I am having the same problem as most of the people on this Topic, Originally my iPhone was on 9.3.3 and it was working beautifully until I noticed my battery life was draining so fast that I didn't do anything to change the setting or anything like that. I decided to turn off location and notification but that didn't make any difference. So I decided to update from 9.3.3(June 2017) to 10.2.1 but that didn't solve the problem, Often will shut down when I open a specific app like SnapChat when I had like 43 percent battery life. It won't turn on until i use the charger and it will finally turn on then jump back to 43 percent. This has happened all the time. So I went by UbreakIfix to have them replace the battery, that solved the problem but shortly after that, they messed up my BlueTooth, Wifi(2.4ghz), and GPS. None of it works right and UbreakIfix WON'T fix it what they broke So Do not go by New Port Richey, Florida location to do your repair. I have tried to back up and restore to factory setting and even updated it to 10.3.3, that didn't solve the problem.
My iphone was from ATT then switched to Verizon with the same phone and unlocked. Today, My wife is facing the same issue where hers will drain down quickly for no reason. She upgraded to 10.3.3. a few days ago.