ELI5, how do devices that use batteries, like phones, know the percentage of the charge remaining?

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ELI5, how do devices that use batteries, like phones, know the percentage of the charge remaining?

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Hi 🙂

TL;DR: A full battery has more volts than an empty one. So for example, 4.2v=100%, 3v=0%. But it’s not linear.

 

Voltage of a battery drops when discharged over time. Some phone apps (or programs for your laptop) will even let you see the voltage.

You probably have seen the 1.5 volt label on regular AA/Mignon alkaline batteries. It’s actually a little higher initially. Then it will gradually go to 1.4…1.2v. Once it reaches 1 volt or less (drop off happens more suddenly), many devices stop working. Battery testers or battery circuits measure this voltage (or any $3 multimeter can to some extend if you add a load).

Same with lithium batteries. Some may be labled 3.6V, but when full, they’re over 4v. Under 3v they would get damaged, so devices (or a circuit on the battery pack) will cut off.

This is the reason why old phone or laptop batteries will sometimes cause the device to just shut off at, say, 20%, as the old battery’s voltage will drop unexpectedly fast compared to a new one (EDIT2: And when the load is high).

So the percentage is just a “guesstimate” using the common discharge properties of a battery. Some smart programs will recognize old battery behavior though and compensate.

EDIT: [Example discharge curve of an alkaline battery](https://commons.wikimedia.org/wiki/File:AA_Alkaline_battery_energy_usage_-_discharge_current_100mA.svg)

the less charge a battery holds, the less voltage it outputs. the device just checks the voltage and determines the percent.

The battery pack or the android main board has a “battery fuel gauge” IC which keeps track of the charge state of battery. You cannot achieve that level of precision by only measuring voltage. Basically, it uses fuel gauge. There are several algorithms for Li-Ion chemistry, and the gauge might use one or more of them.

To be a little more more ELI5 than some of the other comments, a battery is like a water balloon. When the battery is fully charged, it’s like having a water balloon that is full to bursting. When you let the water out, it comes out harder and faster when the balloon is full than when the balloon is almost empty. The phone looks at how hard and fast the water is coming out to figure out how full the balloon is.

They measure the voltage. Think of batteries as tubes of angry pixies. Voltage is just how angry the pixies are. The more voltage, the angrier the pixies are. When the pixies get tired, the voltage drops. When the pixies get all tuckered out, the battery is dead.

Every battery has a known voltage when full. The device either computes a ratio to flat dead, or to “low enough” that the pixies can’t do any work for the device