ExtraCells is the first Applied Energistics Add-On, implementing quite some very usefull functions. EC began with just being an Add-On for bigger Storage Cells, but after that it evolved into the AE Fluid System. Thanks to EC you can now store Fluids/Liquids in the network, on storage cells or in real tanks, using the storage bus. Apart from fluids, EC also contains some usefull things like abackup battery, a ME Dropper and a fully customizable storage cell, which you can upgrade. And finally, my Blast Resistant ME Drive protects your storagecells against creeper explosions :P
You can find more functions in the wiki and in the mod itself, since the wiki isnt always up to date, but I'm working on it ;)
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