Let me guess, are you using ios13 by any chance?
The problem is Apple & iOS 13 being an absolute waste of space.
Ive got an iPhone SE which is basically an iPhone 6 just smaller & I've had the battery go from 100% to empty in less than 40 minutes because of iOS 13. It never happened on iOS 12 & my battery health is 87% & is performing at peak capacity apparently, which if it was designed to help keep your hands & ears warm for very short times it works a treat.
Are you having other problems like emails not always syncing, rebooting, incorrect battery percentage before reboot etc?