Batmon is, as stated by the title, a really simple battery monitor. It is developed mainly with an x220, Arch Linux and FVWM.
To compile batmon you need qmake
Go in the project root and run the following commands:
$ qmake
$ make
Then, launching the batmon
command, you should see a battery icon in
your traybar.
An example configuration file is provided with the sources.