This guide will show you where to find every Energy Cell in End of Abyss. These give you more slots, allowing you to equip more Suit Mods.
Once you collect all 6, you will unlock the 
Fully Suited Up trophy.
None of them are missable, and they can all be found after finishing the main story.
Energy Cell #1
Maintenance > F2.
You will find this one inside the large chest in plain sight.
Energy Cell #2
Factory West > F3.
Use an Explosive Charge on the wall to the left of the switch that extends the bridge. Then go through into the small hall, head to the bottom of the screen and place another charge on the left wall to reveal a hole leading to the room with the Energy Cell inside.
Energy Cell #3
Habitation Sector > F2.
Start by burning down the wall on the left side of F1 (you will need the Incinerator, a story-related weapon obtained in the final area of the game). Then go through and burn the Parasitic Growth to get the Block A Keycard.
Next, head back to where you burned down the wall and use the Block A Keycard to go through the door just above it. Head up to F3, go through the door, then make your way down the hall and through the bottom right door. After that, head through the vent, and once you come out the other end, drop down to the floor below. Step out into the hall and take the door below you on the left side of the hall. Here, use an Explosive Charge to blow a hole in the floor, then drop down and exit through the door in the bottom right corner. Next, enter another room on the right side of the hall, and leave it through the door in the top left to reach another hall. Here, go through the last door on the right to find the Janitor Key.
Now that you have the Janitor Key, head over to Block B (the area on the right side of the Habitation Sector). Here, use the Janitor Key to enter the room with the Energy Cell.
Energy Cell #4
Reactor > B3.
Head up the ladder in the top left corner of B4 to find it.
Energy Cell #5
Waterworks > B3.
Use the Grappling Hook on the switch above the door, then go through it to find the Energy Cell.
Energy Cell #6
Containment Sector > F2.
Destroy the petrified worker by shooting or hitting it.
