Thursday, May 31, 2012

Introduction to ACPI Control Method Battery (1)

All laptop PCs come with a battery. However, how does an operating system get information from a battery pack?

First of all, it must be understood that a battery pack can power a computer whether an OS is aware of its presence. All information past to an OS and shown to users is reported.

ACPI defines two types of battery interfaces.
  1. Smart Battery Subsystem Interface
    • Controlled by OS directly through EC
    • Solution is expensive
  2. Control Method Battery Interface
    • Reported by BIOS
    • Solution is inexpensive
It would not be surprising to see most of laptop PCs use Control Method Battery interface.

Battery Capacity and Battery Life

When a battery pack is produced, it has a "design capacity" that is the maximum capacity of the battery pack in theory, and "last full charged capacity" that is the actual maximum capacity. At the initial state, it can be assume that "design capacity" equals "last full charged capacity".
However, the last full charged capacity ages over time, and therefore the battery life will also be decreased.

Battery life can be calculated as

  • Remaining Battery Percentage [%] =

  •  Remaining Battery Life [h] = 

In operating systems, remaining battery percentage is also calculated by (Remaining Capacity / Design Capacity). In this case, it is very soon that you will not see remaining battery percentage equal 100%. Technical speaking, this is not incorrect but often this will be reported as a bug. In my opinions, this depends on interpretation but I prefer (Remaining Capacity / Last Full Charaged Capacity).

(To be continued...)

No comments:

Post a Comment