In addition to the portable brainwave detector, Dianthus also provides Barbarella with a number of weapons from the Museum of Conflict for her mission. All of these weapons are powered by a single energy box.
We only see it in use after she fires a single shot from the smallest of the weapons. She tries a second shot, but when it doesn’t work, she glances at a device on the cuff of her boot. The device is designed in a taijitu, a yin-yang set of lights: one red, one white. They are blinking in an alternating pattern, and after viewing it she tells Pygar, “My energy box is completely dead.”
Though having a visual signal is quite useful to understand the state of an invisible resource like power, the signal would be much more useful if it showed the amount of energy remaining, and gave warnings before the power was completely out. Failing all that, it would be more useful if she just put the device on the glove of her shooting hand so it was in her field of view at all times.
And though Barbarella’s culture doesn’t understand war, even a peaceful person can quickly come to realize the risk in making your available resources—like power for your weapons—wholly visible to your enemies.