system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does the battery on your laptop or phone degrade if you charge it too much or for too long?
gpt
Lithium Ion batteries rely on very small molecular structures to store charge, and these slowly become damaged. The more energy in the battery (both keeping it fully charged and keeping it warm) causes the structures to break down quicker. 

It's a completely different reaction, but it's a little like how iron rusts quicker if exposed to salt water. 

We don't actually know exactly why the structures break down, but research as worked out that to save battery capacity you should avoid keeping a battery fully changed and keep it cool, so storing batteries at 50% charge in a cool place will make them to last much longer.