Measuring Quiescent and Load Current with One Sensor: Why the Shunt Leads the Way in Low-Voltage Vehicle Electrical Systems

In 12, 24 and 48 V vehicle electrical systems, currents ranging from a few milliamps to several hundred amps must be measured reliably – in the vehicle as well as on the test bench. An overview of shunts, Hall sensors and current clamps – and why a single shunt sensor can handle both measurement tasks.

The challenge: dynamic range

The quiescent current of a parked vehicle is in the milliamp range once the ECUs have gone to sleep. Just a few milliamps over budget can deep-discharge the battery within a few days. Load currents from high-current consumers, by contrast, briefly reach several hundred amps. Examples include the starter motor, electric power steering, PTC auxiliary heaters and refrigerant compressors, and in 24 V commercial vehicles also hydraulic pumps and tail lifts. Peaks are particularly high during switch-on and start-up events and during cold cranking.

Between 1 mA and 750 A lies a dynamic range of 1:750,000. Sensors designed for load peaks usually offer only limited resolution at the low end – yet that is exactly where quiescent current lies. In practice, two sensors are therefore often installed. That means twice the effort for installation, wiring, calibration and evaluation.

Environmental conditions

In a test vehicle, the measurement equipment is exposed to splash water, dust, humidity and temperature cycling. A permanently installed sensor therefore needs at least IP67 protection and a temperature range that covers cold- and hot-weather testing, typically −40 °C to +85 °C. Temperature drift is decisive here: for load current, an offset of a few milliamps is negligible; for quiescent current, it renders the measurement useless.

Hall sensors and current clamps: strong at high currents, weak in the milliamp range

Hall sensors and current clamps measure contactlessly via the magnetic field. They are quick to install and galvanically isolated. For high-current measurement and diagnostics, they are proven tools.

When it comes to quiescent current, however, they reach physical limits. Offset, temperature drift and remanence of the magnetic core typically mask a few milliamps. With current clamps, the result also depends on the conductor position and zero adjustment. Moreover, clamps are rarely designed for permanent installation. Anyone who wants to measure quiescent current precisely with these principles usually needs a second sensor.

Shunt: one sensor for the entire measuring range

A shunt measures current directly in the current path via the voltage drop across a precision resistor. This offers decisive advantages:

• No magnetic offset, no remanence: The zero point remains stable. This is the prerequisite for reliable quiescent current measurements over hours and days.
• Low temperature drift: Resistance alloys such as manganin keep the measured value stable across the temperature range of vehicle testing.
• One sensor instead of two: Quiescent current and load peaks are captured with the same sensor at a single measuring point. This saves hardware, installation space, wiring and evaluation effort.
• Suitable for permanent installation: Compact, encapsulated and with a high IP rating, the shunt stays in the vehicle for the entire test campaign.

The real challenge lies in the electronics. Across a typical 100 µΩ shunt, 1 mA produces a voltage drop of only 100 nV. Capturing these signals reliably while also withstanding load peaks of several hundred amps requires measurement electronics designed specifically for this task.

PowerTrace HCS 75060: from quiescent current to cold cranking

This is exactly what BRIGHT developed the PowerTrace HCS 75060 for. The CAN current and voltage sensor captures quiescent current in the milliamp range and load peaks up to ±750 A – with one compact device instead of two.

• Current: ±750 A peak, ±225 A continuous
• Resolution: 1 mA in the fine current range
• Accuracy: ±0.5 to 1 % in the continuous current range
• Voltage: ±60 V, for 12, 24 and 48 V vehicle electrical systems
• Interface: CAN, up to 1 kHz, configurable via CAN
• Protection rating and temperature: IP67, −40 °C to +85 °C
• Dimensions: approx. 39.5 × 30 × 49 mm

In addition to current, the PowerTrace also measures the system voltage at the same measuring point, making voltage dips during cold cranking, for example, immediately visible. Via CAN, the sensor integrates into existing measurement chains without additional hardware. The software can be adapted to the specific measurement task.

Typical applications include quiescent current analysis, engine start and load peak measurements, 48 V systems, test fleets, test benches and HiL, as well as special-purpose and defence vehicles.

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