Simultaneous measurements of the potential and high-frequency resistance, R HF, of a lead-acid battery cell of 1200Ah capacity used in photovoltaic applications have been carried out during the charge and discharge of the cell, owing to an original experimental set-up devised in the laboratory providing real-time analog signals. It was shown that R HF depended on the state-of-charge in a non-linear way, with a significant increase at low state-of-charge. In complementary measurements on cells of smaller capacities (45Ah), it was observed that the final value of R HF after a complete discharge was higher for a low discharge rate. The R HF variations were explained by the blocking effect of insulating PbSO4 crystals whose morphology and size depend on both state-of-charge and (dis)charge rate. It was also shown that the resistivity of the bulk electrolyte, measured above the plates, had no influence on the R HF value.