There are several ways to save power on the Galaxy S8, all of them conveniently inside the device management settings. You’ll find them under Settings > Device maintenance > Battery. There are “Mid” “Max” power saving modes, both of which can be modified with custom settings. Simply tap one hit the “Customize” link at the top. You can pick a screen resolution, brightness level, C limits, toggle background data on/off, so on. To activate the power saving mode, hit the Apply button instead of Customize. The Mid power saving mode can be accessed from the quick settings, too. There’s also an advanced section, accessible via the menu button in the upper right corner. There, you can tweak the app power monitor, increasing or decreasing length of time before an app goes to sleep (other Android devices call this app stby).