An energy level conversion circuit for a portable energy storage device, the energy level conversion circuit comprising a motherboard circuit, the mother board circuit including: i) a female connector unit in which one or more female connectors are formed, wherein the motherboard circuit is adapted to receive a direct current power and to output the power obtained by boosting an energy level thereof into a specified energy level through an output unit formed in the motherboard circuit; wherein the mother board circuit further including ii) a plurality of sub-board circuits, each sub-board circuit including a) a male connector unit comprising one or more male connectors and, - b) a switching unit having a plurality of switching transistors (Q1 and Q2) turned on and off by receiving a pair of switching pulses (P1 and P2) applied from the motherboard circuit, and c) a transformer, the mother board circuit being further adapted to output a power source transformed by the transformers of the sub-board circuits through secondary-side trans-tap connection terminals (T1 and T2) of the female connector unit as the switching transistors (Q1 and Q2) are turned on and off, and characterized in that the plurality of sub-board circuits are connected to the female connector unit through the one or more female connectors and the one or more male connectors, and wherein the sub-board circuits are connected in serial through the female connector unit of the motherboard circuit, so that energy level output from the mother board circuit is varied according to the number of the sub-board circuits. a motherboard circuit, the mother board circuit including: i) a female connector unit in which one or more female connectors are formed, wherein the motherboard circuit is adapted to receive a direct current power and to output the power obtained by boosting an energy level thereof into a specified energy level through an output unit formed in the motherboard circuit; wherein the mother board circuit further including ii) a plurality of sub-board circuits, each sub-board circuit including a) a male connector unit comprising one or more male connectors and, - b) a switching unit having a plurality of switching transistors (Q1 and Q2) turned on and off by receiving a pair of switching pulses (P1 and P2) applied from the motherboard circuit, and c) a transformer, the mother board circuit being further adapted to output a power source transformed by the transformers of the sub-board circuits through secondary-side trans-tap connection terminals (T1 and T2) of the female connector unit as the switching transistors (Q1 and Q2) are turned on and off, and characterized in that the plurality of sub-board circuits are connected to the female connector unit through the one or more female connectors and the one or more male connectors, and wherein the sub-board circuits are connected in serial through the female connector unit of the motherboard circuit, so that energy level output from the mother board circuit is varied according to the number of the sub-board circuits. i) a female connector unit in which one or more female connectors are formed, wherein the motherboard circuit is adapted to receive a direct current power and to output the power obtained by boosting an energy level thereof into a specified energy level through an output unit formed in the motherboard circuit; wherein the mother board circuit further including ii) a plurality of sub-board circuits, each sub-board circuit including a) a male connector unit comprising one or more male connectors and, - b) a switching unit having a plurality of switching transistors (Q1 and Q2) turned on and off by receiving a pair of switching pulses (P1 and P2) applied from the motherboard circuit, and c) a transformer, the mother board circuit being further adapted to output a power source transformed by the transformers of the sub-board circuits through secondary-side trans-tap connection terminals (T1 and T2) of the female connector unit as the switching transistors (Q1 and Q2) are turned on and off, and characterized in that the plurality of sub-board circuits are connected to the female connector unit through the one or more female connectors and the one or more male connectors, and wherein the sub-board circuits are connected in serial through the female connector unit of the motherboard circuit, so that energy level output from the mother board circuit is varied according to the number of the sub-board circuits.