A battery is like a balloon. You charge it up like you’d fill a balloon with air. When it’s full, it’s at a higher pressure than before. Then when it’s being used, the air escapes and the pressure decreases over time. Eventually the balloon is empty of all its air and its pressure is at 0.
A battery uses voltage instead of pressure. You can measure the voltage and get a general idea of the remaining capacity. A fully charged lithium ion cell is at around 4.2V, and when fully discharged, is at around 3V.
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