An update on What is causing the iOS11 battery drain problems.
The problem did not clear up. I’ve been in contact with Apple support. They had me do a complete factory reset on my iPhone. They accessed it remotely and did some battery tests. The hardware is fine. The battery dropped from 100% to 75% while I was on the phone to Apple support! I have every possible feature turned off, no mobile data, no Location Services, no Bluetooth, no Background App refresh etc. I now am waiting a few hours for the phone to be able to be able to collect some usage data and then Apple tech support will call me back. I’ll keep you updated.
Another Update: The factory reset did not work. Apple remotely ran some more diagnostics on my phone. They did a test where I had to let the phone battery drain for 4 hours without using any apps in airplane mode. Then Apple got me to download a diagnostic app to my computer which downloaded a 350Mb file from my iPhone and these results were sent to Apple engineers who are analysing them. Apple will call me back soon.
Yet Another Update: Apple need to do more tests. I took my phone into an Apple repairer today.
Final Update: Apple Sent me a new iPhone. So there’s a major problem with iOS11 that can cause an iPhone do die such that Apple need to replace it to fix the problem. If you have an iPhone SE or 6 that’s battery life is problematic since iOS11 and it’s under warranty I’d suggest you take it back.