When the machine boots up, we should use the Balanced profile by default (unless the battery is low); we don't want a case where someone used High Performance mode the other day, turns their computer back on and loses their battery life. Or conversely where they turn their computer on and it feels super slow because they don't realize Battery Saver is on.
When switching to Battery Life, dim the display backlight a bit (let's try 50% of the current level, with a sane bottom threshold like 10%) and turn off the keyboard backlight by default, but allow the user to raise them back up. Switch to a more conservative processor profile.
Switching to Battery Life should dim by a relative amount (instead of to a set value) because:
- It will almost always reduce the backlight, which is a HUGE aspect of power draw,
- It communicates to the user that, "hey, turning this down a little will go a long way",
- It best respects the user's preference and current environment. If they have it at 100% due to being in the sunlight, 10% brightness is useless. But 50% will save a lot of power while also being more visible.
Balanced should not modify backlight settings at all, but should turn the processor tuning back to default.
Should be automatically selected when disconnecting from power, unless the battery is still low.
High Performance should not modify backlight settings at all, but should turn the processor tuning to the high performance mode.
The only time we should be touching the backlight is to bring it down to a more power efficient level when we switch into Battery Saver mode.
We shouldn't try to be smart about restoring backlight settings or anything—we don't know if the context has changed from when they last used the other profile, so changing the backlight would end up feeling unpredicatable in a lot of cases. Instead, we should always be relative to the current state, because it's the best signal we have regarding the user's preference in the current context without having an ambient light sensor.
We should never turn the backlight up automatically—if they needed a brighter screen in that environment, they can always turn it up. Then we don't want to change it on them again, because they've likely already adjusted as needed.