I leveraged my current battery state/level script to color the window border(s) accordingly. Also customized rainbow to pulse red when at critical level. All colors used for borders are sourced from xrdb.

• bbdr for active border colors based on battery state/level. This will give you a static border color as such. Source bbdr in focus, then set active border color as '$bb'
  rnbw for pulsing red when critical. To use this you don't necessarily need to change the active border color in focus, because the colors are the same (they actually work better together, coloring the newly focused window border with a bit less lag). By default rnbw will only pulse at critical level. I left in commented code to easily change to pulse always. Easily customizable to fit your needs.