Extend Your Battery Life By Using Your Screen With Low Brightness
Everytime your iPhone is in use, it uses a lot of battery to keep the screen lit. The higher the brightness, the higher would be the battery drainage. Turning down the brightness of the screen helps in extending the battery life.
The brightness of the screen can be adjusted directly from the control center. Simply swipe down the brightness slider to dim the screen light.
Another way to enhance the battery life is to reduce the auto-lock time. This will ensure that your screen is turned off within a few seconds of non activity. To do this, go to Settings>>Display and Brightness>>Auto-Lock. From here you can adjust when your screen is turned off when not in use.
Auto Brightness
Your iPhone is designed smartly to automatically adjust the brightness of the screen according to your ambient light. By default, this setting is activated to ensure ease of use and battery saving. To disable auto brightness adjustment, go to Settings>>Accessibility>>Display and Text size. From here, you can directly disable auto brightness.