Low-power self-powered equipment has gradually become the mainstream of development with the continuous advancement of Internet of Things technology. Energy storage module based on supercapacitors can be applied to low-power self-powered smart devices, but the self-discharge characteristics of supercapacitors will cause the loss of stored energy and the drop in terminal voltage, which is not conducive to the power consumption of low-power self-powered devices management and measurement. A variable leakage resistance model which is used to analyze the introduced energy storage module is established in this paper. The relevant parameters in the equivalent model are determined through experiments and the self-discharge process of the energy storage module is analyzed by Matlab/Simulink. Finally, the simulation results are fitted with piecewise linearization in order to facilitate subsequent test error correction or mathematical calculations in the future, and the fitted equivalent circuit model is used to predict the voltage of the energy storage module during self-discharge. The voltage variation of the energy storage module predicted by the equivalent model fits well with the experimental data. The work in this paper completes the basical steps for the designation of power management unit and the power consumption measurement in low-power and energy autonomous devices.