Note 3 N9005 draining amazingly fast? Nothing helps?

[AC Question]

Hello everyone,

I've bought a Note 3 N9005 LTE from a close friend about 4 days ago. He owned the device for approximately a year before moving on. The device itself is in good condition and it works fine except for the battery. I'm talking about a drain of +/- 30-60% an hour. I've red countless forums yet none of them really solved the problem. Here's a quick summary of what I've tried so far:

He rooted the phone for me and that's where it all began. I downgraded back to Kitkat (UK version) but it didn't help. No matter which apps I try (Greenify, Battery saver, .. ) the battery status shows 1 hour or less in the remaining usage time (Battery status).

I've tried countless other ROM's without succes either. He bought me another official Samsung battery and flashed the "Smokkierom lite v5" ROM on it which solved the problem partially. It shows 5-7 hours in the estimated usage time but my battery is still draining like hell.

Is there any (common) solution to this problem? Do I need to "calibrate" my phone? Is my newly acquired phone ready for the trash can?

Thank you for your time.