Battery Fully Dies Overnight

My iPhone 8 was at 82% battery health and would fully die from 100% to shut off dead in about 8 hours. The phone is not used once during this and the battery section does not show any apps running in the background. So, figured it was the battery and replaced it with an iFixit battery kit. To my surprise, it still dies overnight, and the battery graph shows the same steady but rapid decline to 0%. I am now worried there is a board issue. Any ideas?