Amey-Thakur Low power design
Battery life is an arithmetic problem: average current = (active current x active fraction) + (sleep current x sleep fraction), and the sleep term usually dominates. Design the duty cycle first, then make sleep genuinely cheap, then measure because estimates always lie.
Method
- Build the power budget spreadsheet first. Battery capacity (derated for temperature and aging), target lifetime, and the resulting average-current ceiling; then allocate: sleep floor, per-wake cost (sensor read, radio transmission), wakes per hour. This arithmetic decides the architecture (see capacity-planning's spirit in microamps): a 5uA budget with a 20mA radio means the radio runs seconds per day, which shapes everything (see iot-messaging's batching).
- Sleep as deep as state allows, wake on events. Map the MCU's sleep states (stop/standby with RAM retention tradeoffs: see embedded-memory-constraints' retention note) and choose per situation; wake sources are interrupts (RTC, GPIO edge, comparator), never polling loops; RTOS tickless idle so the scheduler itself stops the tick (see rtos-task-design's idle hooks). The design question per feature: "what is the cheapest state that can notice this event".
- Kill the parasitic draws. Floating GPIOs (configure every pin), pull-ups fighting drivers, sensors and regulators left enabled, debug interfaces powered in the field, LEDs (a single LED can dwarf your MCU budget): power-gate peripheral rails where hardware allows. Most "impossible" sleep-current numbers are one forgotten peripheral; the hunt-order is a checklist, not intuition.
- Make radio time the scarcest resource. Transmit costs orders of magnitude over compute: batch readings (see sensor-data-handling), compress/delta-encode, prefer connectionless or long-interval protocols suited to the link (see iot-messaging's QoS and keepalive costs), and back off reconnect attempts exponentially: a device that retries a dead network every second dies in days (see timeouts-and-retries, translated to milliamp-seconds).
- Measure with real instruments across modes. A power profiler (Joulescope/PPK-class) or precision shunt across the actual operating modes: sleep floor, wake spike shape, radio bursts: not a multimeter average. Automate a power regression test (scripted device cycle on the profiler, CI or nightly): power regressions arrive silently with any firmware change (see performance-testing's gate ethic, in microamps).
- Account for the analog realities. Brown-out behavior at battery end-of-life (clean shutdown, state saved: see firmware-ota-updates' power-loss machine), cold-temperature capacity collapse, self-discharge and quiescent regulator draw in the total, and duty-cycle jitter so synchronized fleets do not all wake together (see scheduled-jobs' jitter).
Boundaries
- Energy harvesting (solar, kinetic) changes the problem to power management (make progress when energy exists, checkpoint always); the measurement discipline transfers, the budget becomes stochastic.
- Wall-powered devices still care (thermals, efficiency standards) but the microamp obsession does not transfer; do not tax those designs with battery ceremony.
- Battery chemistry selection, fuel gauging, and charging are electrical engineering with safety implications; firmware consumes their outputs but the design belongs to hardware review.