A battery isolator unit is disclosed for controlling a switching means having a first contact for connection to a terminal of a first battery, a second contact for connection to a corresponding terminal of a second battery, and an actuating input for biasing a switch element of the switching means switch in a closed position. The battery isolator unit includes a sensing circuit and a switch controller. The sensing circuit periodically determines a first and second value attributable to terminal voltage values of the first battery and the second battery respectively. The switch controller is responsive to detecting a predetermined condition of the first battery and/or the second battery to provide to the actuating input a control signal having a characteristic for biasing the switch element to the closed position. The switch controller periodically determines a difference between the first and second values when the switch element is in the closed position to obtain a obtain a sequence of difference values, and controls the characteristic of the control signal according to a comparison of a present difference value with a previous difference value to modify the bias of the switch element.