The purpose of this test is to evaluate the level of parasitic current draw from the battery during a vehicle's shutdown phase and sleep periods.
To obtain the correct parasitic drain values it is essential to make sure that the vehicle battery is serviceable and fully charged with all accessories switched off. The vehicle must be locked (security systems armed), and for keyless and Smart Entry System (SES) vehicles make sure that all keys are out of the detection range of the vehicle.
Make sure that the current clamp’s internal battery voltage is sufficient for extended parasitic measurements. The Internal battery voltage should be above 8 V to ensure stable readings during prolonged measurements. If the battery is below 8 V then a new battery is recommended..
Switch on the current clamp and let it warm up internally while it acclimatizes to the workshop temperature (approximately 10 minutes). Make sure that all doors and windows are closed and that the vehicle battery remains accessible. Make sure that the ignition is switched off and that the key fob/set is removed from the vehicle.
Run the PicoScope software by pressing either the space bar on your keyboard or the Go button in PicoScope. Zero the current clamp with the thumb wheel and connect it around the battery negative lead in order to measure the parasitic current drain.
Note the orientation of the current clamp in relation to the current flow.
Before you take any measurements with PicoScope, you need to make sure that the opening of the TA018 current clamp jaw (8 mm) is wide enough to connect around the battery negative cable. Where the diameter of the battery lead exceeds 8 mm, you will need to insert a 10 A fused jumper wire between the battery's negative terminal and the battery lead (see image).
To avoid momentary disconnection of the battery (and excessive current draw), insert the fused jumper wire by following the procedure listed below:
Zero the TA018 current clamp with the push button on the clamp, and connect it around the fused jumper wire (note the orientation of the current clamp in relation to the current flow).
Note: Once the test is complete, re-establish a partial connection between the negative battery terminal and battery lead (as in Fig 2 above). Once partial connection is complete, remove the fused jumper wire, press the battery clamp down to seat it correctly, and tighten securely.
All values included in the example waveforms are typical and not specific to all vehicle types.
Refer to technical data of the vehicle for specific test conditions and results.
The vehicle is prepared as indicated per items 1, 2 and 3 in section How to connect the scope.
You would normally evaluate the vehicle battery with automated devices, or better still, the battery test procedure in PicoDiagnostics (where you can test not only the battery, but the starting and charging systems as well). When the starting and charging system components are confirmed to be working, you can investigate battery parasitic current draw.
Parasitic battery drain testing is often overlooked, but should conclude any customer complaints regarding non-starts as a result of a discharged battery. Traditionally, evaluating the battery and charging systems is very quick indeed, but parasitic drain demands an extended test.
The demand of power from the vehicle battery has increased in direct proportion to the technology now installed by all manufacturers. Not only is the battery required to perform under repeated engine start conditions and periods of heavy electrical load, but also to maintain sufficient charge levels (reserve) relative to prolonged periods of inactivity (standing vehicle).
The challenges therefore faced by all manufacturers is to limit the parasitic current drain of the vehicle battery that is directly associated with on-board computer activity.
High specification vehicles may contain over 60 computers, linked by multiple networks, responsible for the seamless operation of the drivetrain and comfort control systems.
These computers will require a period of time in order to complete a variety of functions, long after the customer has exited and locked the vehicle. Monitoring functions, initialising, security arming, system integrity tests, and memory writing are typical computer activities that will draw current from the battery without replenishment by the vehicle charging system.
This is referred to as the shutdown period where all computers will carry out functions in order to ensure the correct operation of the vehicle when the driver returns.
By using the current clamp you can monitor the various stages of shutdown to make sure that the vehicle moves into sleep mode. You should expect to measure no more than 80 mA of parasitic drain until the vehicle is woken up by the intervention of the key fob/set (unlocking of the vehicle).
When you monitor parasitic drain, you are looking for the average of very low current values (below 80 mA). Low values like these are always susceptible to noise and so you will find that lowpass filtering will be essential to measure the average of the value in question.
Periodic spikes visible during parasitic drain measurements are often attributed to security LED operation or keyless entry systems looking for keys. While 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 to protect the vehicle battery during prolonged periods of vehicle inactivity.
Lowpass filtering is incorporated into the PicoScope software and can be applied to any channel to remove high frequency noise and reveal the true signal level beneath (see Example waveform 1 and Example waveform 2).
A 1 Hz lowpass filter applied to any signal will reject all frequency signals above 1 Hz. Only signal values of 1 Hz and below will pass through and be displayed.
To activate/deactivate the lowpass filtering feature:
In addition to lowpass filtering, owners of the PicoScope 4425 or PicoScope 4225 can take advantage of an additional filtering option called Bandwidth Limit. This is an inbuilt hardware filter that will reject all noise above 20 kHz before the signal is passed from the scope to the software. In effect you convert the selected scope channel bandwidth from 20 MHz to 20 kHz (see image).
This bandwidth is ideally suited to current clamps as they do not detect signals over 20 kHz (which is too high a frequency to be hiding any issues of concern during parasitic current measurements [maximum current clamp bandwidth = 20 kHz]).
With a PicoScope 4425 or 4225 connected, select "Tools > Preferences > Options" and check the box marked Show Analog Options, followed by OK. This will activate the bandwidth limit in the channel options menu, and limit the bandwidth to 20 kHz for all incoming signals on your chosen channel.
Traditionally, any offending parasitic drain could be located by removing circuit fuses, in an attempt to locate the source of parasitic current draw. However, with today’s technologically advanced vehicles, removing fuses while looking for offending circuits and components will have the opposite effect and could increase parasitic drain as a result of intrusion into computer memory supply voltages.
Momentarily disconnecting a memory supply and then reconnecting (fuse pulled out and then re-installed) will wake a computer and so wake the network.
In this scenario the shutdown period could commence due to disconnection of the fuse. This may no longer be a true representation of the actual shutdown procedure (remember, you must simulate the identical conditions to those experienced by the customer).
In order to identify offending circuits or components you need to monitor the voltage drop across system fuses. Any component drawing current via a fuse will certainly generate a voltage drop across the relevant fuse. Vehicle manufacturers specify the voltage drop across their fuses relevant to current flow. These charts should be available via technical information or on the manufacturers’ websites.
A typical example is a 10 amp fuse which measures 0.0001 mV across the fuse check points indicating that the circuit protected by this fuse is drawing 13 mA (no disconnection required and so no intrusion into the network). Resistance values of fuses differ between manufacturers so please ensure you have the correct volt drop table.
A standard Multimeter is sufficient to measure these volt drop values while your scope continues to accurately monitor the parasitic drain.
By using the parasitic current values obtained during the sleep mode of the vehicle, you can calculate an indication of the approximate time a customer can expect to leave their vehicle standing before the health of the battery is affected. To do this, use the following formula:
Fully charged battery at 21 degrees centigrade
Example (77 Ah battery):
(77/100) x 70 = 53.9% of stated Ah rating of vehicle battery
53.9%/0.040 A = 1347.5 hours
1347.5 hours/24 = 56 days before the battery will require recharging
This help topic is subject to changes without notification. The information within is carefully checked and considered to be correct. This information is an example of our investigations and findings and is not a definitive procedure. Pico Technology accepts no responsibility for inaccuracies. Each vehicle may be different and require unique test settings.
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