I often use my laptops as portable desktops: they are plugged into AC power and an external monitor/keyboard 95% of time. Unfortunately, continuous charging is hard on the battery. To mitigate this, ThinkPads have customizable start and stop charging thresholds, such that the battery will only start charging if its level falls below the start threshold, and it will stop charging as soon as it reaches the stop threshold. Suggested thresholds from Lenovo’s battery team can be found in this comment.

You can set these thresholds on Linux using tlp-stat(8), and you can make the values persist across reboots by setting START_CHARGE_THRESH_BAT0 and STOP_CHARGE_THERSH_BAT0 in /etc/tlp.conf.

I recently installed OpenBSD on my work ThinkPad, but struggled to find any information on how to set the thresholds under OpenBSD. After only finding a dead-end thread from 2021 on misc@, I started digging around on how to implement it myself. The acpithinkpad and acpibat drivers looked promising, and a bit of Google-fu lead me to the following small announcement in the OpenBSD 7.4 release notes:

New sysctl(2) nodes for battery management, hw.battery.charge*. Support them with acpithinkpad(4) and aplsmc(4).

Lo and behold, setting the start and stop threshold in OpenBSD is simply a matter of setting hw.battery.chargestart and hw.battery.chargestop with sysctl. The documentation was not committed in time for the 7.4 release, but you can read it in -CURRENT’s sysctl(2). I personally set the following values in /etc/sysctl.conf: