iOS 18.3.1 battery drain and heating issue

Your battery health is about right for 171 cycles.

If you just updated give it time for iOS to complete its update “housekeeping”, a day or two (less if you leave it plugged in)

if you fast charge your iPhone it may get very warm, and may pause charging at 80% and display a message saying that charging has paused, and will resume when the phone cools down. This is NOT overheating, and is normal when fast charging. It also won’t prevent you from using any app you have on your phone.

Some apps that use a lot of energy or that use cellular data rather than Wi-Fi can cause the phone to run warmer. Worst offenders are Facebook, interactive games, watching streaming videos, and the Camera app. And having VPN installed, whether using it or not, can also consume a lot of energy.

All of these are manifestations of the 2nd Law of Thermodynamics.