Question: why do phone batteries run out after a certain amount of time?

Lynne Thomas answered on 20 Jun 2014:

Batteries are basically a way of storing electrical charge which can be discharged over a period of time to power a device like a mobile phone. Once the stored charge has been used up, the battery needs to be recharged so that it can do it again.

Inside the battery, it is a chemical process taking place where electrons are moved between different materials an anode and a cathode. The chemical process that is taking place is not just the one that is giving us the energy that we are wanting to use, there are also side reactions which take place. At first, you don’t notice that these are happening, but after a period of time, more and more side reactions will have happened and they destroy parts of the materials that we need to work as the battery. So batteries will always work less well (or less efficiently) after they have been used for a while.