Yes, you need to replace the hardware (S6) with something else.

It's a feature of the S6, making the small and non-replaceable battery slim enough so you can put it in a sturdy case, to protect the glass back.

O-0

 

Try this https://play.google.com/store/apps/details?id=nextapp.systempanel app to see what's going on. The app is discussed in the bloatware thread atop this S6 forum page; recommended reading. It might give you a clue as to what processes are running in your phone.

 

My S6 has been working great until a couple of weeks ago it started draining the battery overnight. It's really frustrating because I use the phone as an alarm for work and I've slept in twice in 2 weeks when the battery died.

This screenshot is what happened overnight. Luckily today is a public holiday and no worries about the alarm!

Any help? This is an almost brand new phone for me...surely this is unacceptable from Samsung?