system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does the battery in my phone that I've had for 2 years not hold a charge as well as it did when I first got it?
gpt
Batteries degrade over time.  The interior parts of the battery are subject to fairly intense chemical reactions...you drive it one way when you charge, and the other way when you use the device.  The chemicals and parts in the battery are not perfect, and eventually impurities cause a tiny failure. This begins to increase over time...slowly at first, but eventually it becomes significant.