Skill: Battery (Energy) Optimization

Overview

Battery drain comes from sustained CPU/GPU use, frequent networking (radio wake-ups), location/sensor usage, and work that prevents the device from idling. The OS rewards batching, coalescing, and deferring work. Use the Energy Log / os_signpost and MetricKit to find drains, then reduce wake-ups, respect Low Power Mode, and offload deferrable work to the system (BGTaskScheduler, discretionary URLSession).

Use Cases

• Apps flagged for high background energy use.
• Location/realtime features draining battery.
• Chatty networking causing constant radio activity.

Best Practices

• Batch and coalesce network requests; avoid frequent small polls (prefer push/WebSocket).
• Defer non-urgent work to BGTaskScheduler / discretionary background sessions.
• Right-size location: lowest accuracy that works, significant-location/region monitoring, stop updates when not needed.
• Respect Low Power Mode (ProcessInfo.isLowPowerModeEnabled): reduce refresh/animation.
• Avoid wakeful timers/polling; throttle background refresh.

Anti-Patterns

• ❌ Polling an API every few seconds instead of using push/sockets.
• ❌ Continuous high-accuracy GPS when coarse location suffices.
• ❌ Keeping timers/connections alive in the background unnecessarily.
• ❌ Ignoring Low Power Mode.
• ❌ Heavy work on every scroll/keystroke without throttling/debouncing.

Checklist

• [ ] Network requests batched/coalesced; polling minimized.
• [ ] Deferrable work uses BGTaskScheduler/discretionary sessions.
• [ ] Location accuracy right-sized; updates stopped when idle.
• [ ] Low Power Mode respected.
• [ ] No needless background timers/connections.

Swift Examples

// Respect Low Power Mode and adapt behavior
if ProcessInfo.processInfo.isLowPowerModeEnabled {
    liveUpdates.setRefreshInterval(.seconds(30))   // back off when battery is constrained
} else {
    liveUpdates.setRefreshInterval(.seconds(5))
}

// Schedule deferrable background refresh instead of polling
import BackgroundTasks

func scheduleRefresh() {
    let request = BGAppRefreshTaskRequest(identifier: "com.app.refresh")
    request.earliestBeginDate = Date(timeIntervalSinceNow: 60 * 30) // ~30 min, system-coalesced
    try? BGTaskScheduler.shared.submit(request)
}

// Discretionary download lets the system pick an energy-efficient time
func makeDiscretionarySession() -> URLSession {
    let config = URLSessionConfiguration.background(withIdentifier: "com.app.bg")
    config.isDiscretionary = true
    config.sessionSendsLaunchEvents = true
    return URLSession(configuration: config, delegate: nil, delegateQueue: nil)
}

Common Interview Questions

• What are the biggest sources of battery drain on iOS?
• Why is polling worse than push for battery?
• How does BGTaskScheduler help energy use?
• How do you reduce location energy cost?
• How should an app react to Low Power Mode?

AI Implementation Notes

• Prefer push/WebSocket over polling; batch network work.
• Use BGTaskScheduler/discretionary sessions for deferrable work; check Low Power Mode.
• Right-size location accuracy and stop updates when idle.
• Related: startup_optimization.md, ../../networking/ios/websocket.md.