How about this battery / energy storage device? It will charge and discharge an infinite number of times. Has infinite shelf life. Infinite charge cycles. The charge motor and discharge motor can be the same device to save space.
You can even tell how much charge is left.
Click on picture to make it bigger
https://drive.google.com/file/d/1Vn-DfkEl-ZEwygzx9DliNxPuEZ-EGLla/view?usp=drivesdk [ Guests cannot view attachments ]