iPhone 6s 100% batt But dies

Hello, I know this question has been asked before I could not find a post with a satisfying answer for my particular problem so hence this post. My iPhone 6s running on iOS 11.2.5 has since the update to this version been showing problems. The battery indicator always shows a full 100% charge. But the as earlier problems described here then suddenly dies. If I then plug it in the phone boots up and sometimes show again a full charge or sometimes the actually low percentage of charge. However even with the powercord plugged in the phone still dies. If I get lucky the phone charges fully the indicator does not drop till entirely dead. Before the update to 11.2.5 I had no issues of this kind. Sometimes even could work more then one day with one charge. Right now im doubting throwing the iPhone and irritating problem in the bin, or try to solve this issue. Im not used to being irritated by any apple product, Please help Anyone?