This guide, written by our expert Steve Smith, will teach you an effective method for identifying intermittent parasitic drains in vehicles. While current clamps are convenient, they can have limitations during prolonged testing. We’ll show you how to use a shunt resistor and PicoScope’s powerful Math channels to perform accurate, continuous measurements, so you can catch those elusive drains that often go undetected.
Why a shunt resistor? The advantage of prolonged current testing #
For those tricky, intermittent parasitic drains that only appear after a vehicle has been asleep for a long time, a traditional current clamp may not be the ideal tool. A shunt resistor provides a more stable, non-intrusive way to measure current draw over an extended period. We’ll dive into the setup and demonstrate how to use Ohm’s Law and a math channel to effortlessly calculate and display the current flow.
Step-by-step guide: Setting up your parasitic drain test #
Follow along as we walk you through the process of connecting a shunt resistor to your vehicle and configuring the Math channel in PicoScope 7 Automotive. You’ll learn how to set up the formula to convert the voltage drop across the shunt into a current waveform, allowing you to accurately monitor the vehicle’s sleep mode and quickly identify any unexpected current spikes.
The following Math Channel once again uses Ohm’s law to divide voltage by resistance to obtain current. I am thinking here of prolonged parasitic drain measurements via a precision 0.1 Ohm 100W shunt resistor placed in series (via a fuse and in a heat sink) with the battery negative lead and battery negative post. Be aware that there are a lot of low-cost shunt resistors on the market that don’t have proper heat sinks, and this could be a real safety issue with some of these high wake-up current levels in today’s premium vehicles.
Here is an example of a low-cost (bottom) versus a quality (top) shunt resistor.
Whilst the current clamp method remains an accurate and rapid non-intrusive test (see links below), prolonged measurements in excess of 12 hours can become challenging at best. (Clamp internal battery and thermal drift spring to mind)
Whilst this method is intrusive, we free ourselves from all the limitations presented when using a non-BNC+ current clamp over prolonged measurements.
It is vital to insert the resistor at precisely the correct time during the shutdown period of the vehicle, whilst maintaining contact between the battery negative terminal and negative lead. Momentary disconnection of the battery negative lead (to insert the resistor) will most probably shut down the device we are attempting to diagnose.
Be aware that the current flow through the 0.1 ohm resistor should be a device wake-up or activate during your measurement. Ensure your resistor has a wattage rating sufficient to carry a specified amount of current to allow for such events. A 0.1 ohm resistor rated at 100 watts should carry 7 amps comfortably if mounted in a heat sink. (Watts = Volts x Amps) 100 W / 12.6 V = 7.93 A.
NOTE: Many higher-end vehicles now have wake-up currents over 30 Amps, so this method is not suggested for those.
Measuring the volt drop across the resistor and dividing this voltage value by 0.1 we can display current using a Math Channel
Given the small voltage levels concerned, filtering will be essential. Most certainly, bandwidth limit (4425 and 4225) accompanied by low-pass filtering (1 kHz minimum). You can read more about filtering here.
Here is a great example of the resistor volt drop method capturing parasitic drain over long periods.
The Math channel performs a component waking up every 15 minutes for approximately 2 minutes and 16 seconds. Total test time, 13 hours and 16 seconds, with no concerns surrounding current clamp battery failure or thermal drift.
For increased accuracy, you could modify your math channel to allow for the resistance of the inline fuse, too (Food for thought)
With the new BNC+ current clamps (aka TA408: https://www.picoauto.com/accessories/current-clamps/picobnc-100-a-ac-dc-current-clamp) and the new extended long-time base options in PS7 (over 2 days and 7 hours, but only with a scope connected), the shunt resistor method is not as crucial as it once was, but still a tool for the toolbox.
