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This feels like a stack exchange EE question, but it seems like it belongs here instead.

How is the charge of a notebook computer's battery reported to the operating system? Since all laptop batteries I've seen seem to be cross-platform, there must be some standard procedure to inform the kernel of the charge level.

The best I can think of is that some microcontroller has to maintain a serial connection and inform the OS of the battery's voltage, but I can't figure out how this applies to determining things like whether the AC is plugged in or how this information is reported to the OS.

This seems like such a basic problem, but Googling it just led me to consumers trying to increase their battery life.

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Here's a page with good info: How laptop batteries work –  TFM Jan 21 '13 at 18:57
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up vote 2 down vote accepted

The specification you are referring to is the Smart Battery Specification, which reports the data to the System Management Bus. The SMBus is responsible for a whole host of things in addition to battery information such as temperatures, fan speed, etc.

There was a BlackHat presentation (link is PDF) that goes into the real details of how this system reports data, and how you can hack it for fun.

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