Battery draining, phone heating after ios11 upgrade

The problem with my iPhone started since I upgraded the software to version 11.0.1. Before the upgrade, I had to charge the device once in one and half days but just after I upgraded the software, the device is not holding charge for more than 3/3.5 hours and the device is heating up. The phone is constantly warm to touch, even when it is not in use, the screen is off and it is kept aside.

For the past four weeks I have been in touch with Apple senior advisors (telephonic-ally) and have tried multiple things as recommended by them.

• They have performed a diagnostic and informed me that the battery condition is in stage 1 and there is no other hardware issue. The problem is with the ios.
• I have restored the devise to factory settings and used it without restoring backup for two days. Still the battery completely drained in 4 hours.
• I upgraded to version 11.0.3 (which was the latest available version at that time). Still the problem was not resolved.
• As advised by Apple, I placed the device in flight mode and took screen shots for battery condition. In airplane mode and standby condition, the battery completely drained in 6 hours.
• I collected logs and screen shots and shared with Apple.
• I was advised to upgrade to version 11.1, which I did. Still the problem persists.
• I have again collected logs and screen shots and shared on 2^nd November 2017. Yet to hear any update on it.
• I have tried the service center also. Apparently they can help out with hardware issues only and not software issues.

Since the device is not holding charge for more than a few hours. I have to constantly keep it attached to a power source. It is no better than a landline phone. Moreover, there is a lot of delay in opening even contacts and messages. I have wasted enough time and effort talking to Apple advisors and trying out ‘this and that’ as advised by them. I am tired and frustrated now. Every time I call them they ask for couple of more days to resolve the issue. I am afraid that Apple is only procrastinating the case and have no intention of resolving.

What do I do now? Who can I seek help from? Have I wasted my money by buying an Apple device?