After an update of ios 18 in iphone 11 Battery drains too faster as usual.

See information in If the battery in your iPhone drains too quickly - Apple Support including the following excerpt:

If battery life is less than expected after an update

If you notice that your battery life has decreased after updating your iPhone, wait a few days and then check again.

Even though you can use your iPhone immediately after an update, certain tasks related to the update continue in the background and might affect battery life and thermal performance.

This is primarily a user-to-user support forum; other Apple users like you.

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. By the way, you won’t receive a free iPhone for this.

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