My iPhone 6 is having the same issue. Started when I updated to 10.2. At first it would jump between different percentages so I never knew what it was really at until I plugged it in. It could die at 90% or at 10% I never really knew. I also noticed this happening more often when I used the camera. My phone would die instantly when I would try to take a picture.
After a few weeks of that it then started to die faster. The percentage was always accurate at this point, but I could be at 100% one minute and 15 minutes later I'd be at 40%. That's with no apps running in the background and just having the phone sit there with the screen off playing music.
Yesterday it started draining even faster. It was draining 1% about every minute, I timed it. Now today it's even faster than that.
My phone is impossible to use unless it's being charged. I've tried everything that has been suggested besides getting a new battery. I'm not contacting Apple as every time I've had an issue with a product in the past they would just tell me there's nothing they can do and to buy a new phone/computer. Because apparently everyone has thousands of dollars to throw out every few months when their device starts acting up.
I'm really considering going over to an Android. This to me is just ridiculous. My phone isn't even two years old. I purchased it spring of 2015. It's February 2017, there's no reason for this to be happening. Completely unreal. I guess I shouldn't expect much more from a clearly money hungry company.
If anyone ever finds a solution to this please let me know. I really don't feel like throwing out $100 on a battery that can't even be guaranteed to fix the problem. If I have to spend that much I might as well just save up the extra cash and buy a Samsung.