Cool battery shell script to get the battery level:
Make sure that script is in you path somewhere and then include this in your .tmux.conf
set -g status-right "#(battery Discharging)"
#(battery Discharging) is the important part that runs the command and swaps in the output. You can mix it in with other stuff or into a different part of your status bar.
I prefer to see the amount of battery and the time in the lower right corner, so here's mine:
set -g status-right "#(battery Discharging) | #[fg=cyan]%d %b %R"
Will show something like:
83% | 15 Nov 22:10