Battery of iphone 13 is draining too fast after 16.4 update

If you're experiencing issues with the battery in your iPhone draining faster than expected I would first recommend updating to iOS 16.5 (the latest release at this date) to see if this may fix the issue. If this doesn't help the below documents outline a few of the best ways to troubleshoot the issue.

I have seen some reports that battery life on iOS 16.4 did degrade from 16.3 so this may be related to as software bug that will likely have been fixed in future updates if not in 16.5.

Batteries - Maximising Performance - Apple (AU)

Optimize your settings.

There are two simple ways you can preserve battery life — no matter how you use your device: adjust your screen brightness and use Wi‑Fi.

Dim the screen or turn on Auto-Brightness to extend battery life.

- To dim, open Control Center and drag the Brightness slider to the bottom.

- Auto-Brightness adjusts your screen to lighting conditions automatically. To activate it, go to Settings > Accessibility. Tap Display & Text Size, then turn on Auto-Brightness.

When you use your device to access data, a Wi‑Fi connection uses less power than a cellular network — so keep Wi‑Fi on at all times. To turn on Wi‑Fi, go to Settings > Wi‑Fi to access a Wi‑Fi network.

and

View Battery Usage information

With iOS, you can easily manage your device’s battery life, because you can see the proportion of your battery used by each app (unless the device is charging). To view your usage, go to Settings > Battery.

Here are the messages you may see listed below the apps you’ve been using:

Background Activity. This indicates that the battery was used by the app while it was in the background — that is, while you were using another app.

- To improve battery life, you can turn off the feature that allows apps to refresh in the background. Go to Settings > General > Background App Refresh and select Wi-Fi, Wi-Fi & Cellular Data, or Off to turn off Background App Refresh entirely.

- If the Mail app lists Background Activity, you can choose to fetch data manually or increase the fetch interval. Go to Settings > Accounts & Passwords > Fetch New Data.

Location and Background Location. This indicates that the app is using location services.

- You can optimize your battery life by turning off Location Services for the app. Turn off in Settings > Privacy > Location Services.

- In Location Services, you can see each app listed with its permission setting. Apps that recently used location services have an indicator next to the on/off switch.

Home & Lock Screen. This indicates that the Home screen or Lock screen was displayed on your device. For example, the display was awakened by pressing the Home button or by a notification.

- If an app frequently wakes your display with notifications, you can turn off push notifications for the app in Settings > Notifications. Tap the app and set Allow Notifications to Off.

No Cell Coverage and Low Signal. This indicates either that you are in a poor cell coverage area and your iOS device is searching for a better signal or that you’ve used your device in low-signal conditions, which has affected your battery life.

- You can optimize your battery life by turning on Airplane mode. Open Control Center and tap the Airplane mode icon. Note that you cannot make or receive calls while in Airplane mode.

If you need further assistance with the battery life issues you've noticed, you'll want to reach out to an Apple Support representative. Contact — Official Apple Support (AU)