Poor standby battery life, even after battery replacement.

Hello

I recently switched to a second-hand iPhone 6, the seller told me about device's poor battery life, i was fine with that as i was going to replace the battery anyway.

However, even after the replacement, standby battery life is atrocious. The phone goes from 100% to 0% in around 8 hours.

Tried two different batteries (one of which was an original apple battery that came from another iphone) and the results are not different.

Things i've already tried

>checking battery stats (nothing out of usual)

>turning off all synchronization/background services

>soft and hard resets

>switching to iOS 10 beta

>restoring as a new iPhone

none of that worked, so i'm not really sure what else could be left

Surprisingly, i can pull off 3-4 hours of screen on time easily, even if the battery dies in 8 hours from standby drain. The new battery also reports cycle count, capacity and other stuff correctly when i examine it in ibackupbot.