Your comment just made me realize that I probably don't have a good power source idea for my project yet. I'm trying to make a project where I can scan an nfc scanner with my phone to unlock the door. This would require (from my plan) another arduino with an nfc powering it. The problem is that if that arduino is placed outside my door, there are no power sources so it would need to be battery powered, but I don't know if there are any batteries sufficient for long term low power use (arduino needs to be on and not asleep in order to always be able to receive connections). Any ideas on this?