# How is state-of-charge measured for Li-ion batteries?

**URL:** <https://boards.straightdope.com/t/how-is-state-of-charge-measured-for-li-ion-batteries/806995>\
**Category:** Factual Questions\
**Created:** [January 19, 2018, 8:27pm UTC](https://boards.straightdope.com/t/how-is-state-of-charge-measured-for-li-ion-batteries/806995 "2018-01-19T20:27:42Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\
**Post date:** [January 19, 2018, 8:27pm UTC](https://boards.straightdope.com/t/how-is-state-of-charge-measured-for-li-ion-batteries/806995/1 "2018-01-19T20:27:42Z")

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My 2-year-old cell phone is going nutty with regard to battery charge level. the reported state of charge sometimes drops suddenly from well over 50% to 1%. That’s when I catch it, anyway. If I don’t catch it, then the phone will shut itself down, presumably because it thinks the battery is completely empty. When I start it up again, it’ll report a reasonable state of charge, something like what it should be for that time of day.

So what’s happening in there? How is the state of charge measured on a Li-ion battery in the first place? Why might it be getting flaky?

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**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\
**Post date:** [January 19, 2018, 8:42pm UTC](https://boards.straightdope.com/t/how-is-state-of-charge-measured-for-li-ion-batteries/806995/2 "2018-01-19T20:42:48Z")

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It depends on the battery management chip.  
Most modern ICs do “coulomb counting” - they measure the charge and discharge power and figure out the power remaining in the pack from that. But, if the battery is going bad, the IC may do a bad job of estimating the charge remaining.

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**Author:** ![rat\_avatar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rat_avatar/32/255_2.png) [@rat\_avatar](https://boards.straightdope.com/u/rat_avatar)\
**Post date:** [January 19, 2018, 9:00pm UTC](https://boards.straightdope.com/t/how-is-state-of-charge-measured-for-li-ion-batteries/806995/3 "2018-01-19T21:00:19Z")

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The state of charge is also calculated based on the “voltage discharge curve”. If it is dropping off quickly that is typically a sign of old age and that the build in functions that calculate the capacity are no longer valid.

figure 2 in this link will show how most basic terms are calculated and it can be a simple mapping of voltage to percentage charge in many cases.

> **[BU-204: How do Lithium Batteries Work?](https://batteryuniversity.com/article/bu-204-how-do-lithium-batteries-work)**
>
> Learn about lithium-ion batteries and their different types. They have high energy density, relatively low self-discharge but they also have limitations.