Investigation of the high-frequency resistance of a lead-acid battery

The high-frequency resistance, R_HF, or internal resistance, of 45 Ah flooded tubular lead-acid battery (LAB) cells was monitored during cycling at constant rates between C/100 and C/10 in order to understand the origin of the R_HF variations and to evaluate the feasibility of monitoring the state-of-charge (SOC) with this parameter. It is shown that the R_HF variations did not depend on the electrolyte-conductivity variations, as usually indicated in the literature. At a low discharging rate (C/100), R_HF increased only at low SOCs, with a high final value, while at a high rate (C/10) R_HF increased progressively from the beginning of the discharge, with a low final value. This has been interpreted as the consequence of the shape and size of the PbSO₄ crystals in the pores of the active material, which are the result of the continuous competition between crystal nucleation and crystal growth. R_HF informs on the structure of the PbSO₄ layer, and therefore, depends strongly on the history of the previous cyclings of the cell. Monitoring the SOC from the single value of R_HF was found to be impossible since, for different cycling rates, distinct values of SOC may correspond to the same value of R_HF. Fluctuations of R_HF were also measured at different SOC: they allow detection of gas evolution in the cell and could provide complementary information for estimating the SOC of LABs.