system
Given the following post, respond as if you're commenting on Reddit:
human
Why do rechargeable batteries lose the ability to hold a charge over time?

When I bought my laptop, it had about 3 hours of battery life. Now it has about an hour. People say not to leave things charging, and to let them go completely dead before doing so. What is happening, chemically, when a rechargeable battery loses its potential for charge?
gpt
The primary reason batteries lose their charge, is gradual cell oxidation.  In lithium ion batteries (like those in most laptops), the charge is stored in the anode, and as you use the battery, lithium ions move through an electrolyte to the cathode.  When you charge your battery, this process is reversed.  Over time, the process is rather harsh on the anode and cathode, which become coarse, covered in patches of oxidation.

More in depth investigation in the last few years has also revealed that the anodes and cathodes which (in some Li-ion batteries) are covered in nano-filaments to increase efficiency, can see those nano-filaments get coarse and knobby, decreasing surface area exposed to the electrolyte, and increasing their resistance to lithium ions trying to transfer a charge.