Emacs can show the status of your system battery in the ModeLine.

    UUU:@**--F1  *scratch*   All L5     (Lisp Interaction)--[100.0%]---

Interactively, `M-x display-battery-mode' or put ##(display-battery-mode 1)##
in your InitFile.

Available in Emacs 22 and later.  Supports GNU/Linux APM, ACPI, Mac OS
X 10.3.x, FreeBSD and OpenBSD.

See also ModeLineConfiguration.

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CategoryDisplay
CategoryModeLine