A number of users are experiencing that updating to iOS 9 seems to have reduced the battery life of their iOS devices.
For instance, one user stated that:
“I installed iOS 9 on my iPhone 6 and the battery life is horrible. I hope they come out with a patch soon.”
Also note that battery drain issues are usually reported by users after major iOS updates, like iOS 9. Here is how to fix bad iOS 9 battery life:
If you think that your iPhone feels warmer than usual, this indicates that there are CPU activities that produce heat (and a drain to battery) when running demanding apps. A force reboot may get things back to normal. To to this, press and hold the power button and home button simultaneously until you see the Apple logo.
2-If this does not resolve your battery drain issue, reset all Settings by going to Settings > General > Reset > Reset All Settings.
Other tips to increase iOS 9 battery life:
1-Turn off Location Services. Location Services are very useful. But I only turn on Location Services when I need. Go to Settings > Privacy > Location Services. If you do not want to turn off Location Services entirely, you can also individually control which apps have access to Location Services. You may turn off Location Services for certain apps.
2- Disable background app refresh. Background App Refresh lets apps check for updates and content. Although this may be very useful, this will increase battery use. Simply tap Settings > General > Background App refresh and turn off Background App Refresh.
3-Disable Automatic Downloads. Go to Settings > App and iTunes Stores and turn off Automatic Downloads.
4-Turn off Push Notifications. Go to Settings > Notifications, and turn off any app you do not want to be notified. Simple select the apps that you do not care so you would like to turn off.
Also note that iOS 9 includes a new setting called Battery. Simple tap Settings > Battery. This section includes some useful information.