It sounds like you've updated to iOS 11 and now your battery is draining faster than it did post
Maximizing Battery Life and Lifespan
Auto-brightness
Adjust the brightness on your iPhone, iPad, or iPod touch - Apple Support
In iOS 11, you can turn auto-brightness on or off in Settings > General > Accessibility > Display Accommodations. In iOS 10 and earlier, go to Settings > Display & Brightness.
Have a great day!