Battery drains heavily without using the mobile while connected mobile data only

If your iPhone's battery is draining even when it's not in use, there could be a few possible reasons for this issue. Here are some steps you can take to troubleshoot the problem:

1. Go to Settings > Battery and see which apps or processes are consuming the most battery. If you notice any particular app using an unusually high amount of battery in the background, it might be the cause of the drain. Check the iPhone battery health and usage - Apple Support (IN)
2. Some apps refresh their content in the background, which can consume battery. You can disable background app refresh for specific apps by going to Settings > General > Background App Refresh and toggle off the apps you don't need to update in the background. go to Settings > General > Background App Refresh and turn on Background App Refresh. -->Switch apps on your iPhone, iPad, or iPod touch - Apple Support
3. If you have multiple email accounts set up on your iPhone and use the push email feature, it can drain your battery. Instead, consider changing your email settings to fetch data manually or at longer intervals. Go to Settings > Mail > Accounts > Fetch New Data and adjust the settings accordingly.
   1. Push Mail: Whenever possible the email client will push mails to all your devices.
   2. Fetch Mail: You need to set How you want it to be fetched, Automatically, Manually , Hourly,Every 30 mins, every 15 mins
   3. Automatically: will fetch when the iPhone is powered and on WiFi
   4. Manually: with fetch when you open email, open Inbox and swipe down
4. You get an email notification as soon as the client server receives an email and your device is connected to internet, irrespective of whether the mail is in your device's Inbox or not
5. Some apps constantly track your location in the background, which can use significant battery power. You can manage and restrict the location access for apps by going to Settings > Privacy > Location Services and adjust the settings for individual apps.
6. Keep an eye on Wi-Fi and Bluetooth settings. Sometimes, if Wi-Fi or Bluetooth is left on even when not in use, it can contribute to battery drain. Make sure to turn off these features when you don't need them.
7. Sometimes, a simple restart can resolve temporary software glitches. Turn off your iPhone, wait for a few seconds, and then turn it back on.
   1. Restart the device. That's it.
   2. Still not solved --> Force restart iPhone - Apple Support
8. If the issue persists, you can try resetting all settings on your iPhone. This will not erase your data but will reset all personalized settings to their default values. Go to Settings > General > Transfer & Reset > Reset > Reset All Settings and confirm the action. Note that you will need to reconfigure your settings afterward. All settings—including network settings, the keyboard dictionary, the Home screen layout, location settings, privacy settings, and Apple Pay cards—are removed or reset to their defaults. No data or media are deleted.
   1. Reset All Settings: Return iPhone settings to their defaults - Apple Support:- Return iPhone settings to their defaults
   2. You can return settings to their defaults without erasing your content.
   3. If you want to save your settings, back up your iPhone before returning them to their defaults. For example, if you’re trying to solve a problem but returning settings to their defaults doesn’t help, you might want to restore your previous settings from a backup.
9. Make sure you have the latest updates installed.
   1. Update Over-the-Air (OTA) —> Update your iPhone or iPad - Apple Support (IN)
   2. Update using iTunes/Finder —> Update your iPhone, iPad, or iPod touch - Apple Support (IN)

If none of the above steps resolve the issue, it is recommended to reach out to Apple Support or visit an authorized service center for further assistance. They can provide specific guidance based on your device model and iOS version.

User Tip to test for self-discharge, a common ailment in older batteries

Check your iPhone battery for Self-Discha… - Apple Community 

The measure of battery health deterioration/drainage is subjective and can vary depending on individual usage patterns. There is no universal standard to determine whether the battery health deterioration/drainage is normal or abnormal because it heavily relies on how the battery is used and maintained.

Protect & Prolong the Life of your iPhone's Battery:

1. About the battery usage on your iPhone, iPad, and iPod touch ...
2. Batteries - Maximising Performance - Apple (IN)
3. iPhone Battery and Performance