iPhone 6 battery draining from 100% to 0% overnight

My iphone has just in the last week started to drain overnight to 0%. I have done all the suggested things, turn off location services, turn off background app refresh etc but it’s still happening. I downloaded IOS 12.1.3 but that hasn’t helped either. Maximum capita for battery is 82% so don’t think that ps the problem as it’s been that for a few months now. Can see previous posts for same issue but they all date back to 2018 and different version of iOS. But I have done everything suggested in those posts. HELP please