An electric propulsion system for a mobile platform includes a battery system connected to positive and negative bus rails, an accessory load having a rotary electric machine, a traction power inverter module (“TPIM”), and an accessory load, switches configured to transition the battery modules to a series-connected (“S-connected”) configuration during a direct current fast-charging (“DCFC”) operation of the battery system, and a controller. When battery modules of the battery system are connected in series during a direct current fast-charging (“DCFC”) operation, the controller executes a diagnostic method to determine bus rail voltages on the positive and negative bus rails and a mid-bus voltage, identifies a diagnosed electrical condition of the electric propulsion system by comparing the voltages to expected values or ranges, and executes a control action in response to the diagnosed electrical condition.