Energy & Power · Study deck

Battery-Life Target Contracts

Picture a pasture sensor expected to run for five years.

Battery Bruno is your guide for this deck.

awareconsiderations

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

After studying this chapter

Learning objectives

You will be able to:

Convert a battery-life target into a maximum average-current budget.
Build a current ledger for sleep, sensing, compute, radio, and exception states.
Explain why source derating, peak current, and end-of-life voltage matter.
Choose design changes based on the dominant energy drain rather than intuition.

Major section

Start With a Battery-Life Contract

Firmware is stored device software.

Budget the Bad Day, Not Only the Quiet Hour: Picture a pasture sensor expected to run for five years.
On the bench it sleeps well, but a weak field link makes it wake, search, and send again.
A protocol means the agreed rules used to exchange messages.

Major section

Start With a Battery-Life Contract (continued)

A lifetime claim based only on the calm average can fail in the first difficult season.

The largest drain should lead the next design change, and every estimate should keep its measured condition.
This budget does not guarantee field life or replace ageing tests.
The deeper sections show average-current arithmetic, source resistance, duty changes, network trade-offs, derating, and release evidence for the final claim.

Battery lifetime calculator compares 2 × AA and CR2032 capacities and the average current allowed for ten years: 11.4 µA versus 2.6 µA.

Major section

The Energy Design Gate

In particular, Average-current budget must be reconciled with: Load ledger before the result can guide implementation.

With those jobs separated, the chapter's the energy design gate claim remains testable.

Energy-aware design gate showing lifetime target, average-current budget, load ledger, architecture choices, and measured margin.

Major section

Build the Load Ledger

A useful energy review separates states by current and time.

The example below uses one hourly sensing and reporting cycle.
The visible relationship between 38 uA and: Sleep baseline identifies what must remain true in the real system.
The ledger shows why both sleep and active states matter.

Energy load ledger showing sleep baseline, sensor warm-up, radio transaction, exception mode, and remaining margin.

Major section

Review Trap: Treating mAh as a Complete Battery Specification

The printed 3 V identifies a nominal voltage, but the photo cannot show how far that voltage will sag during a radio pulse.

The picture confirms the cell's compact form; it does not measure internal resistance or temperature performance.
The mAh rating is measured under defined conditions.

CR2032 coin cell battery in a real physical setting

Major section

Protocol and Workload Decisions

The right choice trades lowest cost against highest reliability; it is not free to get both.

Communication energy depends on the workload, not only the protocol name.
A protocol that is efficient for one-byte status messages may be inefficient for firmware updates, and a protocol with low transmit current may still waste energy if it retries often in the deployment environment.
The common "send less, sleep more" rule is useful, but it is incomplete.

Major section

Measurement Loop

A spreadsheet can guide design, but a power trace catches firmware states, board leakage, retry behavior, and timing that the spreadsheet missed.

At this point, measurement loop needs an inspectable chain rather than another rule of thumb.
The first establishes the starting condition, the second adds a distinct review condition, and the third adds a distinct review condition.

Energy measurement loop showing estimate, implement, measure, compare, fix the dominant state, and preserve margin.

Major section

Energy Design Review Checklist

Requirement: The service target states lifetime, reporting interval, latency, storage behavior, maintenance model, and acceptable data loss.

Budget: The source capacity is derated, and the maximum average-current budget is written as a number in uA or mA.
Ledger: The current ledger includes sleep, active work, radio behavior, exceptions, indicators, pull-ups, regulators, and disabled-but-powered peripherals.
Peak Load: Radio bursts and startup current do not pull the supply below the device minimum voltage at end of life.

Major section

A Coin Cell's Real Limit Is Its Resistance, Not Its mAh

The separated layers show that the coin cell has more than an outer metal case, but they do not reveal its pulse-current limit.

Average capacity can make a coin-cell design look comfortable while a short radio pulse still causes brownout.

Real photograph of cr2032 coin cell battery

Major section

A Coin Cell's Real Limit Is Its Resistance, Not Its mAh (continued)

In Figure: A CR2032's wide, the stamped CR2032 identifies the chemistry and package class, while 3V states nominal voltage rather than the voltage guaranteed during a burst.

Those markings support part selection and polarity checks, but the running design still needs aged internal resistance and pulse-current measurements to prove the radio stays above cutoff.
The risk in a coin cell's real limit is its resistance, not its mah lies between Parameter and 2 x AA.
That conclusion rejoins the chapter's a coin cell's real limit is its resistance, not its mah thread.

A CR2032's wide, thin case stores about 220 mAh, but the small internal electrode area and rising cell resistance limit how hard it can be pulsed. The engraved plus side also matters during assembly: reversing this cell is a physical fault, not a software setting. Photo: Krzysztof Woznica (KyloDee), Public domain

Deck summary

Key takeaways

Firmware is stored device software.

A lifetime claim based only on the calm average can fail in the first difficult season.
In particular, Average-current budget must be reconciled with: Load ledger before the result can guide implementation.
A useful energy review separates states by current and time.
The printed 3 V identifies a nominal voltage, but the photo cannot show how far that voltage will sag during a radio pulse.

Retrieval practice

Recall check 1 of 6

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

Q1A battery-life estimate uses one successful lab cycle. What measurement evidence is still needed?

AAnother copy of the clean lab trace as proof of retry, cold-source and late-life behavior.

BNominal cell capacity alone without board or radio current measurements.

CMicrocontroller sleep-current tables alone without board or radio load traces.

DWhole-device traces for weak links, retries, restarts, cold source and late-life pulses.

Show answer

Answer: D Those traces expose states missing from the clean cycle.

Retrieval practice

Recall check 2 of 6

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

Q2A radio choice has a low energy figure for a clean send. What else must the energy review test?

AUse clean-send energy alone as proof of the installed radio budget.

BUse protocol name alone as a substitute for installed-link energy tests.

CUse the largest battery capacity alone as proof the installed radio path is affordable.

DTest payload, report interval, retries and response deadline on the installed link.

Show answer

Answer: D Service timing and failure traffic can dominate the real duty cycle.

Retrieval practice

Recall check 3 of 6

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

Q3A node has a 40 uA average-current budget. Its measured cycle is: 12 uA sleep baseline, 8 mA sensor warm-up for 1 second every 10 minutes, and 100 mA radio transmit for 0.1 seconds every 10 minutes. Which change most directly brings the design under budget?

AReduce sensor warm-up from 1 second to 0.2 seconds if the reading remains valid

BIncrease battery capacity by 10 percent without changing the load ledger

CReduce sleep current by 1 uA and leave every active state unchanged

DSend two radio messages per cycle so each message can be smaller

Show answer

Answer: A Energy review starts with the measured ledger.

Retrieval practice

Recall check 4 of 6

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

Q4A CR2032-powered beacon resets during radio transmissions while a coulomb counter says over half the rated 220 mAh remains. What is the most likely cause?

APulse current through internal resistance is sagging the rail below brownout.

BThe coulomb counter proves the cell is empty and the radio event is incidental.

CThe firmware sleep current is too low for the regulator to hold the rail.

DThe 220 mAh label guarantees stable voltage until all charge is used.

Show answer

Answer: A The 220 mAh label is a low-drain figure.

Retrieval practice

Recall check 5 of 6

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

Q5A coin cell near end of life shows an internal resistance of about 40 ohm. A radio pulse draws 10 mA. Approximately how much does the terminal voltage sag during the pulse, and does the capacity label protect against it?

AAbout 0.04 V, and yes; the label keeps the pulse rail essentially fixed.

BAbout 4 V, and no; the pulse exceeds the cell voltage before resistance matters.

CAbout 0.4 V, and yes; remaining mAh prevents brownout during the pulse.

DAbout 0.4 V, and no; the label is an energy figure, not a pulse-sag guarantee.

Show answer

Answer: D see answers page

Retrieval practice

Recall check 6 of 6

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

Q6Two CR2032 designs share the same average current and the same 220 mAh cell, but design A has a 2.0 V brownout and design B has a 2.4 V brownout. Which lasts longer under a pulsed radio load, and why?

ADesign B, because its higher cutoff avoids deeper discharge.

BExpect equal life because matching capacity and average current outweigh the cutoff difference.

CNeither, because coin cells cannot drive any pulsed radio.

DDesign A, because lower brownout tolerates more radio sag.

Show answer

Answer: D Usable capacity for a pulsed load is set by when the sagging pulse voltage hits brownout.

Print reference

Answers 1 of 2

Answer key.

D · Those traces expose states missing from the clean cycle.
D · Service timing and failure traffic can dominate the real duty cycle.
A · Energy review starts with the measured ledger.
A · The 220 mAh label is a low-drain figure.

Print reference

Answers 2 of 2

Answer key.

D · V = I x R = 0.010 x 40 = 0.4 V. This drop happens regardless of how many mAh remain, so a healthy capacity reading gives no protection. Only reducing the pulse current, lowering the resistance seen by the pulse, or adding a reservoir capacitor helps.
D · Usable capacity for a pulsed load is set by when the sagging pulse voltage hits brownout.