There are several methods you can try to improve battery life after iOS 12 update:
# Check the battery consumption of each app
# Turn down the brightness of the screen
# Turn on Battery Saving Mode
# Turn off Emails
# Turn off the Bluetooth if you do not use it
# Disable “Hey Siri”
# Set iOS to turn off the screen before there is no activity
# …
Complete solution: [https://www.iossystemrecovery.com/how-to-improve-battery-life-of-iphone-ipad-in-ios-12/|How to Improve Battery Life of iPhone/iPad in iOS 12]