coconutBattery indicates iPhone 6 problem
The issue here is that coconut battery shows that the design capacity is 45% and that app is probably correct.

This means that the battery is dying and needs to be replaced. 

This can be done by Apple or a third party. 

This question happened to be the first I read  on the way home after getting my battery replaced at an Apple Store. They need the phone for 2-3 hours. So book an appointment on line or as I did turn up and see if they have a spare slot, they will message you when they do (in my case after an hour). It cost £79.