Should I just replace my battery

So recently, my phones battery has been lasting less and less throughout the day, so I have to constantly charge it-even after 20 of a full charge. This morning I unplugged my phone at 100% and went downstairs. After a good 5 minutes of being off the charger, it turned off, then showed the battery symbol. I left it on the charger all day, and it still shows the dead battery symbol. Should I just replace the battery? Or is this a system issue? What can the apple store do to fix this(preferably me not spending money but thats not how life works) Thank you!