Energy Crystal (IndustrialCraft 2)

The Energy Crystal is a rechargeable energy storage unit similar to an RE Battery or Lithium Battery. It can hold 100,000 EU. It requires an MV-tier interface to charge. Right clicking an energy crystal will NOT cause it to recharge electrical powered items in your inventory.

Uses
The Energy crystal is used as a power storage unit much like an RE Battery.

Or in the crafting of MFEs, Nano Armor, Lapotron Crystals, and other electric based tools.