Coin-Cell Power for MCUs: Measure Pulses Before Selection
By PlainBattery Team
Key takeaways
- Measure the assembled board instead of relying on MCU sleep specifications.
- Check minimum rail voltage during the largest pulse.
- Validate cold start and aged-cell behavior on hardware.
A microcontroller data sheet may advertise nanoamp sleep while the finished board consumes tens of microamps. Radio bursts, sensor warm-up, flash writes, pull-ups, regulators, and indicator LEDs determine whether a coin cell works.
Choose by workload, not capacity alone
CR2032 and CR2450 are both 3 V primary lithium coin cells, but they have different dimensions and capacity. A larger cell generally offers more stored energy and often supports the workload better, but neither should be selected from mAh alone.
| Question | Measurement |
|---|---|
| Will firmware meet the runtime target? | Charge consumed over a complete duty cycle |
| Will the MCU reset during transmit? | Minimum voltage during the largest pulse |
| Will it work near end of life? | Pulse test with an aged-cell model or series resistance |
| Will it work outdoors? | Cold-start and cold-transmit test |
| Is the regulator helping? | Efficiency and quiescent current at the real load |
Measure the assembled board
Use release firmware and disconnect the debugger. Capture startup, sleep, sensor sampling, processing, radio transmit and receive, retry, and nonvolatile-memory operations. A profiler that spans microamps to milliamps is more useful than a slow multimeter for this job.
Nordic’s Power Profiler Kit II guide describes a profiler that can capture both low current and fast events. Measure voltage at the MCU pins as well as current. Long traces, a weak holder, contamination, and connector resistance can create a local dip that is invisible at the battery terminals.
Check the brownout margin
Record four values: fresh-cell voltage, minimum pulse voltage, brownout-reset threshold, and the lowest voltage at which the application still completes its task. Repeat with realistic radio retries and sensors enabled.
A fresh cell passing one transmission proves little about end-of-life behavior. Internal resistance increases during discharge, which deepens pulse sag. Emulate that condition with a programmable source or a documented series resistance, then repeat the test across temperature.
Size a reservoir capacitor
For a short pulse, an initial capacitance estimate is:
C = current × pulse duration ÷ allowed voltage drop
For a 15 mA, 8 ms pulse with 0.2 V allowed droop, the ideal calculation is 600 µF. Real capacitors have tolerance, leakage, equivalent series resistance, and temperature dependence. The battery still supplies average energy and must recharge the capacitor between events.
Place the capacitor near the pulsed load and verify the result on an oscilloscope. Increasing capacitance cannot solve continuous excess current or frequent retries.
Audit hidden current
Common causes of disappointing runtime include:
- a regulator with high quiescent current at light load;
- floating GPIO pins or pull resistors that remain active;
- sensor heater or warm-up current omitted from the budget;
- debug, level-shifter, or USB-interface circuitry left powered;
- an LED that draws more than the sleeping MCU;
- a network stack that scans or retries more often than planned.
Validate the holder and environment
Confirm holder depth and contact force using the exact production cell. CR2032, CR2025, and CR2016 share a 20 mm diameter but have different thicknesses. CR2450 is wider and thicker. A mechanically loose cell can look like a firmware reset problem.
Run cold-start, transmit, and brownout tests at the lowest intended temperature. The Panasonic lithium handbook provides manufacturer discharge and temperature data, but a board-level test is still needed because cutoff and pulse profile are product-specific.
Use CR2032 vs CR2450 for physical and electrical differences and Battery Runtime for the full calculation method.
Related charts
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.
- Panasonic Lithium Technical Handbook — Panasonic Energy, 2020 handbook (2020). CR coin-cell construction, capacity, dimensions and discharge characteristics.
- Power Profiler Kit II User Guide — Nordic Semiconductor. Measuring sleep current, pulse current and accumulated charge.
Revision record: source citations and field-level review scope added September 11, 2026. See our sources and methodology and corrections policy.