I live in rural area where I usually have 0 or 1 bars, but in the last 5 years since I moved there, I never had a problem that the battery on the phone dies overnight if it's not connected to the charger, but it happened twice this week. I checked the Battery section in the Settings and "No Cell Coverage" uses 57% of the battery in last 24 hours. I suspect this is related to the latest iOS 14.6, because, as I said, it never happened before. I use iPhone 12 Pro Max.