To begin with, are you sure your phone is charging correctly? I would check to make sure that your charging cord is working. Similarly, if you find that the charging cord is working, then you might be running on Performance Mode. To fix this, just simply change the mode in the battery settings to either Smart-saving or Ultra-saving (depending on your preferences). Ultimately, if this doesn't work, you may have a faulty battery. If you need help replacing the battery, check out the replacement guide on the page.