Why is my battery draining so quickly?

A couple of reasons might be why the phone is drained of battery quickly.

The main reason why is usually due to high consumption of battery power from apps with large processing power. In that case either delete apps that are normally not used or turn off apps such as GPS, Bluetooth, any cloud service as well as screen brightness.

You might also want to try using Power Save Mode to increase battery life. Power Save Mode should be located either in Settings, or even on the main screen.

Resetting your phone might help solve your problem using either a soft reset (pressing and holding the Power and Volume Down keys simultaneously for 15 seconds) or a hard reset, located on Settings (which will return your phone to factory default).

Another reason might be the charger and where the phone is being charged. Keep in mind that using your computer to charge your phone takes longer than a regular wall plug.

And last but not least, it may be the battery itself, in which case you may need to replace it.