Battery capacity monitoring

From postmarketOS Wiki


Battery state can be monitored periodically. When the capacity is below certain level, an action can be triggered.

Battery capacity change doesn't trigger udev events, so udev can't be used for this purpose. Monitoring is performed periodically, using crond to perform the job.


Create an executable script for action at various capacity level. The example script below will blink the notification light when the battery is at 25 percent and will blink the notification light again when the battery is at 15 percent and shutdown. The script below is for Xiaomi Redmi 1S (xiaomi-armani), so modification of sysfs path is needed for use on another devices.

# Replace BATTERY variable with the actual battery device
# found under sysfs

# Turn off notification leds
# Replace the value of leds according to the sysfs entry
ledsoff() {
local leds="red green blue"
for led in $leds; do
	echo 0 > /sys/class/leds/"$led"/brightness

# Blink notification leds for 100 times
# Replace the red variable with actual red led device
# found under sysfs
blink() {
local countdown=100
local red=red
until [ "$countdown" -eq 0 ]; do
	countdown="$((countdown-1))" &&
	echo 100 > /sys/class/leds/"$red"/brightness && sleep 1
	echo 0 > /sys/class/leds/"$red"/brightness

if [ -d /sys/class/power_supply/"$BATTERY" ]; then
	CAPACITY="$(cat /sys/class/power_supply/${BATTERY}/capacity)"

if [ "$CAPACITY" -lt 25 ]; then
	logger "Battery capacity is less than 25 percent"

if [ "$CAPACITY" -lt 15 ]; then
	logger "Battery capacity is less than 15 percent"
	logger "Battery low. Powering off"


Place the script above as inside /etc/periodic/2min directory. Create the directory if needed.

# mkdir -p /etc/periodic/2min

Make the script file executable.

# chmod 755 /etc/periodic/2min/

Adding a crontab entry to do the job

A crontab entry is needed to perform the scheduled battery check. Modify root's crontab to do the scheduled task.

# crontab -u root -e

The command will open root's crontab file /var/spool/crontabs/root inside vi text editor.

Note that the busybox crontab command doesn't parse EDITOR environment variable, so if another text editor is pereferred, open the root crontab file directly.

# nano /var/spool/crontabs/root

Add a line to indicate that a new 2 minutes action will be performed.

# min   hour    day     month   weekday command
*/2     *       *       *       *       run-parts /etc/periodic/2min

Enabling crond service

To enable crond service so that crontab will be executed, execute the following command.

# rc-update add crond default

The crond service will be started automatically after rebooting.

To start the service immediately, run the following command.

# /etc/init.d/crond start

The crond will execute every 2 minutes and the predefined low battery actions will be performed.

See also