Why is my phone battery draining so fast?

@priyamdutta sounds like a bad battery. Replace the battery and then reevaluate. Since iFixit does not yet have a specific guide for that, use something like this video to change the battery.

While you work on your phone, take lots of pictures. Once you are done with it, take those pictures and create a guide for iFixit. That will help the next person that has a device with the same issues as you do. It's easy and fun to create these guides. https://ifixit.com/Guide/new The iFixit community will appreciate this.

Repair is War on Entropy!