An active isolation detection method may be used with an electrical system having a battery pack connected to a high-voltage bus. The bus has positive and negative bus rails, each having a respective rail-to-ground voltage. The method may include connecting variable resistance element to the high-voltage bus, and determining input information indicative electrical characteristics of the battery pack, the high-voltage bus, and/or a charging station. The method includes varying a bias resistance of the high-voltage bus, via control of the variable resistance element, e.g., via duty cycle control of a binary switch in series with a bias resistor, to produce a varied bias resistance based on the input information. A target voltage shift is achieved on the high-voltage bus as a target level of change in one of the rail-to-ground voltages. An isolation resistance of the electrical system is determined via the controller using the varied bias resistance.