Is that what was wrong with your first Plus? Just wondering how i should bring this up this Tuesday for my Genius Bar appt since i already tried last week but they wouldnt replace due to their diagnostics tests showing my battery to be in the 100% "green level"
The diagnostic test will show that your battery health to be in green level. It's matter of what may be caused that drains the battery.
Couple weeks back, I had this similar issue where battery would drain and phone felt warm even though I have not even touched my phone. So that night I did a factory reset and set it up as new. This of course fixed my drain issue and went back to normal.
However, I did go to the genius bar for a different reason, noticing a dark band on my 6 Plus. But I also mentioned to them that I experienced a battery drainage before the factory reset and wanted to make sure that my battery was fine.
Even with my brand new phone, I noticed battery drain and ran the diagnostic, and Apple told me that there were two apps that gave me the jetsam (low memory) issue so it seemed like those two apps that I check was somehow running and not freezing itself when I exit the app.
Go to Settings > Privacy > Diagnostics and Usage and check what you are getting.
Also go to Settings > General > Usage and see what is using up most of the battery.
For me Angry Birds Transformers was number 1 issue so I ended up deleting the app and so far my phone is back to normal.
Also I guess it depends on who you get for the genius, but most likely they may suggest you do a factory restore and set it up as new and see if that will fix any issue. If that doesn't that it is possibility of a hardware failure that may cause the battery to drain.
