To elucidate the deterioration mechanism of valve regulated lead-acid battery (VRLA) under high-rate partial-state-of-charge (HRPSoC) duty, the cyclic performance and the direct-current internal resistance (DCIR) of VRLA with addition of a granular carbon additive, (Vulcan 72, VC 72) in the negative active materials (NAMs) are investigated specifically. The DCIR during the whole charging process rises linearly up to a threshold value due to consecutive formation of lead sulfate. According to Percolation theory, the relationship between compositions of negative plates and charging DCIR is clarified. The charging DCIR behaviors are changed via addition of VC 72. Granular carbon particles in nanometer size distribute uniformly in the NAMs, and the reduction of Pb2+ ions to Pb is directly found to occur on the surfaces of both metallic lead and carbon particles. Granular carbon particles provide more active sites for the electrodeposition and nucleation of PbSO4 during the discharging process, while in the charging process, those carbon particles provide conductive pathways for electrons exchange between insulating PbSO4 . The modified microstructure of the NAMs suppresses the generation of irreversible lead sulfate and delays the rapid growth of the charging DCIR under HRPSoC duty, therefore extending the service life by 3 times. (C) 2018 The Electrochemical Society.