system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How can cell phones and other electronic devices tell how much battery life they have left.

do batteries put off more energy when full?
gpt
Batteries are kind of like tanks filled with water attached to a faucet. When you turn on the faucet, the water comes out. You can tell how much water is left in the tank by measuring the force at which the water comes out. Same thing with batteries, except the water is electricity and the force is the voltage.