If you are planning to store your device for a long period of time, the best thing is to partially charge the battery and then store it. By storing in the discharged state, you may face problems with recharging. Again, storage of a battery in completely charged state can reduce its life span. https://goo.gl/ybNwx4
A film type capacitor made from this simple approach would create a large component. To prevent the waste of space the plates are “rolled” together, with an extra layer of dielectric to prevent the plates from touching, which results in a higher capacitance and smaller size. https://goo.gl/niJDtR
