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Call Apple's technical support
If you use an Apple mobile device (iPhone, iPad, iPod), contact their technical support on 1-800-275-2273, 24/7
They can determine what is draining your device's battery even if you don't take it to one of their stores. They will ask you to connect your device to a computer via iTunes and through a utility they can inform you about the actual condition of the battery and will tell you what to do next.
One iPhone service that requires Bluetooth is AirDrop. AirDrop allows you to transfer photos and other files to and from nearby iPhones with the same feature (AirDrop) switched on. Unfortunately, it drains your battery because of the way it seeks out nearby iPhones to connect to.
You can switch off AirDrop in the Control Center (From the Home screen, select and hold the control center ->swipe upwards from the bottom of the screen -> tap AirDrop -> Select OFF). Use AirDrop only when need it.
Siri is a built-in intelligent assistant that enables users of iPhone 4S and later verions and users of newer iPad and iPod Touch devices to speak natural-language voice commands in order to operate the mobile device and its applications.
Even though it is very useful you should disable it if you don't use it, because the sensor which detects that you are holding your phone next to your face and that makes Siri listen automatically, is active whenever your phone is unlocked.
To turn Siri off:
Settings -> General -> Siri -> Make sure that Raise to Speak isn't switched on.
Before iOS 7, if you switched between apps by double tapping the home button, the old application (the one you used before) would be frozen, with limited access to system resources.
iOS 7 allowed background applications to periodically refresh their data, and so does iOS 8, so that when you open the application again, you’ll see the latest updates immediately. This can be very useful in certain circumstances, but most of the time is just a waste of battery power to update applications that you don't really need at the moment.
If you want to get the most out of your battery, turning off Background App Refresh should help.
Go to Settings -> General -> Background App Refresh -> choose which applications you want to turn off or you can turn off all of them at the same time (By moving the turn off button in OFF position).