In order to transport the vast numbers of Mini-Cons safely inside the Exodus during its journey through transwarp space, most Mini-Cons were converted into energy and stored inside glowing green pentagonal capacitors that bear the Mini-Cons' sigil.
These storage panels are generally very sturdy, able to survive extreme conditions, including the heat of atmospheric re-entry. Mini-Con panels have been found at the bottom of the ocean, buried in tons of solid rock, in arctic ice fields, even on Earth's Moon, without even a hint of damage, the Mini-Cons within as healthy as the day they first entered the panels. While it takes a lot to damage a Mini-Con panel, it can be done.
Freeing a Mini-Con from its panel is not always easy. Sometimes all it takes is the will of the one who holds the panel. In some cases, such as that of the Mini-Cons who composed the Star Saber, all three Mini-Cons needed to be activated at once. And sometimes, the panels simply release the Mini-Con within on its own without a definite outside influence.
In some respects, the panels had something of a "reset" program attached to them. When a Mini-Con was awakened from their panel, their built-in "servility program" kicked in, forming a mental bond to the being that released them from the panel, regardless of their mental state and programming directives before they entered the panel. Mini-Cons awakened by other Mini-Cons did not seem to form any special bond, however. Since the activation of the Master Key and the elimination of the servility program and other directives in the Mini-Cons' core program, this "reset" feature of the panels was eliminated as well, and any Mini-Cons who still rest inside the panels will awaken as truly free machines.
Mini-Cons that have been captured and reformatted by Fixer Bugs are stored in a different style of storage panel, notable for its more sinister shape and ominous glowing-red Mini-Con sigil. Once released from these panels, the Mini-Cons have lost their souls (cutting them off from the Linkage forever), and are ultimately little more than slaves to the will of Unicron.
These panels are created from the same energy/matter as the Fixer Bug itself; once the Bug attaches to its victim, it absorbs the Mini-Con as energy, then converts its own mass into the shape of the storage panel.
Playstation 2 Game
In the Armada video game, Mini-Con storage panels retain their same basic pentagonal shape. However, when activated, they project a large paneled-sphere "container" above them, which dissipates as the top of the panel proper opens to reveal the Mini-Con within. It is unknown if the panel itself dissipates, or if it can be used again to store another Mini-Con.
The glow from a storage panel can be seen from great distances, but normally cannot be distinguished from the glow of a Data-Con disc.