on the following link to open and download the chart.

We advise you to print a copy so you can use it as a quick reference guide:-

Click here to Open Fuse Current Chart PDF


Any values obtained which are greater than 50mV indicates that the fuse is faulty.

In order to complete Parasitic Drain Testing with the PicoScope Automotive Oscilloscope the following equipment is required:-

  • PicoScope Automotive Oscilloscope
  • CAN Test Box (Not Essential)
  • 60/20 Amp Current Clamp
  • BNC to 4mm Test Lead
  • Unshrouded Adaptor (Used with CAN Test Box)
  • Flexible Back Pinning Probe (Red)
  • Flexible Back Pinning Probe (Black)

Setting up the PicoScope Hardware

CAN Test Box

The CAN Test box was used to give a reliable Chassis Ground inside the vehicle when taking voltage measurements of components and ECU feeds. This can easily be done with the use of the 'Large Dolphin Clip' or the NEW 'Battery Clip' instead and clipping it to a suitable ground point, however we found it more easier with the CAN Test box.

Channel A - Current Measurement

Connect the 60/20 Amp Current Clamp to Channel A of the PicoScope as shown in the illustration below. Place the Current Clamp around the Ground Cable and ensure the jaws of the clamp are fully closed. Switch the Current Clamp on and ensure its set to the 20 Amp Range and then press the Zero Button to ensure the clamp is fully zero'd for the test.

Channel B - Voltage Measurements

Connect a 'BNC to 4mm' Test lead to Channel B of the PicoScope as shown in the illustration below. Connect a 'Flexible Back Pinning Probe' to each of the 4mm ends of the 'Test lead'.

This will be used to test for voltages across a fuse within the Vehicles Fuse boxes.

For the purpose of this test this channel was connected to the Main Ignition Relay (KL15) to show the Ignition being switched off and the Electrical System shutting down.

Connection Diagram

We did this on a 2001 Vauxhall Vectra 2.2 CDX Z22SE

Testing Notes

When testing interior fuse boxes you need to prevent the Vehicle Electrical System from waking up. If you require the bonnet, boot and vehicle doors to remain open to access fuse boxes then it is good practice to close the switch assembly so that the Door ECU and Body ECU thinks that the car is fully closed. This will ensure that no CAN messages can be sent in detection of an open car and this should help keep the system from waking up.

You can do this by either unplugging the switch or by using electrical tape and taping it down i.e. contacts open.

Waveform 1 - Vehicle Electrical Shutdown Waveform

Click to Open and Download Waveform

Waveform 2 - Alternative Vehicle Shutdown Waveform

Click to Open and Download Waveform

Waveform 1

It can be seen that on the Vauxhall Vectra waveform the system shut-down period is extremely quick and is at zero within 1.5 seconds.

It can be seen that the current, Channel B, has fully zero'd indicating all ECU's and circuits are shut down.

Please note there are no known issues with this vehicle.

When system fully shut down, current draw dropped to average of 30mA.

Waveform 2

It can be seen on this waveform that the shut-down period is a lot longer taking up to 5 minutes 50 seconds to fully shut down. This Vehicle is fitted with an enhanced level of equipment compared to the Vauxhall Vectra and has a large number of ECU's to shut down in order for the Electrical System to be fully 'asleep'.

General Waveform Notes

When looking at a Shut-down current waveform its important to identify that the system has fully shut down. If a substantial amount of current can be seen on the current trace then this indicates that a component on the Electrical System has not Electrically shut off.

There will be a small amount of current (mA) that will be seen as this will simply be a small amount of power needed to keep the clock and other minor circuits awake but this should not ideally exceed 20mA to 30mA. This is often controlled by the vehicles KAM system (Keep Alive Memory).

Its best practice to check OEM Vehicle Information for correct shut down status and sometimes 'Live Data' maybe required to check CAN information for Bus traffic that's actively keeping the system awake.

Current Clamps

Click to view Pico Current Clamps

Diagnostic Kits

Click to view

Pico Diagnostic Kits

Support Section

Click to view our Support Section

Follow Us on the Following Social Networks

Automotive Specialist Kit

LMH-AS Automotive Specialist Kit


View Product

PP619 CAN Test Box


View Product

PP967 Fuse extension leads kit


View Product

TA018 20 A / 60 A DC Current Clamp


View Product

TA019 600 A DC current clamp


View Product

TA167 200 A / 2000 A DC current clamp


View Product

TA189 30A Current Clamp


View Product

TA234 30 A Current Clamp


View Product