system
Given the following post, respond as if you're commenting on Reddit:
human
[ELI5] How does charging a smartphone work? what happens after it reaches 100% and left plugged in, say, for 8 hours?
gpt
In all batteries, there is a chemical reaction that causes electrons to flow from one terminal to the other.  When batteries are recharged, electrons are forced to flow through the battery in the opposite direction, which reverses the reaction so that it can happen again.

Smart phones use lithium ion batteries.  Unlike some older types of batteries, these batteries can explode if they are overcharged.  Because of this, phones have a built in circuit to prevent overcharging.  When the battery becomes fully charged, the circuit will automatically stop the battery from charging.  It is safe to leave your phone plugged in for 8+ hours thanks to that circuit.