Phone is draining even after battery replacment

My Iphone 6 was functioning properly battery wise on my first year of usage, after i updated to IOS 9.2 the problem started showing, where i left it on 100% off charge one night and woke up the next day 8 hours later with 20% .

Methods tried

I followed and contacted apple in which they tell you what to switch and what not to switch in order to maximise your batter life (It only wasted my time and DID NOT WORK).

Disabling settings (DID NOT WORK)

Deleting certain apps (DID NOT WORK)

Setting up the phone as new (DID NOT WORK)

They advised me to replace my battery and i did (Again it DID NOT WORK).

My battery during usage only lasts to 3 - 4 hours max

And dies when it is on standby mode in approximately 7-8 hours without using the phone.

Kindly if your phone has started battery draining after updating into a certain IOS HELP A BROTHER UP ! :(