The Plasma Battery was the Covenant equivalent to the UNSC Fusion coil, and is possibly a reverse-engineered Forerunner Power Core. Their sole purpose in the game is so they can be used as improvised explosive devices, due to their explosive reaction to weapons fire. They require more shots than the Fusion coil to explode. They also require a higher drop to detonate them on impact. It is possible they are a Covenant power source for Plasma Weapons which recharges them when they run out of battery power, although it is more likely that they are there as a power source for covenant encampments and bases for communications and shielding etc.