Self-discharge significantly impacts the performance of supercapacitors, particularly in double-layer capacitors, by causing energy loss over time. This paper presents a numerical investigation into the self-discharge mechanisms in supercapacitors, incorporating both experimental observations and theoretical models. We examine three key factors influencing self-discharge: ion diffusion, side reactions, and ohmic leakage, based on electric double layer theory. Through simulations and experimental validation, we highlight the effects of initial voltage, charge duration, and current on self-discharge behavior. The study reveals that ion diffusion dominates at lower initial potentials and short holding times, while ohmic leakage becomes more significant at higher initial potentials and longer charge durations. Additionally, the paper investigates the impact of temperature on charge redistribution and self-discharge, emphasizing the role of current during the relaxation period. We find that the rapid energy loss observed at the beginning of the discharge process is due to an unbalanced ion distribution, which can be mitigated by applying a small charging current. The results suggest that understanding these mechanisms, specifically the interactions between charge redistribution, side reactions, and ion movement is crucial for optimizing supercapacitor design and reducing self-discharge rates.