iPhone 6s+ Battery discharges when turned off

Hello…Here are the facts:

Iphone 6s+ brought in because of no power

I installed new battery.

Phone powered on and showed positive charging state

turned off phone

next day, phone battery is dead.

plugged in and let charge all day. no charge, no plugged in icon on the screen.

removed new battery, charged with Kaisai external charger to 100%

put battery back in phone. powers on, 100%, showed charging after i let it

discharge a bit

Turned off phone.

No power today.

What do you all think? (it also seems to be warmer to the touch than normal)