Parasitic Drain Testing
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 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 on the vehicle battery that is directly associated with onboard 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, initializing, 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.
A timeframe of 30 minutes to 2 hours is not uncommon for a modern vehicle to shut down and enter sleep mode.
By using the current clamp to conduct Parasitic Drain Testing, you can monitor the various stages of the shutdown to make sure that the vehicle moves into sleep mode. You should expect to measure no more than 80mA of parasitic drain until the vehicle is woken up by the intervention of the key fob/set (unlocking of the vehicle).
Be aware that at this point, during the sleep mode of the vehicle, a simple pull on a door handle or operation of the tail door release button (vehicle locked) will be sufficient to wake a network and will introduce another shutdown period before it returns to sleep mode.
When you monitor parasitic drain, you are looking for the average of very low current values (below 80mA). 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.
The specification of the current clamp in use must be considered as we approach small current levels (below 10mA). Our TA018 and TA234 Current Clamps have low current detection specifications of 10mA and 5mA respectively.
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 1Hz lowpass filter applied to any signal will reject all frequency signals above 1Hz. Only signal values of 1Hz and below will pass through and be displayed.
To activate / deactivate the low pass filtering feature:
- Click on the channel options button (B in this case).
- Check the Activate filtering check box.
- Select the level of lowpass filtering required (in this case 1Hz).
Click anywhere on the scope grid to exit the channel options menu.
Bandwidth Limit - PicoScope 4425 & 4225 only
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 20kHz before the signal is passed from the scope to the software. In effect, you convert the selected scope channel bandwidth from 20MHz to 20kHz (see image).
This bandwidth is ideally suited to current clamps as they do not detect signals over 20kHz (which is too high a frequency to be hiding any issues of concern during parasitic current measurements - maximum current clamp bandwidth = 20kHz).
Activating the Hardware Filter
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 20kHz for all incoming signals on your chosen channel.
Note: When you use the Bandwidth Limit feature, it cannot be adjusted after the capture to reveal the original signal (unlike software filters that can be applied or removed post capture).
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 when Parasitic Drain Testing, 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 0.0001mV across the fuse check points indicating that the circuit protected by this fuse is drawing 13mA (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 monitor the parasitic drain accurately.
By using the parasitic current values obtained during the sleep mode of the vehicle when parasitic drain testing, 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
- Amp hour rating (stated on the battery) /100 x 70 = 70% of Amp hour value
- Calculated 70% of Amp/hour rating/parasitic value obtained during sleep mode = Hours before battery recharge is required
- Calculated hours before battery recharge is required /24 = days before battery recharge is required
Example - 77Ah battery:
- (77/100) x 70 = 53.9% of stated Ah rating of the vehicle battery
- 53.9%/0.040 A = 1 347.5 hours
- 1 347.5 hours/24 = 56 days before the battery will require recharging