Why does my iPhone XS battery drop by 15% overnight?
That can easily happen if you have a lot of things running. Even when the screen has gone blank with the iPhone in standby mode things are still running in the background.

Here are some things that can eat up battery power that you can change:
- Close all unused/unneeded Apps before you go to bed. Lots of Apps running in the background really eats through battery power.
- Turn off automatic refresh on Apps you don’t need it to be turned on. Leaving it turned on means your iPhone is constantly refreshing in the background and the result of this is it uses up battery power and data. Why do people buy the iPhone?
- Turn Bluetooth off when you are not using it. When it is on it is constantly looking for devices to pair with and uses a considerable amount of power.
- Turn off WIFI if you don’t need it one or if the signal is poor. If is constantly looking for WIFI networks to join and if the signal is poor your iPhone will be constantly at full power trying to make a connection which will eat through your battery fast.
- If your mobile carrier has poor reception at your home your iPhone will go onto full power to try and connect to the network and as a result of your iPhone constantly trying to connect at full power it will eat through your battery fast. Turn your phone to flight mode.
- Turn off location services if you don’t need it on. This helps have your battery.
- Your iPhone should have turned the screen brightness down at night. However, if you are happy with it being even darker turn it down. Screens use a lot of power. However, don’t turn it down so much you strain your eyes to look at it as that is bad for your health.
- Turn off “raise to wake.” Sometimes, when you pick the phone up when you don’t want to use it the screen will light up. This of course eats though more battery.
- Turn off “Hey Siri.” To respond to you saying “Hey Siri” your phone has to always be listening and for it to be listening it has to use a little bit of battery power to do so.
If you are happy to go without these features (personally I am not happy to go without some of them) you can save a lot of battery power. Hope this helps. iPhone XS
PS: This is not just true for iPhones either, it also applies to all Androids as well.
source : david