The Standard Empyraen Power Cell Mk-4 is used in everything from weapons, to light enhancement gear, fònos, and scanners. They are cheap to make, their storage crystals and other materials abundant and they can survive pretty much any treatment. Soldiers 'cook' them on camp fires to give extra charge by heating the crystals inside, use them to 'spark' fusion cells, and even as overload troublesome electronic door locks.
In emergency conditions, you can even rig a power cell, or whole bank of them, to power things normally meant to work on a more permanent even different source of energy. Hoplite legend speaks of a nameless soldier who got his dying or seriously wounded unit back to base by rigging enough spare cells to the engines of a Stymphalion to let it fly the five miles over rough country. He got them back although the Stymphalion apparently caught fire shortly after the last man was unloaded, killing the nameless hero.