Energy & Power · Study deck

Energy Measurement and Profiling

Picture a battery sensor whose average current looks low while short radio bursts drain it early.

Battery Bruno is your guide for this deck.

awaremeasurement

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

After studying this chapter

Learning objectives

You will be able to:

Choose a measurement method for sleep current, active current, radio bursts, and startup transients.
Explain burden voltage, shunt power, dynamic range, and why one fixed shunt often cannot cover the whole profile.
Build a repeatable measurement setup that isolates the target device from USB, LEDs, debug adapters, and regulator leakage.
Capture a full duty-cycle trace and connect current spikes to firmware states.

Major section

Start With the Trace, Not the Datasheet

One average can hide the states that decide real life.

Duty cycle means the share of time a device spends active instead of asleep.
Firmware means the software stored on the device.
This runway does not prove lifetime in every field condition.
Both matter, but only the trace can prove the budget.

Major section

Measurement Evidence Loop

Good energy profiling is an iterative loop.

The first trace rarely confirms the original model; it usually reveals hidden loads, extra wake time, radio retries, or a board path that was not in the spreadsheet.
The visible relationship between state budget and and duty cycle identifies what must remain true in the real system.

Energy measurement evidence loop from predicted budget through instrument setup, trace capture, anomaly explanation, and revised battery-life estimate.

Major section

Recalculate

Sleep validation, state-cost diagnosis, battery survival, and before-and-after fixes require different measurement points.

This keeps the lifetime estimate tied to observed behavior instead of one attractive current value.

Major section

Current Measurement Chain

The instrument may be a multimeter, oscilloscope, current-sense amplifier, auto-ranging power profiler, or source-measure unit.

At this point, current measurement chain 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.

Current measurement chain showing supply, series shunt, sense leads, instrument, device under test, and optional firmware marker.

Major section

Current Measurement Chain (continued)

If the burden voltage is too high, the device sees a lower supply voltage and may behave differently.

For example, a 100 ohm shunt gives useful resolution at 10 uA, but it would drop 5 V at 50 mA.
The abstract shunt symbol becomes useful only when its physical resistance and terminals are understood.
The photograph therefore closes the loop back to the equations: choose a value high enough to resolve the quiet state but low enough that the wake burst does not lose unacceptable rail voltage.

Major section

Tool Selection

Tool choice is about current range, time resolution, burden voltage, and repeatability.

Steady current checks, simple shunt voltage readings, and quick sanity tests.
Too slow for short duty-cycle pulses and often adds noticeable burden in current mode.
Low-side or high-side sensing when the shunt voltage is too small for direct measurement.

An oscilloscope like this is the "oscilloscope plus shunt" entry in the table above: it is the tool that actually shows current versus time, catching the short radio bursts and wakeup pulses a multimeter blinks straight past. Photo: BekundaOff, CC BY 4.0

Major section

Tool Selection (continued)

The distinct: CH1 and: CH2 inputs also allow a state marker to be aligned with the current trace, which is the evidence the later power-profile analysis needs.

Improves small-signal measurement while keeping the shunt low.
Bandwidth, offset, gain error, and common-mode range must match the device.
Captures a wide dynamic range with fewer manual range changes.

Major section

Capturing a Power Profile

The trace is only useful if every region can be explained.

10 µA changes the question by adds a distinct review condition, and 1 mA completes it by adds a distinct review condition.

Annotated current profile for an IoT node showing sleep baseline, wake, sensor stabilization, processing, radio transmission, and return-to-sleep tail.

Major section

From Trace to Battery-Life Estimate

The most useful output of profiling is charge per useful cycle.

Total charge per cycle is about 52 uAh.
A 2400 mAh cell with 80% usable capacity would be estimated at roughly 256 days before adding battery derating, temperature effects, self-discharge, and reserve margin.
The point is not the exact number.

Major section

Common Findings and Fixes

Sleep current is far above budget.

Active window is longer than expected.
Batch work, shorten timeouts, remove debug logging, or change the reporting policy.
Peripheral shutdown sequence, capacitor discharge, regulator mode change, or radio not fully off.
Life: a slow return-to-sleep tail often means a shutdown sequence never actually finished.

Major section

Validation Conditions

A single room-temperature bench trace is useful, but it is not final validation.

Final hardware: development boards include loads that production hardware may not have, and production boards include leakage paths that prototypes may hide.
Battery voltage range: radio transmit current, regulator efficiency, brownout margin, and sensor behavior can change as voltage falls.
Temperature range: leakage, battery capacity, sensor settling, and oscillator behavior can shift at temperature extremes.
Multiple units: one good board does not prove a manufacturing population.

Deck summary

Key takeaways

One average can hide the states that decide real life.

Good energy profiling is an iterative loop.
Sleep validation, state-cost diagnosis, battery survival, and before-and-after fixes require different measurement points.
The instrument may be a multimeter, oscilloscope, current-sense amplifier, auto-ranging power profiler, or source-measure unit.
If the burden voltage is too high, the device sees a lower supply voltage and may behave differently.

Retrieval practice

Recall check 1 of 6

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

Q1A battery sensor measures 12 uA sleep current on the isolated target rail, but 6 mA when powered through the development board USB connector. What is the best interpretation?

AThe target sleep mode is probably working, and the USB-board path includes extra loads.

BThe isolated target-rail measurement should be ignored because USB is the normal development connection.

CThe instrument must be broken because the two readings disagree.

DThe battery-life estimate should use the lower number without checking the final product rail.

Show answer

Answer: A Low-power measurements must state exactly which rail is measured.

Retrieval practice

Recall check 2 of 6

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

Q2Why can a 100 ohm shunt be a poor choice for a device that wakes to 50 mA, even if it helps measure 10 uA sleep current?

AIt makes the 10 uA sleep signal too small to see, so only wake current is measured.

BIt drops about 5 V at the wake current, starving the device instead of observing it.

CIt converts the DC current into AC ripple, so Ohm's law no longer applies.

DIt protects the battery by limiting every wake pulse to the planned current budget.

Show answer

Answer: B Shunt selection is a tradeoff between low-current resolution and high-current burden voltage.

Retrieval practice

Recall check 3 of 6

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

Q3Place each measurement component where it lives so you can connect a trustworthy current trace to the device event that caused it.

ABattery or supply

BRadio antenna

CSensor interrupt

DGround loop

Show answer

Answer: A Follow source path, measurement bridge, and device correlation so you can distinguish supplied energy, sensed current, and the firmware event that explains the trace.

Retrieval practice

Recall check 4 of 6

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

Q4Why can a single fixed shunt resistor not accurately measure both a 10 uA sleep current and a 200 mA transmit current?

AThe battery chemistry changes between sleep and transmit, so each phase needs a different supply model.

BThe meter ADC can sample only one current level per boot, so separate tests are always required.

CSleep needs a larger shunt for resolution, while transmit needs a smaller shunt for low burden.

DThe radio bypasses the sense resistor during transmit, so only the sleep interval is recorded.

Show answer

Answer: C Resolution wants a large shunt; low burden wants a small one.

Retrieval practice

Recall check 5 of 6

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

Q5You need burden under 50 mV at a 200 mA peak but also a 100 uV signal at 10 uA sleep. What do the two bounds tell you?

AA single 5 ohm shunt satisfies both bounds.

BBurden requires R <= 0.25 ohm while resolution requires R >= 10 ohm.

COnly the burden bound matters; resolution is irrelevant.

DThe bounds agree, so a 0.25 ohm shunt reads sleep fine.

Show answer

Answer: B

Retrieval practice

Recall check 6 of 6

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

Q6A bench multimeter reads 0.4 mA average for a duty-cycled radio node, and an engineer concludes a coin cell will be fine. Why is that conclusion unsafe?

AThe average proves the node never transmits, so the radio duty cycle must be disabled.

BCoin cells respond only to total charge, so the peak current has no effect on voltage sag.

CA multimeter cannot measure milliamp averages, so the 0.4 mA reading is unusable.

DThe average can hide a short transmit peak that may exceed what the coin cell can supply.

Show answer

Answer: D

Print reference

Answers

Answer key.

A · Low-power measurements must state exactly which rail is measured.
B · Shunt selection is a tradeoff between low-current resolution and high-current burden voltage.
A · Follow source path, measurement bridge, and device correlation so you can distinguish supplied energy, sensed current, and the firmware event that explains the trace.
C · Resolution wants a large shunt; low burden wants a small one.
B
D