@danamoako make sure that all of your settings are geared toward power savings mode. Turn off any Apps that are running in the background. If none of that makes a difference, replace the battery and re-evaluate. Since iFixit does not yet have a guide for this, use a video [https://www.youtube.com/watch?v=GF7YoiYgiEs|like this] to help you out. Batteries are readily available at many online stores. Check a vendor in your geographic region since shipping of batteries can be tricky.
While you work on your phone, take lots of pictures. Once you are done with it, take those pictures and create a guide for iFixit. That will help the next person that has the same problem as you do. It's easy and fun to create these guides. https://ifixit.com/Guide/new The iFixit community will appreciate this.