Think you missed the point, if nothing has been changed it needs to be changed - when you start a new career it defaults to EASY, the lowest difficulty setting which has unlimited fuel and unlimited battery.
As CapnScurvy said, click the radio at upper left, that opens this menu. You can change the difficulty from low to a higher setting, and/or customize it by checking/unchecking the boxes.
This has to be done in this office for a career, but it's not cast in stone at the beginning - you can change settings between patrols, so if you still have the save game return to base, end patrol, then change the settings in the office before starting the next patrol.