Battery Runtime: Measure Current and Use mAh Correctly
By PlainBattery Team
Key takeaways
- Measure a complete operating cycle, including retries and startup.
- Use capacity measured under conditions close to the real load.
- Validate the estimate with a recorded discharge test.
The formula runtime = capacity ÷ current works only when both numbers describe the same conditions. A printed milliamp-hour rating uses a specified load, temperature, and cutoff voltage. A real device sleeps, wakes, transmits, lights a display, and shuts down at its own minimum voltage.
Capture a complete current profile
Measure the device in series with its supply. A power analyzer can capture microamp sleep and milliamps of radio current in one trace. Nordic’s Power Profiler documentation explains source-meter and ammeter setups. A multimeter works for steady loads, but its display may miss short pulses.
- Program release firmware and disconnect the debugger and USB interface.
- Measure sleep current after startup has settled.
- Capture the current and duration of every wake event.
- Include regulator quiescent current, sensors, pull-ups, and indicator LEDs.
- Record several complete cycles, including retries and error paths.
Use the runtime measurement worksheet to keep every state, duration, event count, and measurement method in one auditable record.
For each state, multiply current by time. Add the charge contributions and divide by cycle time.
Average current = Σ(current × duration) ÷ cycle duration
Runtime hours = usable capacity in mAh ÷ average current in mA
Suppose a sensor sleeps at 12 µA for 59 seconds and transmits at 18 mA for one second:
Sleep: 0.012 mA × 59 s = 0.708 mA·s
Transmit: 18 mA × 1 s = 18 mA·s
Average: 18.708 ÷ 60 = 0.312 mA
A nominal 235 mAh battery produces an ideal result of about 753 hours.
Turn rated capacity into usable capacity
The Energizer CR2032 sheet rates 235 mAh under a 15 kΩ load at 21°C down to 2.0 V. A product that resets at 2.7 V cannot use all capacity measured to 2.0 V. Cold temperature, storage age, and rising internal resistance reduce usable energy further.
Calculate multiple cases and state the assumption:
| Case | Usable fraction | Typical reason |
|---|---|---|
| Optimistic | 90% | Light load, mild temperature, low cutoff |
| Planning | 70% | Normal uncertainty and pulse load |
| Conservative | 50% | Cold, aged stock, high cutoff or large pulses |
At 70%, the example becomes 235 × 0.70 ÷ 0.312, or about 527 hours. The percentage is a scenario, not a universal rule; replace it with measured discharge data when possible.
Include voltage conversion
For a regulated supply, use energy and converter efficiency:
Battery Wh = nominal voltage × capacity in Ah
Load W = output voltage × average output current
Runtime = battery Wh × efficiency ÷ load W
Read efficiency at the expected input voltage and load. Add regulator quiescent current separately because it can exceed the sleeping circuit’s current.
Validate the result
Log terminal voltage and resets during an accelerated run using the real enclosure, firmware, radio interval, and temperature range. Record the cell model and lot, starting voltage, instrument, sampling rate, cutoff condition, and test duration. A resistor test characterizes a cell; a complete-device test shows whether the product can still perform its task.
For a radio-specific workflow, use the IoT duty-cycle test method. Model pages for CR2032, AA, and 18650 provide the input specifications.
Review record and primary sources
Reviewed by PlainBattery Technical Review Desk on 2026-09-11. Checked: instructions and calculations, battery specifications used in examples, safety limits and linked primary references.
- Energizer CR2032 Product Datasheet — Energizer Brands, CR2032EU0118 (2018). Capacity test conditions, cutoff voltage, continuous load and pulse behavior.
- Power Profiler Kit II User Guide — Nordic Semiconductor. Current-profile measurement, source-meter operation and accumulated charge.
Revision record: source citations and field-level review scope added September 11, 2026. See our sources and methodology and corrections policy.