Abstract: A voltage monitoring circuit (4) detects when the output battery of a power source (e.g., battery) falls below a predetermined reference voltage. A controller (3), in response to the monitoring circuit, performs a load operation. In the load operation, a switch (8-1) in a pseudo-load circuit 8 is operated to connect a dummy load (8-2). On the basis of the output voltage of the power source when the dummy load is operable, the controller determines whether to allow a further drain on the power supply, e.g., connection of a peripheral device interface (10). In one embodiment, the pseudo-load circuit 8 has plural dummy loads which can be invoked to determine with which of plural possible loads (e.g., peripheral devices) the power source can be utilized. Also disclosed are both digital and analog methods and circuits for determining utilizable load magnitude by gradually raising power supply capacity.