Policy
Reduce display brightness (on battery)
This policy setting allows you to specify the period of inactivity before Windows automatically reduces brightness of the display. If you enable this policy setting, you must provide a value, in seconds, indicating how much idle time should elapse before Windows automatically reduces the brightness of the display. Windows will only reduce the brightness of the primary display integrated into the computer. If you disable or do not configure this policy setting, users control this setting.
677d07e7e8ef DisplayDimTimeOutDC Registry
Copy registry mappings
HKLM\Software\Policies\Microsoft\Power\PowerSettings\17aaa29b-8b43-4b94-aafe-35f64daaf1ee\DCSettingIndex Policy notes
This policy setting allows you to specify the period of inactivity before Windows automatically reduces brightness of the display. If you enable this policy setting, you must provide a value, in seconds, indicating how much idle time should elapse before Windows automatically reduces the brightness of the display. Windows will only reduce the brightness of the primary display integrated into the computer. If you disable or do not configure this policy setting, users control this setting.