Trace-to-Lifetime Calculation Audit

Trace-to-Lifetime Calculation Audit

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

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Ada ADA · CALCULATION AUDIT

Trace-to-Lifetime Calculation Audit

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.

See the relationship before changing it

The figure reads from left to right. The blue card is measured radio duration. The middle card applies this page's rule. The green card is radio-event charge. Walk the arrows once: set the input, apply the rule, then read the result with its unit.

The retained audit below checks several chapter fixtures. This model keeps those stated values fixed and changes only measured radio duration, so the numeric fixture does not switch without explanation.

Measured radio duration changes radio-event charge An input card leads through the rule radio charge = 120,000 uA x duration / 3,600 to the radio-event charge result. INPUT PAGE INPUT APPLY THE RULE predict calculate check units OUTPUT RESULT
Walk the arrows. A longer measured radio event adds charge directly and dominates the ten-minute trace.

Derive the baseline in four named moves

  1. 1

    Name the input. The chapter baseline is 1.4 s.

  2. 2

    Name the relationship. radio charge = 120,000 uA x duration / 3,600

  3. 3

    Substitute with units. 120,000 x 1.4 / 3,600 = 46.67 uAh

  4. 4

    Read the result. Keep the unit beside the value. Use it only inside the technical boundary on this page.

Predict, then change measured radio duration

Try Predict the direction of radio charge = 120,000 uA x duration / 3,600. Test another measured radio duration, then compare radio-event charge.

1.4 s
Chapter baseline
Radio-event charge

Observe A longer measured radio event adds charge directly and dominates the ten-minute trace. Reset measured radio duration to 1.4 and compare radio-event charge.

Explain A longer measured radio event adds charge directly and dominates the ten-minute trace.

Check yourself

What should you do before trusting a moved-control result?
Answer: Predict its direction, apply the shown relationship, keep the units, and reset to the worked baseline.
What does this small model leave out?
Answer: Only measured radio duration moves here. Field effects named in the technical boundary stay fixed.

Honesty note. This teaching model changes only measured radio duration. It is not a field prediction.

TryConvert the 120 mA radio plateau lasting 1.4 s into uAh, add every trace row, and only then press Check derivation. Apply the stated 80% usable-capacity factor.
ObserveThe trace sums to 52.2 uAh per ten-minute cycle, or about 313 uA average. Dividing the usable 1920 mAh by that average gives roughly 256 days.
ExplainThe radio's 46.7 uAh is about 90% of measured cycle charge. Because lifetime divides usable capacity by average current, radio timing dominates this estimate.

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.

Technical boundaries
The trace conversion deliberately does not simulate probe bandwidth limits, measurement burden, unobserved startup spikes, retries, regulator loss, cell aging, or temperature; it integrates the listed current plateaus only.

Work the audit first, then check the displayed derivation.

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