Never lose your credit card again!
A project we did a few years ago that incorporated a bunch of different ways to power a small battery without having to ever charge it manually.
Here's the cool thing about "Power Harvesting" for low power devices. They can get energy from a lot of different sources to keep them charged up. This one gets it from your cell phone (NFC and Cellular) , the chip reader, (EMV harvesting) , available wireless signals (Bluetooth and Wifi Harvesting). All these little sources can keep this small device working worry free for a very long time.
You never have to actively charge it, and it will use available power sources to keep the battery running theoretically "forever".