An auxiliary power unit of the present invention has an auxiliary lithium-ion secondary battery, a charge connector connected to the auxiliary lithium-ion secondary battery and adapted to receive power from an external charger, and a supply connector connected to the auxiliary lithium-ion secondary battery and adapted to supply power of the auxiliary lithium-ion secondary battery to an external portable device, and the auxiliary lithium-ion secondary battery is constructed so that each of thicknesses of cathode active material and anode active material layers is in the range of 10 to 40 μm.