I have an unusual battery life on my note 4 (n910c)

[AC Question]

When I first bought the phone the battery life was excellent,but now it's so bad that it takes 4 Hrs or 5 max on medium usage ( Youtube , browsing , reddit etc).I have Googled a couple of times of how to increase battery life but non has worked.And usually when my battery is at 20% or 30% it goes down very fast and when I reach to the charger and plug it in it goes to 0%!! I did everything to reduce battery usage (Doze, power savving mode *always*greenify , black wallpaper , GPS and Bluetooth and location off etc) Heck I even use WiFi mostly as it uses less power than 4G.And one thing is see that uses my battery usage s what's app and android OS *strange*!! I even calibrated my battery when my phone was rooted a while back but now it's formatted and still the same..