HOW DO SMARTPHONES KNOW THE AMOUNT OF CHARGE LEFT IN THEIR BATTERIES?

This is actually a complex problem. The job is typically delegated to a device called a coulomb counter. This is built into the battery management system in the phone.

The base principle is just to count how much energy is going in and out of the battery to guess how much is left. It’s pretty much like putting an ammeter (a tool that measures electrical flow) on the connection to the battery. It’s hard to do this without wasting power, and it’s even harder to predict how a battery will discharge over a variety of circumstances. Extra heat, higher discharge, and old age will all waste power inside the battery in ways that the coulomb counter can’t easily measure.

So, in addition to just measuring the flow, a coulomb counter watches the battery’s voltage (the total electrical “pressure”) to take a guess as to how much power is left in the battery and learn its capacity as it ages. This is complex too, because chemical reactions hold the energy, and that voltage is just a side-effect that suggests maybe how much energy is left in the battery. The voltage fluctuates a lot depending on other factors, such as discharge rate (sometimes call the “C” rate).

And last, a battery that has been damaged from deep discharge (sitting around while dead), excess heat, short-circuit and overcharging will discharge in a very different way that is also hard to predict. (Note: cell phones protect against overcharging and short-circuit, so don’t worry about these!)

Thank you for reading, hope you enjoyed.