The Galaxy S8 has a couple of battery saver modes you can use to squeeze out extra juice when you need it. You shouldn't use these modes all the time, but they can come in handy in a pinch.
- Go to Settings > Device maintenance
- Tap Battery
- Choose a battery saver mode
- Mid - decreases brightness, limits performance, reduces screen resolution
- Max - Limits apps, decreases brightness, limits performance, reduces screen resolution
- Tap Apply