How Cell Phone Batteries Calculate Remaining Charge

[vivekrai]

I do not know to which section this question belongs correctly.

The question is :

How does our cellphone calculates or knows the percentage of battery left?

 

[Bobbywhy]

A typical cell phone battery is the lithium-ion type. It is fully charged when the battery open circuit voltage is around 4 volts. As energy is used the voltage drops off towards around 3.5 volts. This 0.5 volt change corresponds to going from fully charged 100% to around 20%. Somewhere just below 3.5 volts the unit will reach “low battery” and turn off automatically. The state-of-charge monitor circuit of the cell phone simply measures the battery voltage and indicates to the user by means of a display of “percent charge remaining”, often with a bar graph.

http://batteryuniversity.com/learn/article/lithium_based_batteries

 

[Borek]

More or less by measuring exact voltage and applying the http://en.wikipedia.org/wiki/Nernst_equation

 

[Topher925]

Voltage measurement (at least voltage measurement alone) is rarely used for determining Li battery SOC. This is due to modern Li batteries having relatively flat voltage curves as they are discharge along with the voltage being dependent upon battery temperature, health, and life, as well as SOC. The most common method used is coulomb counting where the current discharged from the battery is integrated over time starting from a full charge.

True SOC of a Li battery is actually a very very difficult thing to determine and is a huge engineering challenge for automotive and similar applications.