Iphone battery draining completely in less than 1h

Specs: iPhone 6 64GB refurbished iOS 12.3.1

Hi guys,

I have a serious problem with my IPhone 6 battery. It drains so fast (2% per minute) even my 4 years old iPhone 5S has better battery performance...

I bought it refurbished from an authorized dealer. It behaved quite well when I got it, charging normally, until I installed the 12.3.1 update and I think that’s when it started behaving abnormally.

First it went from 94% to shutting down and restarting with 12% only. Performance management was automatically applied by OS after that. Still, it charges and drains tremendously fast.

I have changed every possible setting to help, screen lock after 30s, auto brightness on, everything set on fetch, background app refresh off, location services off, using WiFi only, Bluetooth is off and Low Power Mode on, I restored factory settings, did a hard reset, I let it drain completely and charged it back to 100%, etc, etc... You got it nothing worked. Even when the phone is turned off the battery drains ridiculously.

Battery health is going down two like crazy in a matter of hours...

Has it happened to you? Is it the OS or does the battery needs to be changed?