After the room with the dark eco pool from Reach the center of the complex, take the path to the right to lead you to a room filled with numerous platforms and electrified water, as well as a single puffer flying around in the center.
The idea for this cell is to light all platforms up (they flick on and off when you jump on them, but only for the first jump) while making sure you end up at the other side of the room with the closed door. The key is to make an N shape in your approach, start left and go into the left corner. Then make your way back to the bottom right jumping on unlit platforms as you go. You'll encounter the lurker mentioned previously around here so take care.
Then, go forwards again into the right end corner and finally one or two platforms to the left where you'll hopefully land on the platform in front of the door. If not it might still be possible to roll jump straight into the door if you're still close enough. The power cell is straight ahead, behind the newly opened door.