Vehicular electrical system and control method therefor

A vehicular electrical system for a hybrid electric vehicle (HEV) comprising:

• a high voltage electrical system having a high power electrical energy storage device (HPD) and a power inverter that cooperate to provide power to a HEV propulsion system;
• a low voltage electrical system having a low voltage power source, at least one accessory power load, and a control switch operated by a control switch logic to one of an opened and a closed state;
• a DC to DC converter between the low voltage electrical system and the high voltage electrical system;
• said control switch logic controlling the control switch to the opened state electrically isolating the low voltage electrical system from the high voltage electrical system when a key is not engaged with an ignition switch in a key on position until one of a plurality of hierarchical conditions being met indicates an impending propulsion request;
• said control switch logic controlling the control switch to the closed state electrically connecting the low voltage electrical system to the high voltage electrical system to charge the high voltage power source from the low voltage power source when at least one of a plurality of hierarchical conditions indicates an impending propulsion request;
• and
• said control switch logic controlling the control switch to the opened state electrically isolating the low voltage electrical system from the high voltage electrical system wherein none of the plurality of hierarchical conditions have resulted in a propulsion request at the expiration of a predetermined duration.