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

Question

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

In: Technology

Answer 1

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)