The Energy Crystal is an upgrade of the Advanced Charging Battery added by IndustrialCraft 2. It acts much like an Energy Crystal, although it can store more energy. Secondly, having one in your inventory will automatically charge any other (LV-HV) EU-powered tools or batteries in your inventory, similar to an Energypack. This is particularly usefully when compared to the Energypack, as it frees an armor slot for a Jetpack or something more protective.
An Energypack cannot charge a Charging Energy Crystal. Likewise, a Charging Energy Crystal cannot charge an Energypack.
The Charging Energy Crystal originally was a part of Aroma1997's Portable Recharger mod. However, it was annexed into IndustrialCraft when Aroma joined the IndustrialCraft 2 Dev Team.