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Hi Scott71,
Welcome to the community,
I'm sorry to hear the frustration, in many cases third party apps cause power issue, I'm by no means saying that this is the root cause of this issue, it is however one big factor.
You could attempt a software repair on the device, this at least would remove any software related issues which could be present.
Make sure to back up your device however, as you will lose content. Backup device
Regarding the warranty, I would first suggest you speak with Sony Mobile Customer Care, you may actually still be within warranty, Terms, conditions and timeframe differ from country to country.
Use my signature to find contact details, and let us know how you get on.
Check this out whenever you experience a significant battery drain
charge the phone 100% use it as you normally would then once the battery reaches 10%
settings > power management > battery > take screenshots of:
1. the list
2. tap on the graph and take a screenshot
3. back > tap on the top 4 items on the list > take a screenshot of each
post them all here or upload the screenshots to box or dropbox or google drive or etc and post the link