Solution need for Battery drain

If you want to avoid automatic updates for the future, you need to disable that option in Settings>General>Software Updates.

As far as your battery is concerned, how is your maximum capacity % in Settings>Battery>Battery Health (tapping there)?

Regards

Giulio

Setting the phone back to factory settings solved this issue for some users, that updated via Wi-Fi:

Restore your iPhone, iPad or iPod touch to factory settings

If you create an encrypted backup in iTunes before you start, your backup also contains Account- and Wi-Fi passwords.

Your battery has reached the critical point of it's life (and could have unpredictable behaviors).

Make a full backup of your iPhone (before the battery dies completely) and have your battery replaced at an Apple Store or Authorized Service Provider:-> Apple – Location Selector

Regards

Giulio

I also ran into this problem after updating to 13.3.1. The battery was dropping from 100% to 50% within a few hours. The battery health app showed various apps were draining such as Contacts or Maps. After a lengthy Apple Support conversation the conclusion was the dreaded Factory Reset.

Although I had a full B/U I opted to start fresh. I screenshot all my Apps for reference and did a fresh install.

Battery life has been Stellar. Actually better than when the phone was new.

I hated to do it it is time consuming but the outcome was well worth it.

I'm far from being a Specialist and did need help but Apple was extremely accommodating and tried a number of options via Screen Share.

Battery continues to hold a charge well into two days with minimal calls, quite a few texts, music playback.

I hope that helps you some. It really is nice to have a fresh phone.

Check this Apple page about it:

iPhone Battery Replacement - Official Apple Support

Do you have any idea how much a battery replacement would cost?

In US Apple Stores is $49 (Tax excluded, work included) for iPhone 7.

Check those prices in your country, if your'out of USA.

Regards

Giulio