system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What happens to a battery when it reaches 100% charge and stays charging?
gpt
Dumb old batteries like lead acid and nickel cadmium can handle trickle charging indefinitely; excess charge will just turn into heat.  Trickle charging generally means a rate that will take at least ten hours to fully recharge the battery.  Anything charging faster than that has to have an automated system to work out when to stop.  Or else overheating and battery destruction with the potential for fire or even explosions.

Modern lithium based batteries are more finicky and over charging them, even at slow rates can cause fires.  Battery packs and/or the devices they go in contain intelligence to monitor the charging and have it stop at the appropriate time based on things like voltage, time spent charging, rates of charge and temperature.  Devices were long battery life is required, e.g., electric cars, might not ever fully charge or fully discharge the batteries.