Abstract: The widespread use of lithium-ion batteries has sparked interest in swiftly assessing their state of health. Aiming at the problem that traditional impedance diagnostic methods cannot diagnose the state of health of battery cells in a battery pack online, a health estimation method for lithium-ion batteries based on bridging circuits is proposed. The symmetry and balance of the bridging circuit stimulate the battery cells on two strings simultaneously, simplifying the system and solving the harsh voltage resistance requirements of the excitation equipment, which are easily interfered with by the load in the traditional method. Based on the AC voltage response of the reference battery, the estimation of the internal resistance of the battery cells and the excitation current shunt in the bridging circuit is realized, and its validity is verified by simulation and experiment. To enhance the accuracy of the state of health estimation, the relaxation time distribution method is used to decouple the internal polarization response of the battery, and the corresponding characteristic frequency points are screened out. Experiments show that the characteristic frequency points are not changed after the battery is cascaded into a group, and the power electronic converter minimally impacts the excitation shunt at the characteristic frequency point. Experimental validation is conducted at different charging rates using the bridge circuit, and the results confirm the effectiveness of the proposed method, which enables accurate online estimation of the state of health of individual cells in a battery pack based on the growth of the internal resistance of polarization in the high-frequency band.