Checking Laptop Battery Status via Sysfs on Arch Linux

TL;DR

cat /sys/class/power_supply/BAT0/capacity  # Shows current charge percentage
cat /sys/class/power_supply/BAT0/status    # Shows Charging, Discharging, or Full

Understanding the Sysfs Power Supply Interface

Sysfs is a pseudo-filesystem that provides a structured interface to kernel data structures, particularly hardware information and configuration. For power supplies like batteries, this interface is exposed under /sys/class/power_supply/.

Each battery and AC adapter connected to your system appears as a separate directory in this location. The primary battery is typically named BAT0, with additional batteries potentially named BAT1, etc. You can list all power supplies with:

ls /sys/class/power_supply/

Locating and Interpreting Key Battery Files

Within each battery directory, numerous files contain specific information about the battery’s state and characteristics. Here are the most critical files for battery monitoring:

Reading Basic Battery Status

The most immediate battery information can be obtained by reading simple text files:

# Check current battery percentage
cat /sys/class/power_supply/BAT0/capacity

# Check charging status
cat /sys/class/power_supply/BAT0/status

These commands will return the raw values, making them perfect for scripting and automation.

Calculating Battery Health

Battery health degradation can be calculated by comparing the current maximum capacity (energy_full) with the design capacity (energy_full_design):

# Read the capacity values
current_full=$(cat /sys/class/power_supply/BAT0/energy_full)
design_full=$(cat /sys/class/power_supply/BAT0/energy_full_design)

# Calculate health percentage
health_percentage=$(echo "scale=2; $current_full / $design_full * 100" | bc)
echo "Battery health: $health_percentage%"

Note: bc is a command-line calculator. If bc is not installed, you can install it via your package manager (e.g., sudo pacman -S bc).

Here, scale=2 sets the precision to two decimal places.

Health percentages below 80% typically indicate significant battery wear and may suggest considering a replacement.

Practical Monitoring Scripts

Simple Monitoring with Watch

For real-time battery monitoring, you can combine the watch command with file reading:

watch -n 5 'echo "Battery: $(cat /sys/class/power_supply/BAT0/capacity)% ($(cat /sys/class/power_supply/BAT0/status))"'

This command updates the battery percentage and status every 5 seconds.

Advanced Monitoring Script

For more comprehensive monitoring, create a script that gathers multiple battery metrics:

#!/bin/bash
# Define battery path
BATTERY_PATH="/sys/class/power_supply/BAT0"

if [ ! -d "$BATTERY_PATH" ]; then
    echo "Battery not found at $BATTERY_PATH"
    exit 1
fi

capacity=$(cat "$BATTERY_PATH/capacity")
status=$(cat "$BATTERY_PATH/status")
energy_now=$(cat "$BATTERY_PATH/energy_now")
energy_full=$(cat "$BATTERY_PATH/energy_full")
energy_full_design=$(cat "$BATTERY_PATH/energy_full_design")
voltage_now=$(cat "$BATTERY_PATH/voltage_now")

# Convert energy from µWh to Wh
energy_now_wh=$(echo "scale=2; $energy_now / 1000000" | bc)
energy_full_wh=$(echo "scale=2; $energy_full / 1000000" | bc)

# Calculate health percentage
health_pct=$(echo "scale=2; $energy_full / $energy_full_design * 100" | bc)

echo "Battery Status: $status"
echo "Capacity: $capacity%"
echo "Current Energy: $energy_now_wh Wh"
echo "Full Charge Energy: $energy_full_wh Wh"
echo "Design Capacity: $(echo "scale=2; $energy_full_design / 1000000" | bc) Wh"
echo "Battery Health: $health_pct%"
echo "Voltage: $(echo "scale=2; $voltage_now / 1000000" | bc) V"