The hidden powercell is the one on the rocks near the train tracks just before the passageway to spider caves.
To get the powercell, first make your way to spider caves. At the entrance of spider caves, you should see an orange eco vent(the one that allows you to shoot fireballs) on your upper left. It would be easier if all the spiders and the tree chewers have been eliminated. Get up to that eco vent so that your eco meter is full. Then return to volcano crater by somarsaulting(to do this- move foward then hold L1 or R1 and press x). Be quick to avoid the eco from running out. Once there use your goggles to aim and shoot the box. The powercell will tumble down to you where you can reach it.
darkstar_legend here. more hints to come.