Power Management and Battery

Effective power management extends battery life on laptops and reduces energy consumption on desktops. macOS provides sophisticated power management through pmset, caffeinate, power assertions, and App Nap. Understanding these systems enables optimization for both performance and efficiency.

Power Management Architecture

Power Management Layers

┌─────────────────────────────────────────────────────────────────┐
│                    Applications                                  │
│          (Power assertions, App Nap responses)                  │
├─────────────────────────────────────────────────────────────────┤
│                    WindowServer                                  │
│          (Display sleep, screen brightness)                     │
├─────────────────────────────────────────────────────────────────┤
│                    IOKit Power Management                       │
│          (Device power states, sleep/wake)                      │
├─────────────────────────────────────────────────────────────────┤
│                    Kernel                                        │
│          (CPU power states, thermal management)                 │
├─────────────────────────────────────────────────────────────────┤
│                    SMC (System Management Controller)           │
│          (Battery, fans, sensors - Intel Macs)                  │
│                    or                                            │
│                    Apple Silicon Power Management               │
│          (Integrated power controller)                          │
└─────────────────────────────────────────────────────────────────┘

pmset - Power Management Settings

pmset is the primary tool for configuring power settings.

Viewing Current Settings

# All current settings
$ pmset -g
System-wide power settings:
Currently in use:
 standby              1
 Sleep On Power Button 1
 hibernatefile        /var/vm/sleepimage
 powernap             1
 networkoversleep     0
 disksleep            10
 sleep                1 (sleep prevented by screensharingd)
 hibernatemode        3
 ttyskeepawake        1
 displaysleep         10
 tcpkeepalive         1
 lowpowermode         0
 womp                 0

# Battery status
$ pmset -g batt
Now drawing from 'Battery Power'
 -InternalBattery-0 (id=12345678)    85%; discharging; 4:30 remaining present: true

# Power assertions (what's preventing sleep)
$ pmset -g assertions
2024-01-15 12:00:00 -0800
Assertion status system-wide:
   BackgroundTask                 1
   ApplePushServiceTask           0
   UserIsActive                   1
   PreventUserIdleDisplaySleep    1
   PreventSystemSleep             0
   ExternalMedia                  0
   PreventUserIdleSystemSleep     1
   NetworkClientActive            0
Listed by owning process:
   pid 12345(Safari): [0x0000123400000abc] 01:23:45 PreventUserIdleDisplaySleep named: "Playing Audio"

Power Source Settings

Different settings for battery vs AC power:

# View settings by power source
$ pmset -g custom
Battery Power:
 displaysleep         2
 disksleep            10
 sleep                10
 womp                 0
 powernap             0
AC Power:
 displaysleep         10
 disksleep            10
 sleep                0
 womp                 1
 powernap             1

Configuring Power Settings

# Set display sleep timeout (minutes)
$ sudo pmset -b displaysleep 5    # Battery: 5 minutes
$ sudo pmset -c displaysleep 15   # AC: 15 minutes
$ sudo pmset -a displaysleep 10   # All: 10 minutes

# Set system sleep timeout
$ sudo pmset -a sleep 30

# Disable sleep entirely
$ sudo pmset -a sleep 0
$ sudo pmset -a disablesleep 1

# Enable/disable wake on network (Wake on LAN)
$ sudo pmset -a womp 1  # Enable
$ sudo pmset -a womp 0  # Disable

# Power Nap settings
$ sudo pmset -a powernap 0  # Disable Power Nap
$ sudo pmset -a powernap 1  # Enable Power Nap

Important pmset Keys

Hibernate Modes

# Check current hibernate mode
$ pmset -g | grep hibernatemode

# Hibernate modes:
# 0 = RAM stays powered, no hibernation (desktop default)
# 3 = RAM powered + hibernation file (laptop default, safe sleep)
# 25 = Pure hibernation, RAM powered off (maximum battery preservation)

# Set hibernate mode
$ sudo pmset -a hibernatemode 3

Mode comparison:

Low Power Mode

Available on MacBooks with Apple Silicon and recent Intel:

# Check Low Power Mode
$ pmset -g | grep lowpowermode

# Enable Low Power Mode
$ sudo pmset -a lowpowermode 1

# Disable
$ sudo pmset -a lowpowermode 0

Low Power Mode effects:

• Reduces CPU performance
• Dims display
• Reduces background activity
• Extends battery life significantly

caffeinate - Prevent Sleep

caffeinate prevents the system from sleeping.

Basic Usage

# Prevent sleep while command runs
$ caffeinate -s make all

# Prevent sleep for duration (seconds)
$ caffeinate -t 3600  # 1 hour

# Prevent display sleep
$ caffeinate -d

# Prevent idle sleep
$ caffeinate -i

# Prevent disk sleep
$ caffeinate -m

# Prevent system sleep
$ caffeinate -s

# All of the above
$ caffeinate -dims

caffeinate Options

Practical Examples

# Keep awake during long compile
$ caffeinate -i make -j8 all

# Keep awake while download completes
$ caffeinate -s curl -O https://example.com/large-file.zip

# Keep awake while backup runs
$ caffeinate -s rsync -av /source /destination

# Keep display on during presentation (until Ctrl+C)
$ caffeinate -d

# Keep awake as long as another process runs
$ long_running_process &
$ caffeinate -w $!

# Script that manages its own caffeinate
#!/bin/bash
caffeinate -i -w $$ &
# ... long running work ...
# caffeinate automatically exits when script finishes

Scripted Power Control

#!/bin/bash
# smart-caffeinate.sh - Caffeinate with status

DURATION=${1:-3600}

echo "Preventing sleep for $((DURATION / 60)) minutes"
echo "Press Ctrl+C to cancel"

caffeinate -dims -t $DURATION &
CAFF_PID=$!

trap "kill $CAFF_PID 2>/dev/null; echo 'Sleep prevention cancelled'" EXIT

while kill -0 $CAFF_PID 2>/dev/null; do
    REMAINING=$((DURATION - SECONDS))
    if [[ $REMAINING -lt 0 ]]; then
        break
    fi
    printf "\rRemaining: %02d:%02d" $((REMAINING / 60)) $((REMAINING % 60))
    sleep 1
done

echo -e "\nSleep prevention ended"

Power Assertions

Power assertions are how applications communicate their power needs to the system.

Viewing Assertions

# List all active assertions
$ pmset -g assertions

# Detailed assertion info
$ pmset -g assertionslog

# Who's preventing sleep?
$ pmset -g assertions | grep -A2 "PreventSystemSleep\|PreventUserIdleSystemSleep"

Common Assertion Types

Creating Assertions from Command Line

# caffeinate creates assertions internally

# To create specific assertions programmatically:
# Use IOKit APIs (requires code)
# Or use caffeinate with appropriate flags

Finding Assertion Offenders

#!/bin/bash
# find-assertion-owners.sh - Find processes preventing sleep

echo "Processes with active power assertions:"
echo "======================================="

pmset -g assertions | grep -E "^\s+pid" | while read line; do
    pid=$(echo "$line" | awk -F'[()]' '{print $2}')
    name=$(echo "$line" | awk -F'[()]' '{print $1}' | awk '{print $2}')
    assertion=$(echo "$line" | awk -F'"' '{print $2}')

    echo "Process: $name (PID $pid)"
    echo "  Assertion: $assertion"
    echo ""
done

App Nap

App Nap reduces resource usage for applications not actively being used.

How App Nap Works

┌─────────────────────────────────────────────────────────────────┐
│                    App Nap Conditions                            │
├─────────────────────────────────────────────────────────────────┤
│ App enters App Nap when:                                        │
│ 1. Window is not visible (minimized, behind other windows)     │
│ 2. Not playing audio                                            │
│ 3. No active power assertions                                   │
│ 4. Not explicitly disabled by app                               │
├─────────────────────────────────────────────────────────────────┤
│ App Nap effects:                                                │
│ - Timer coalescing (batched background work)                    │
│ - Reduced I/O priority                                          │
│ - Reduced CPU priority                                          │
│ - Paused or throttled network activity                          │
└─────────────────────────────────────────────────────────────────┘

Checking App Nap Status

In Activity Monitor:

1. View > Columns > App Nap
2. “Yes” = App Nap active, “No” = App running normally

# Check if an app supports App Nap
$ defaults read /Applications/Safari.app/Contents/Info.plist NSAppSleepDisabled 2>/dev/null
# (no output or error = App Nap enabled)
# 1 = App Nap disabled

Disabling App Nap for an App

# Disable App Nap for specific app
$ defaults write com.apple.Safari NSAppSleepDisabled -bool YES

# Via Finder:
# 1. Right-click app in Applications
# 2. Get Info
# 3. Check "Prevent App Nap"

Power Monitoring

Battery Information

# Battery status
$ pmset -g batt
Now drawing from 'Battery Power'
 -InternalBattery-0 (id=12345678)    85%; discharging; 4:30 remaining

# Detailed battery info
$ system_profiler SPPowerDataType

# Battery health (cycles, condition)
$ system_profiler SPPowerDataType | grep -E "Cycle Count|Condition|Maximum Capacity"
      Cycle Count: 234
      Condition: Normal
      Maximum Capacity: 92%

# ioreg for detailed battery data
$ ioreg -rn AppleSmartBattery | grep -E "Cycle|Capacity|Temperature"

Power Consumption Monitoring

# Using powermetrics (requires sudo)
$ sudo powermetrics --samplers cpu_power,battery -n 1

# CPU power consumption
$ sudo powermetrics --samplers cpu_power -n 1 | grep "CPU Power"
CPU Power: 2850 mW

# Battery drain rate
$ sudo powermetrics --samplers battery -n 1 | grep "Amperage"
Amperage: -1234 mA

# Continuous monitoring
$ sudo powermetrics --samplers cpu_power,battery -i 5000 | grep -E "CPU Power|Battery Level"

Energy Impact by Process

# Using top
$ top -o power

# Using powermetrics for detailed per-process
$ sudo powermetrics --samplers tasks -n 1 | head -40

Power Monitoring Script

#!/bin/bash
# power-monitor.sh - Monitor power consumption

echo "Power Monitor - Press Ctrl+C to stop"
echo "Time,Battery%,Amperage(mA),CPU_Power(mW),Drawing_From"

while true; do
    # Get battery info
    batt_info=$(pmset -g batt)
    battery_pct=$(echo "$batt_info" | grep -o '[0-9]*%' | head -1 | tr -d '%')
    power_source=$(echo "$batt_info" | grep "drawing from" | awk -F"'" '{print $2}')

    # Get power metrics (needs sudo)
    power_data=$(sudo powermetrics --samplers cpu_power,battery -n 1 2>/dev/null)
    cpu_power=$(echo "$power_data" | grep "CPU Power:" | awk '{print $3}' | tr -d 'mW')
    amperage=$(echo "$power_data" | grep "Amperage:" | awk '{print $2}' | tr -d 'mA')

    echo "$(date +%H:%M:%S),$battery_pct,$amperage,$cpu_power,$power_source"

    sleep 30
done

Optimizing Battery Life

System Settings

# Maximize battery life settings
$ sudo pmset -b displaysleep 2      # Display off after 2 min on battery
$ sudo pmset -b sleep 5              # System sleep after 5 min
$ sudo pmset -b powernap 0           # Disable Power Nap on battery
$ sudo pmset -b lowpowermode 1       # Enable Low Power Mode
$ sudo pmset -b lessbright 1         # Slightly dim display on battery

# Aggressive standby
$ sudo pmset -b standby 1
$ sudo pmset -b standbydelay 60      # Enter standby after 1 min of sleep
$ sudo pmset -b hibernatemode 25     # Pure hibernation (slowest wake, best battery)

Finding Battery Drains

#!/bin/bash
# find-battery-drains.sh - Identify battery draining activities

echo "=== Power Assertions ==="
pmset -g assertions | grep -E "pid|named"

echo ""
echo "=== High Energy Impact Processes ==="
echo "(Check Activity Monitor > Energy tab for more detail)"
ps aux | sort -nrk 3 | head -6

echo ""
echo "=== Apps Preventing Sleep ==="
pmset -g assertions | grep "PreventUserIdleSystemSleep\|PreventSystemSleep" -A2

echo ""
echo "=== Bluetooth Devices ==="
system_profiler SPBluetoothDataType 2>/dev/null | grep -E "Connected:|Name:" | head -10

echo ""
echo "=== Active Network Connections ==="
lsof -i | grep -E "ESTABLISHED|LISTEN" | awk '{print $1}' | sort | uniq -c | sort -rn | head -10

Quick Battery Tips

1. Reduce screen brightness - Major power consumer
2. Disable Bluetooth/Wi-Fi when not needed
3. Close unused tabs - Browser tabs use memory and CPU
4. Enable Low Power Mode - Significant battery extension
5. Check Activity Monitor Energy tab - Find specific drains
6. Disable Power Nap on battery - Prevents background wake
7. Use native apps - Rosetta 2 uses more power

Thermal Management

macOS manages thermals automatically, but you can monitor:

# Thermal state
$ sudo powermetrics --samplers thermal -n 1
*** Thermal State ***
System Thermal Level: 0 (nominal)
CPU Thermal Level: 0 (nominal)
GPU Thermal Level: 0 (nominal)

# Fan speed (Intel Macs with SMC)
$ sudo powermetrics --samplers smc -n 1 | grep -i fan

# Or use third-party tools
$ brew install osx-cpu-temp
$ osx-cpu-temp  # Shows CPU temperature

Thermal States

Summary

Key power management commands:

Power optimization checklist:

# Check current state
$ pmset -g batt              # Battery status
$ pmset -g assertions        # What's preventing sleep
$ pmset -g                   # Current power settings

# Common optimizations
$ sudo pmset -b lowpowermode 1      # Enable Low Power Mode
$ sudo pmset -b displaysleep 2      # Quick display off
$ sudo pmset -b powernap 0          # Disable Power Nap on battery
$ caffeinate -i long_task           # Prevent sleep during task

# Monitor power use
$ sudo powermetrics --samplers cpu_power,battery -n 1

Effective power management balances performance needs with battery life and energy efficiency.