Why do batteries run out?
Short answer
Batteries make electricity using changes in the materials inside them. As a battery is used, these reactions make it harder to supply electricity. Charging can drive the reactions in a rechargeable battery in the opposite direction, so it can be used again.
How it works
In a disposable battery, the chemical reaction (a change in materials) is not designed to be reversed by charging. In a rechargeable battery, charging can drive the reaction in the opposite direction and restore much of its ability to supply electricity. The change is not fully reversible, though, so repeated use and charging gradually reduce performance.
How long a battery lasts depends on the device's power use, temperature, and battery type. Some batteries also slowly lose charge while stored. This is called self-discharge, and its rate differs by battery type.