My iPhone 15 Pro Max battery is draining very fast after update to iOS 18

Wait a few days, or leave the phone plugged in overnight. A lot of apps restructure their databases after the update, Photos being the most active.

After an iOS update, it's common to experience an increased battery drain as certain tasks related to the update continue in the background including indexing, you should only experience this increased battery drain for a few days before it goes back to normal.

⇢ If the battery in your iPhone drains too quickly - Apple Support