system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:Why do rechargeable batteries get weaker over time? What's "wearing out"?
gpt
The rough idea of how a battery works is that there's two chemicals. They want to merge, creating one waste chemical + electricity.

A rechargeable battery uses chemicals that can go in reverse. If the rechargeable battery has a ton of waste chemical, you can put in electricity and then it will un-merge and make the two original chemicals.

Thing is, processes like these are like any other process in life - they are not perfect. Instead of converting at 100% perfection, it might convert at 99.999%, which means that *tiny* bit of the conversion goes elsewhere - probably a separate unusable chemical, or some of the energy is wasted as heat.

Eventually both the original chemicals and the waste chemicals turn into enough crappy unusable chemicals and everything just clutters up.