Progress up until the point you are allied with the Prydwen. You can pickpocket a fusion core from one the Knights inside the ship (make sure you do not get caught). Once you pickpocket the fusion core from the Knight, he/she will come out of the Powersuit, replenish their fusion core and re-enter the powersuit. Once they are inside their powersuit again, pickpocket the fusion core and repeat. (be sure to quicksave after each successful attempt)

Step1: Find a knight in a secluded area.
Step2: Pickpocket the fusion core.
Step3: If successfull, quicksave (just incase)
Step4: Wait for him to leave his powersuit and go back in it.
Step5: Repeat Step 1-4.