Energy & Power · Study deck

Power Management Tools

A battery-life calculator can make a design look precise even when the inputs are guesses.

Battery Bruno is your guide for this deck.

awareinteractivetools

Battery Bruno, the module guide, in a scene from this chapter.

After studying this chapter

Learning objectives

You will be able to:

Explain: The measured trace shows 8 mA active for 2.0 s, 42 mA transmit for 0.8 s, and 12 uA sleep for the rest of the 900 s period.
Explain: At 2% self-discharge per month, the cell adds an equivalent 27.8 microamps, lifting the total to 54.7 microamps and cutting the estimate to 2.09 years.
Explain: This progression makes the point concrete: Interactive tool review loop showing run tool, capture assumptions, replace guesses with evidence, test fail-closed cases, and record the decision.
Prepare measured inputs before running a battery-life estimate.

Major section

Tool Workflow

The useful path through an energy tool starts before the calculator opens.

The practical question in tool workflow is where inputs hands responsibility to trace + timing.
It therefore connects this diagram directly to the chapter's treatment of tool workflow.

Power-management tool workflow from measured inputs to model, sensitivity sweep, design decision, and validation.

Major section

Worked Ledger

The measured trace shows 8 mA active for 2.0 s, 42 mA transmit for 0.8 s, and 12 uA sleep for the rest of the 900 s period.

With 1500 mAh usable capacity, first-pass lifetime is about 2.6 years.
That number is not a promise.
It becomes useful when the same firmware is measured again after design changes and when field logs confirm retry behavior.

Major section

Sensitivity Sweeps

A sensitivity sweep answers "what should we optimize first?" It is usually more useful than arguing about a single predicted lifetime.

Against it: Average current exposes the energy consequence; the later 0.067 mA adds a distinct review condition.

Battery-life sensitivity sweep: from a base ledger averaging 0.067 mA (about 2.6 years on 1500 mAh), sweeping sleep current, transmit interval, and retry rate shows which measured contributor to optimize first.

Major section

Validation Loop

A calculator should lead to measurement, and measurement should update the calculator.

Their relationship shows which part of the claim still needs evidence.
The next checkpoint, measurement; measurement, states how the claim is checked, while updates the calculator adds a distinct review condition.

Energy budget validation route compares a tool estimate with a prototype trace in mAh/day, then uses later field logs to update the model.

Major section

Tool Review Checklist

Capacity is usable: Battery capacity reflects voltage cutoff, temperature, age, pulse limits, self-discharge, and reserved margin.

Protocol cost is complete: The model includes association, acknowledgments, receive windows, retries, and failed transmissions when relevant.
Output is a range: The design record shows best-case, expected, and conservative estimates, not a single polished number.
Validation is scheduled: The next bench or field check is named before the design is treated as accepted.

Major section

A Calculator's Job Is To Show Sensitivity, Not Just A Number

This progression makes the point concrete: Interactive tool review loop showing run tool, capture assumptions, replace guesses with evidence, test fail-closed cases, and record the decision.

The result feeds the chapter's a calculator's job is to show sensitivity, not just a number decision with the boundary still attached.

A trustworthy calculator pass turns a lifetime number into an evidence loop: run the model, name the assumptions, replace guesses with measurements, test failure cases, and record the release decision.

Major section

Phoebe's Field Notes: The Sweep This Page's Own Baseline Never Ran

The mathematical gist: A 1000 mAh cell powering the chapter's 26.9-microamp device gives a naive 4.24-year result.

At 2% self-discharge per month, the cell adds an equivalent 27.8 microamps, lifting the total to 54.7 microamps and cutting the estimate to 2.09 years.

Deck summary

Key takeaways

The useful path through an energy tool starts before the calculator opens.

The measured trace shows 8 mA active for 2.0 s, 42 mA transmit for 0.8 s, and 12 uA sleep for the rest of the 900 s period.
A sensitivity sweep answers "what should we optimize first?" It is usually more useful than arguing about a single predicted lifetime.
A calculator should lead to measurement, and measurement should update the calculator.
Capacity is usable: Battery capacity reflects voltage cutoff, temperature, age, pulse limits, self-discharge, and reserved margin.

Retrieval practice

Recall check 1 of 4

Battery Bruno says: answer from memory, then check your reasoning.

Q1A battery-life calculator predicts four years, but the model uses datasheet sleep current, nominal battery capacity, and one successful radio transmission per cycle. What should the engineer do before accepting the result?

AAccept the calculator formula because nominal board current and retry rates are representative.

BIncrease battery size until an unverified calculator predicts eight years.

CRemove sleep current from the model because the low-current state contributes negligible charge.

DMeasure full-device cycles, derate usable capacity and include radio retries.

Show answer

Answer: D Power tools support design decisions only when measured device behavior and realistic deployment limits are included.

Retrieval practice

Recall check 2 of 4

Battery Bruno says: answer from memory, then check your reasoning.

Q2What turns measured observations into a reference lifetime or energy prediction?

AMeasured inputs

BBaseline model

CSensitivity sweep

DValidation trace

Show answer

Answer: B The baseline model converts those observations into a reference lifetime or energy prediction.

Q3Beyond a single lifetime figure, what is the most useful thing a battery-life tool provides?

AA prettier chart of the same single number.

BA guarantee that the battery will last exactly that long.

CA sensitivity view of which input moves the lifetime most.

DConfirmation that active current is always what matters.

Show answer

Answer: C The lifetime number alone can mislead you into optimizing a term that barely matters.

Retrieval practice

Recall check 3 of 4

Battery Bruno says: answer from memory, then check your reasoning.

Q4For a node that reports once per hour with a 0.5 s active burst, a sweep shows halving sleep current gives +59% life while halving active current gives +15%. Which optimization should the team pursue first, and why?

AReduce active draw first because its peak current is numerically larger.

BReduce sleep draw first because its long duration dominates average charge.

CPursue either current because both cuts have the same life gain.

DPursue neither because measured current cannot alter battery life.

Show answer

Answer: B The device sleeps for essentially the whole hour, so the sleep term carries about three-quarters of the average.

Retrieval practice

Recall check 4 of 4

Battery Bruno says: answer from memory, then check your reasoning.

Q5A tool predicts 6.7 years using a 10 uA datasheet sleep current, but the board actually sleeps at 200 uA, giving a real life near 0.55 years. Why is this error so large from a single input?

AThe tool makes an arithmetic error when converting microamps to years.

BActive current changes dramatically between the two battery-life runs.

CLifetime rises proportionally with average current, so more sleep drain helps.

DSleep current dominates the average because the device spends most time asleep.

Show answer

Answer: D The answer is most sensitive to the dominant term.

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Answers

Answer key.

D · Power tools support design decisions only when measured device behavior and realistic deployment limits are included.
B · The baseline model converts those observations into a reference lifetime or energy prediction.
C · The lifetime number alone can mislead you into optimizing a term that barely matters.
B · The device sleeps for essentially the whole hour, so the sleep term carries about three-quarters of the average.
D · The answer is most sensitive to the dominant term.