Ada Audits the Trace-to-Lifetime Numbers

Ada converts the measured current trace into charge, average current, and a usable cell-life estimate

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Ada ADA · CALCULATION AUDIT

Ada Audits the Trace-to-Lifetime Numbers

The chapter measures a ten-minute cycle where a 120 mA radio event for 1.4 s costs 46.7 uAh of a 52.2 uAh total — averaging about 313 uA, or roughly 256 days on a derated 1920 mAh cell. The radio alone is about 90% of the cycle charge. This audit converts the trace to lifetime numbers, checking each state against usable capacity rather than a headline rating.

Companion to the chapter Energy Measurement and Profiling — every number here comes from that chapter.

The current trace becomes defensible when every state is converted to charge, summed over the ten-minute cycle, and then checked against usable battery capacity instead of a headline cell rating.

Charge per state, average current, and cell-life estimate

charge (uAh) = current (uA) x time (s) / 3600
Trace row Arithmetic shown Audit result
Sleep baseline 18 uA x 598.1 s / 3600 2.9905 uAh, so the table's 3.0 uAh is rounded correctly.
Process window 25 mA = 25,000 uA; 25,000 x 0.1 / 3600 0.6944 uAh, so the table's 0.7 uAh is rounded correctly.
Radio event 120 mA = 120,000 uA; 120,000 x 1.4 / 3600 46.6667 uAh, so the table's 46.7 uAh is rounded correctly.
Shutdown tail 15 mA = 15,000 uA; 15,000 x 0.1 / 3600 0.4167 uAh, so the table's 0.4 uAh is rounded correctly.
Cycle total from listed rows 3.0 + 1.4 + 0.7 + 46.7 + 0.4 52.2 uAh per ten-minute cycle, consistent with "about 52 uAh".
Average current 52.2 uAh / (10 / 60 h) 313.2 uA, which rounds to about 312-313 uA depending on the row precision used.
Usable cell life 2400 mAh x 0.80 = 1920 mAh; 1920 mAh / 0.3132 mA / 24 255.5 days, so the chapter's roughly 256-day estimate is supported before derating.

Engineering consequence: the radio event alone is 46.7 / 52.2 = 0.895, or about 90% of the measured cycle charge. The next fix should target radio timing, retries, and receive windows before chasing small CPU savings.

Every number above is taken from this chapter's own worked example and re-derived step by step.