Disclosure: I write for OUOAENR, a UK retailer of electrical test tools, including the clamp meter mentioned below.
If you build things that run off a battery (a Raspberry Pi in a shed, a camper van setup, an ESP32 sensor node) you eventually need to know how much current it actually draws. Datasheet numbers are optimistic, and inserting a multimeter in series means cutting or unplugging a wire. A DC clamp meter lets you measure without breaking the circuit.
Why a DC clamp works at all
Basic clamp meters use a current transformer, which only responds to a changing magnetic field, so they read AC only. Clamp meters that read DC use a Hall-effect sensor in the jaw, which responds to a steady field. If you want to measure battery or USB-powered projects, make sure the meter reads DC.
Method: measuring a project's supply current
- Isolate one conductor. Clamp around the positive lead only. If you clamp the whole two-core cable, the opposing currents cancel and you read roughly zero.
- Zero the meter. DC clamps pick up stray fields. With nothing in the jaw, press the zero/REL button.
- Centre the wire in the jaw. Readings are most accurate with the conductor in the middle.
- Read idle and peak. Note the reading while idle, then trigger the expensive operation (Wi-Fi connect, motor start, display backlight) and watch the jump.
- Use loops for small currents. If the current is low, wrap the wire through the jaw several times and divide the reading by the number of turns. Five turns means divide by five.
Turning readings into battery life
A rough estimate:
average_current = (idle_mA * idle_seconds + active_mA * active_seconds) / cycle_seconds
runtime_hours = battery_capacity_mAh * 0.8 / average_current
The 0.8 factor leaves headroom for battery ageing and regulator losses. Sanity-check the estimate against a real run before you trust it.
Logging over time
A handheld meter gives spot readings. For long-term logging, pair spot checks with a shunt-based sensor (for example an INA219 on I2C) and use the clamp reading to verify the sensor is calibrated correctly.
Gotchas
- Keep the jaw away from magnets and motors when zeroing.
- Very low currents (a few mA) are near the resolution limit of most clamps; use the loop trick or a series meter.
- Never work on mains wiring unless you are qualified.
The meter I used
For this kind of work I use a True RMS AC/DC mini clamp meter, the UNI-T UT210E, which reads up to 100A and also does continuity and capacitance. You can find it at OUOAENR.
