On discovering a Martello you must examine it and select the option to replenish fuel. you will be asked to stand on a specific side of the Martello to replenish it. After replenishment you will be given an Empty fuel vat, which you must then return to the Machine Outfitter to complete the quest.
Regular replenishment of the martellos located throughout the La Theine Plateau is vital to the resistance effort in this area. Refuel a martello and return the empty vat to the machine outfitter when your task is complete.