Health monitoring is essential for the photovoltaic (PV) panels to improve the system's efficiency and stability. Existing health monitoring methods are applied either offline or need additional equipment. Impedance spectroscopy (IS) methods can characterize online the internal processes of electrochemical systems with corresponding equivalent circuit model. However, it is rarely used for PV panels. This paper focuses on the online implementation of IS based on existing power converter for real-time health monitoring of PV panels. The injection region and type of perturbation signal for PV panels during IS measuring are analyzed. Based on PV panel's equivalent circuit model, a theoretical analysis of the influence of temperature and irradiance on internal parameters is made. Besides, a cooperative control method of online IS method and maximum power point tracking (MPPT) method is proposed. Even during the IS measuring, the quasi-maximum output power of PV panels can be maintained. The feasibility of the proposed method is verified by both simulation and experiment under different operating conditions.