How do I make my laptop sleep when it reaches some low battery threshold?

Question

I'm using Ubuntu, but I have i3 as my window manager instead of a desktop environment.

When my battery reaches 0%, the computer will just abruptly shut down, no warning or anything.

Is there a simple script or configuration I can set up so that it goes to sleep at, say 4% battery?

Answer 1

Here's a small script that checks for the battery level and calls a custom command, here pm-hibernate, in case the battery level is below a certain threshold.

#!/bin/sh

###########################################################################
#
# Usage: system-low-battery
#
# Checks if the battery level is low. If “low_threshold” is exceeded
# a system notification is displayed, if “critical_threshold” is exceeded
# a popup window is displayed as well. If “OK” is pressed, the system
# shuts down after “timeout” seconds. If “Cancel” is pressed the script
# does nothing.
#
# This script is supposed to be called from a cron job.
#
###########################################################################

# This is required because the script is invoked by cron. Dbus information
# is stored in a file by the following script when a user logs in. Connect
# it to your autostart mechanism of choice.
#
# #!/bin/sh
# touch $HOME/.dbus/Xdbus
# chmod 600 $HOME/.dbus/Xdbus
# env | grep DBUS_SESSION_BUS_ADDRESS > $HOME/.dbus/Xdbus
# echo 'export DBUS_SESSION_BUS_ADDRESS' >> $HOME/.dbus/Xdbus
# exit 0
#
if [ -r ~/.dbus/Xdbus ]; then
  source ~/.dbus/Xdbus
fi

low_threshold=10
critical_threshold=4
timeout=59
shutdown_cmd='/usr/sbin/pm-hibernate'

level=$(cat /sys/devices/platform/smapi/BAT0/remaining_percent)
state=$(cat /sys/devices/platform/smapi/BAT0/state)

if [ x"$state" != x'discharging' ]; then
  exit 0
fi

do_shutdown() {
  sleep $timeout && kill $zenity_pid 2>/dev/null

  if [ x"$state" != x'discharging' ]; then
    exit 0
  else
    $shutdown_cmd
  fi
}

if [ "$level" -gt $critical_threshold ] && [ "$level" -lt $low_threshold ]; then
  notify-send "Battery level is low: $level%"
fi

if [ "$level" -lt $critical_threshold ]; then

  notify-send -u critical -t 20000 "Battery level is low: $level%" \
    'The system is going to shut down in 1 minute.'

  DISPLAY=:0 zenity --question --ok-label 'OK' --cancel-label 'Cancel' \
    --text "Battery level is low: $level%.\n\n The system is going to shut down in 1 minute." &
  zenity_pid=$!

  do_shutdown &
  shutdown_pid=$!

  trap 'kill $shutdown_pid' 1

  if ! wait $zenity_pid; then
    kill $shutdown_pid 2>/dev/null
  fi

fi

exit 0

It's a very simple script, but I think you get the idea and can easily adapt it to your needs. The path to the battery level might be different on your system. A little more portable would probably be to use something like acpi | cut -f2 -d, to obtain the battery level. This script can be scheduled by cron to run every minute. Edit your crontab with crontab -e and add the script:

*/1 * * * * /home/me/usr/bin/low-battery-shutdown

Another solution would be to install a desktop environment like Gnome or Xfce (and change your window manager to i3). Both mentioned destop environments feature power management daemons which take care of powering off the computer. But I assume you deliberately don't use them and are seeking for a more minimalistic solution.

Answer 2

Instead of hacking your own scripts and if you are using Ubuntu as the tag suggests, you could just install the upower package. It should be available on all Debian derivatives including Ubuntu. By default it comes with a configuration in /etc/UPower/UPower.conf which activates hybrid sleep once the battery level reaches critical values. The default for the critical level is 2%.

For users of other distributions, the relevant entries for /etc/UPower/UPower.conf are:

PercentageAction=2
CriticalPowerAction=HybridSleep

You can also use TimeAction together with UsePercentageForPolicy=false to let the action be carried out once only the specified time is left:

TimeAction=120

The valid values for CriticalPowerAction are PowerOff, Hibernate and HybridSleep. If HybridSleep is set but not available, Hibernate will be used. If Hibernate is set but not available, PowerOff will be used.

The advantage of HybridSleep is, that in addition to writing out memory into your swap area, it then suspends the system. Suspend will still consume some battery but if you come back before the battery ran out, you can much more quickly resume from a suspended system than from a hibernated one. In case the battery does run out before you get back to a power socket, you can resume the system from hibernation once you have power again.

Answer 3

As of Debian ≥ 10 (and comparably recent Linux systems), you can just create a file /etc/cron.d/check-battery that contains:

* * * * * root [ "$(cat /sys/class/power_supply/BAT0/status)" != Discharging -o "$(cat /sys/class/power_supply/BAT0/capacity)" -gt 30 ] || systemctl suspend

This will suspend your system whenever the battery level reaches 30%.

Of course, feel free to replace the final suspend with hybrid-sleep, hibernate, poweroff or whatever fits your needs.

No external tools are required, not even the acpi package. This is based on the idea of Matija Nalis' answer, but adjusted to the year 2023.

Answer 4

The currently accepted answer is great, but a little bit outdated for Ubuntu 16.04:

• The commands to get battery status have changed.
• The environment variables required for notify-send to work have changed.
• The script given there no longer works from user cron as hibernate requires root.
• systemctl hibernate is preferred over pm-hibernate.

So, here is the script I use:

#!/usr/bin/env bash

# Notifies the user if the battery is low.
# Executes some command (like hibernate) on critical battery.
# This script is supposed to be called from a cron job.
# If you change this script's name/path, don't forget to update it in crontab !!

level=$(cat /sys/class/power_supply/BAT1/capacity)
status=$(cat /sys/class/power_supply/BAT1/status)

# Exit if not discharging
if [ "${status}" != "Discharging" ]; then
  exit 0
fi

# Source the environment variables required for notify-send to work.
env_vars_path="$HOME/.env_vars"
source "${env_vars_path}"

low_notif_percentage=20
critical_notif_percentage=15
critical_action_percentage=10

if [ "${level}" -le ${critical_action_percentage} ]; then
  # sudo is required when running from cron
  sudo systemctl hibernate
  exit 0
fi

if [ "${level}" -le ${critical_notif_percentage} ]; then
  notify-send -i '/usr/share/icons/gnome/256x256/status/battery-caution.png' "Battery critical: ${level}%"
  exit 0
fi

if [ "${level}" -le ${low_notif_percentage} ]; then
  notify-send -i '/usr/share/icons/gnome/256x256/status/battery-low.png' "Battery low: $level%"
  exit 0
fi

The environment variables required for notify-send to work are created using this script:

#!/usr/bin/env bash

# Create a new file containing the values of the environment variables
# required for cron scripts to work.
# This script is supposed to be scheduled to run at startup.

env_vars_path="$HOME/.env_vars"

rm -f "${env_vars_path}"
touch "${env_vars_path}"
chmod 600 "${env_vars_path}"

# Array of the environment variables.
env_vars=("DBUS_SESSION_BUS_ADDRESS" "XAUTHORITY" "DISPLAY")

for env_var in "${env_vars[@]}"
do
  echo "$env_var"
  env | grep "${env_var}" >> "${env_vars_path}";
  echo "export ${env_var}" >> "${env_vars_path}";
done

This file needs to run at startup (can be done using any method of your choice; I use Ubuntu's builtin Startup Applications).

Note: sudo systemctl hibernate might not work from cron. Follow this to solve it.

Answer 5

There are many ways it could be implemented, as there are many different power managment schemes implemented depending on what you have installed.

This simple one works for me on minimalistic Debian Jessie without any desktop environment, just with small and fast icewm window manager. (It is trimmed down because is just way too slow otherwise, and this way it outperforms GNOME on much better hardware)

Specifically, I DO have installed following packages: acpi acpi-fakekey acpi-support acpi-support-base acpid pm-utils but have NONE of the following (having purged them): gnome* kde* systemd* uswsusp upower laptop-mode-tools hibernate policykit-1

So I just put this in /etc/cron.d/battery_low_check (all in one line, split for readability):

*/5 * * * *   root  acpi --battery | 
   awk -F, '/Discharging/ { if (int($2) < 10) print }' | 
   xargs -ri acpi_fakekey 205

It is quick, low-resource-usage, and does not depend on other deamons (if fact, it will be ignored if they're active - see /usr/share/acpi-support/policy-funcs for details).

What it does: every 5 minutes (*/5 - you can change to every minute by just using * if you need it to check battery more often) it will poll battery status ("acpi --battery") and execute command after xargs -ri only if battery is "Discharging" (that is, you're not connected to AC) and battery status is less than 10% ("int ($2) < 10" - feel free to tune it to your needs)

acpi_fakekey 205 will by default send KEY_SUSPEND ACPI event (like you pressed a key on laptop requesting suspend), which will then do whatever it usually does for you (configured in /etc/default/acpi-support) - for me it hibernates to disk.

You could use other command instead of acpi_fakekey 205 of course: like hibernate (from hibernate package), s2disk or s2mem (from uswsusp package), pm-suspend-hybrid (from pm-utils package) etc.

BTW, magic key numbers like KEY_SUSPEND=205 above are defined in /usr/share/acpi-support/key-constants (other interesting one is probably KEY_SLEEP=142)

Answer 6

I like this solution, that is partly inspired by other answers: https://github.com/jerrinfrncs/batterynotif, namely the script batterynotif(uname).sh.

See the script here: https://github.com/jerrinfrncs/batterynotif/blob/master/batterynotif%28uname%29.sh

For my own use I have changed the script to enter hybrid-sleep instead of shut-down, by using the command systemctl hybrid-sleep. (Swap space is needed by this option.)