ELPM-S3LW / Measurement
From current trace to runtime.
How we prepare and verify this contentRuntime is estimated by integrating current through every phase and multiplying that energy by the real event count. A peak measurement or Deep Stop figure alone is insufficient: boot, sensors, processing, airtime, receive windows, retries and battery losses belong in the same model.
Measure at the battery to estimate the battery.
Measuring only the 3.3 V rail excludes regulator efficiency, quiescent current, protection and sensors. That can be valid for firmware comparisons; product runtime requires the real supply point, with voltage recorded alongside current.
Choose a sample rate that captures radio transients and enough dynamic range to measure both milliamps and nanoamps. Two acquisitions or ranges are often needed: one for peaks and one for waiting.
Segment the cycle before calculating it.
| Phase | Include | Dominant variable |
|---|---|---|
| Boot | Rails, clock, flash initialization | Boot time |
| Sensing | Sensor warm-up and sampling | Sensor time |
| Processing | Filtering, encryption, payload | Firmware and clock |
| LoRa TX/RX | Airtime, waits and retries | SF, power, payload |
| Waiting | Latch, RTC, always-on sensors | Period duration |
Integrate charge, not just current.
Qcycle [mAh] = Σ(Iphase [mA] × tphase [s]) / 3600
Qday = Qcycle × cycles/day + Iwait × twait/day
Usable capacity is not nominal capacity. Apply factors for temperature, ageing, discharge rate, circuit cutoff voltage and design margin. Ideal runtime is usable capacity divided by daily consumption; rounding down is more honest.
Use measured inputs. The ELPM calculator implements this three-phase model. It is a starting point: always replace defaults with your measured traces.
Model the bad day too.
- Retransmissions and radio link quality.
- Updates, diagnostics and occasional Wi-Fi sessions.
- Self-discharge and cold-temperature capacity loss.
- Sensor drift and warm-up time.
- Abnormal events that keep the latch active.
Compare typical and worst-case scenarios. If the product misses its requirement in the worst case, typical runtime is not a defensible specification.
Use the calculator and download the published Otii traces.
Calculate energy