A battery is more or less fancy a capacitor. They don’t (dis)charge linearly but geometrical or exponentially (can’t remember). This is the reason why some devices can ‘quick charge’ for 15 minutes to get to 70-80% but take an hour to reach 100%.

Fun fact, capacitors can actually never charge to 100%. If you looked at a graph of charge over time the graph flattens out and approaches 100% but only reaches it after an infinite amount of time.

Two things are happening. First, it is not easy to tell how charged a battery is. The only thing you can really tell from a battery is the voltage. From that, and previous study of the battery, software in the phone and the battery can estimate the remaining charge.

Secondly, as the charge in the battery gets low, the phone switches of bits of functionality to make it last longer.

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