I'd be surprised if it's a hardware issue although it's possible. It coincided with the release of 8.3. I can confirm a DFU restore didn't fix the issue. Will try a DFU to 8.2 and see if I'm back to where I started.
Yeah, I'm probably 98-99% confident it's not a hardware issue. Now I have been racking my brains as to when I think this problem came about. At first I was sure it was on iOS 8.1.1, but it can't have been!
I got my phone (with 8.0.2 installed) on the 13th Dec, and looking at the release dates of iOS online 8.1.2 was launched on the 9th Dec, so the first thing I would have done is update the OS to that.
My phone had great battery life until not too long ago. It turns out that iOS 8.1.3 was released in January, and I had no battery issues in February, so that rules out 8.1.3 as being the culprit. My problems started in March, and what happened to launch in March.....iOS 8.2!!
So looks like 8.2 started my battery woes. I originally thought it was 8.1.1 due to remembering great battery life on 8.0.2, but I never had issues that far back.
So 8.2 started my problems and 8.3 continues them. Fingers crossed for 8.4 then!
Also, I saw on Redit the other day that one user who had appalling battery life on 8.2 actually gained 2 hours usage and 6 hours standby time when 8.3 came out. So looks like I am not the only with with issues on 8.2, and from what you've said I'm also not the only one who has issues with 8.3.....
Oh, and with 8.3 not only is my Mail App buggy, so is the Health App. I opened it up today and all my data looked like it had gone. Closed the app properly, re-opened, still nothing. Had to turn the phone off, then on again for the data to come back.
If that's not signs that iOS 8.3 is buggy as hell (2 core apps, Mail & Health, are having problems), then I don't know what is.