UX Design · Study deck
Device Power: Energy Budgets
Start with the service promise.
UX Uma is your guide for this deck.
After studying this chapter
Learning objectives
You will be able to:
- review a connected-device power budget from measured states rather than assumptions
- explain how duty cycle, sleep current, radio use, sensing, actuation, and displays affect service interval
- identify checks needed for battery, mains, charging, harvesting, and backup power decisions
- connect power behavior to user experience, maintenance, OTA updates, and lifecycle risk
Major section
Start Simple
A duty cycle is the share of time spent doing a task instead of resting.
- It helps turn a list of power states into a daily energy budget.
- That promise includes sensing, screen use, wireless work, updates, charging, cold weather, and the time a person has to react.
- An average can hide a short peak that resets the device.
Major section
Start Simple (continued)
A large battery rating does not prove useful life if voltage falls too soon.
- More sleep can save energy while making controls or alerts feel late.
- Under the Hood explains the failures hidden by one average-current number.
- A battery goal with no user promise is hard to test.
- A board value may miss loss in the power path.
Major section
Start Simple (continued)
The hard day may include poor signal, many alerts, or long screen use.
- A source can hold enough energy yet fail to supply one sharp peak.
- Wake it from each cause and prove it can return to rest.
- Weak coverage can add scans, joins, waits, and retries.
- A downlink need can hold the radio awake.
Major section
Phoebe's Field Notes: Averaging Four States, Not Two
The mathematical gist.: The chapter's four states average 51.8 µA when the device reports every 300 seconds, but 219 µA when it reports every 60 seconds.
- A 2,400 mAh cell retaining 99% per year has 2,352 mAh after two years, so the shorter interval cuts the modelled life from well over two years to about 1.23 years.
Major section
Overview: Power Is a Service Promise
Connected-device power design is not just choosing a larger cell.
- For a battery sensor, the promise depends on sleep current, wake interval, sensor warm-up, radio join or transmit time, acknowledgement policy, retry behavior, battery chemistry, temperature derating, regulator quiescent current, and the physical service path.
Major section
Overview: Power Is a Service Promise (continued)
For an interface or actuator, it also depends on displays, LEDs, haptics, motors, relays, locks, valves, and whether low-power mode changes the user-visible behavior.
- The same nominal battery capacity can support very different products.
- A CR2032 beacon that advertises every few seconds may be limited by pulse current and radio schedule.
- A Li-SOCl2 field sensor may be limited by cold-temperature voltage sag and passivation after storage.
Major section
Overview: Power Is a Service Promise (continued)
A Li-ion handheld reader may be limited by display brightness, user charging habits, and charger access.
- A mains-powered gateway may still need backup power long enough to flush buffers, preserve endpoint state, and report the outage cleanly.
- The record should show the energy source, measured states, expected event rate, installed radio behavior, derating assumption, low-power threshold, warning path, service owner, and change condition.
- Design degradation: warn early, preserve core function, defer noncritical work, and explain low-power behavior to support staff.
Major section
Budget from Workload
Gate risky operations.: Require enough energy and storage for firmware download, signature verification, flash write, reboot, boot confirmation, rollback, and post-update health before OTA starts.
- For solar or harvested designs, include the worst charging window, load-shedding rule, storage reserve, and what support sees after several low-energy days.
- For rechargeable devices, include charging time, connector wear, cable availability, heat, and whether the user can recover without losing pairing or configuration.
- The decision should include derating.
Major section
Average Current Hides Failures
Average current is useful, but it can hide the state that breaks the product.
- The firmware state machine should make energy decisions explicit.
- It should know when it is manufactured, provisioned, commissioned, normal, degraded, low-battery, charging, updating, rollback, factory-reset, or retired.
- Hardware details can also invalidate a clean average-current spreadsheet.
Deck summary
Key takeaways
A duty cycle is the share of time spent doing a task instead of resting.
- A large battery rating does not prove useful life if voltage falls too soon.
- The hard day may include poor signal, many alerts, or long screen use.
- The mathematical gist.: The chapter's four states average 51.8 µA when the device reports every 300 seconds, but 219 µA when it reports every 60 seconds.
- Connected-device power design is not just choosing a larger cell.
Retrieval practice
Recall check
UX Uma says: answer from memory, then check your reasoning.
Q1A team is reviewing a battery sensor that claims a two-year service interval before a firmware update policy is approved. Which power record makes the service interval credible?
Show answer
Answer: A A reviewable power decision ties the power source, measured states, duty cycle, installed connectivity, service interval, low-power UX, update gate, performed checks, owner, and change condition together before the battery-life claim is trusted.
Print reference
Answers
Answer key.
- A · A reviewable power decision ties the power source, measured states, duty cycle, installed connectivity, service interval, low-power UX, update gate, performed checks, owner, and change condition together before the battery-life claim is trusted.