Battery drains quickly what could be the cause?

I have had a similar problem with my iPhone 6. What worked for me was re-calibrating the battery. Now this may not work for you as there can of course be multiple problems, but is worth a shot.

1. Firstly charge up your phone fully, and once at 100% leave charging for an hour or so more.
2. Now unplug and use your phone like normal.
3. Take it all the way down to 0% and allow it to die.
4. Ideally you want to leave it overnight or for 4+ hours with the battery dead. If you cannot do this allow it to be dead for as long as you can.
5. Now recharge the phone fully and it should have recalibrate the phones battery.

I hope that this helps.

If battery re-calibration does not work, just replace the battery. Considering your phone's symptoms, it is the easiest, cheapest and most rational first choice repair option to do. The ifix it guide is perfect for the task.