Tech Memorandums

Hibernate on low-battery on the Thinkpad x220

When I first got this laptop in 2012, Debian would hibernate the laptop when the battery went too low. I didn’t set this up and it worked well until it didn’t work.

On the X220, the battery LED is not visible to you when you are using the laptop, it is on the top side of the lid.

Figuring this out would mean a lot of waiting and watching the output of acpi_listen and udevadm monitor. The alternative would be to write a cronjob that would check battery status. This workaround offended my sensibilities and I therefore endured unexpected shutdowns. This became steadily worse as the battery capacity degraded.

But not today. I found user svenper on the arch forums who figured out the fact that the firmware does send an ACPI event. This event is sent when,

To adapt his solution to Debian’s acpid, create a file /etc/acpi/events/thinkpad-low-battery with contents:

event=battery PNP0C0A:00 00000080 00000001
action=/etc/acpi/tp-low-battery.sh

And /etc/acpi/tp-low-battery.sh is:

#!/bin/sh

BATTERY=BAT0

CAPACITY=$(cat /sys/class/power_supply/${BATTERY}/capacity)
STATUS=$(cat /sys/class/power_supply/${BATTERY}/status)
/usr/bin/logger -t auto-hibernate -p info Got event $STATUS ${CAPACITY}%
if [ $CAPACITY -le 6 ] && [ $STATUS = Discharging ]
then
    /usr/bin/logger -t auto-hibernate Hibernating due to low battery (${CAPACITY})
    /usr/bin/systemctl hibernate
fi

Test this with by restarting acpid and plugging in the ac adapter. You should see some output in /var/log/messages.

Thanks svenper!