# Eli5 How does my phone know how much % battery is left?

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Eli5 How does my phone know how much % battery is left?

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Batteries behave differently depending on how much current they have, a discharged battery has less voltage than a charged one.

Batteries don’t put out a constant voltage. A fully charged battery has a higher output than a drained one. The power control circuitry measures that output voltage to guesstimate how much useful charge is left in the battery. I say “guesstimate” because that voltage depends on some other factors that you can’t perfectly account for. You can observe the battery through several full charge/discharge cycles to calibrate your measurements for more accurate guesstimations, but at the end of the day it’s not gonna be an exact figure.

Batteries lose strength as they lose charge. Think of it like a barrel filled with water with a small hole in the side of the bottom: when the barrel is full, the pressure of all the water will cause the stream to come out much faster. As the barrel empties, there’s less pressure on the water, so the stream becomes less and less forceful. So if we look at the stream of water, we can estimate roughly how full the barrel is, and we can be more accurate if we’ve filled it up and emptied it out a few times to get used to it a bit.

Imagine your battery to be a bucket of water with a spout at the bottom. The voltage of the battery is analogous to the pressure at the spout. The amount of water is analogous to the percentage charge of the battery. To measure how much water is left, we can look at the pressure at the spout (voltage) and the flow of water coming out (current). We might ask: how long can the bucket sustain the flow of water at a certain pressure, and by calculating this, knowing the flow and pressure, we can determine the percentage of battery left.

Primarily, they use a device called a [coulomb counter](https://learn.sparkfun.com/tutorials/ltc4150-coulomb-counter-hookup-guide/all) to measure how much electrical charge has flowed out of the battery. They also continuously measure the voltage on the battery, which is described by other posters with the “water pressure” analogy.