Samsung Galaxy Note 4 — Note 4 battery problems

Hi,

My T-mobile note 4 wasn't charging properly (not charging fully, dying at 20-30%), so they sent me a new handset under warranty. First 2 charges worked great, charged fully, but recently, the same problem has arisen. My phone went straight from 18% to dead, and then when I tried to charge it back up, it levelled out at around 73% after 8 hours of charging. It is a brand new phone, charger, and battery. I don't know what to try, I have cleared system cache, uninstalled apps, factory reset, tried to calibrate battery, but the problem persists. Any ideas? Thank you.