The Fuel Cell in an unlockable Uncommon item in Risk of Rain 2. Each cell gives the player's activatable item(s) an additional charge and reduces the cooldown time by 15%.
The amount of time left for the item to gain an additional charge is not displayed anywhere, meaning it may be difficult to know how much time is left for a charge to be added.
Completing the "Experimenting" Challenge unlocks this item.
"As humanity began to venture out into the depths of space, high-energy but low-volume fuel sources became critical for interplanetary travel. Stability came later."
-Brief History of Interplanetary Advances, Vol.2