I don’t have deep knowledge about jet aircraft and all their systems but I found this post explaining where in the FMC to activate the ground power for the Cj4:
Starting at KDSM Gate 16, powering up the aircraft you can see that external power is available (even without calling the power supply, that the way the sim is designed regarding external power).
If OFF selected, the battery drains:
If ON selected, battery does not drain:
Hope it can help, please correct me if I am wrong.