Studies of the pulse charge of lead-acid batteries for PV applications: Part II. Impedance of the positive plate revisited

Abstract

In the second part of this publication series, dedicated to the pulse charge of the lead-acid battery, a special attention is paid to the impedance spectrum of the positive plate as a source for estimation of the electrostatic capacitance of the double layer ( C dl) on the surface of the positive active mass. The impedance spectra were measured at open circuit for different states of charge (SoC) in H 2SO 4 with specific gravity 1.24 and 1.28 g ml −1. A substantial difference was observed in the impedance spectra of partially charged and partially discharged positive plates keeping the same value of the SOC. The impedance data were subjected to inductance error correction, followed by differential impedance analysis (DIA). Considering the results from DIA, the recently published equivalent circuits of the positive plate in charged and in discharged state and the gel-crystal model of the lead dioxide, we proposed a model of the positive plate in partial state of charge (PSoC). The analysis of the obtained experimental results using this model and DIA show that the double layer capacitance is not frequency distributed. The influence of the state of charge and state of health on the model parameters is discussed. One of the most interesting results is the dependence of C dl on SOC—it features a hysteresis at which the values of C dl during the charge are 5–6 times higher than the corresponding ones during the discharge. This result was discussed in terms of changes in the double layer structure considering the gel-crystal model of the lead dioxide. During the discharge in H 2SO 4 with specific gravity 1.28 g ml −1 a passivation process was detected as a high frequency pseudo-inductive loop in the Nyquist plots in PSoC. The passivation time constant is higher at 50–60% SOC and decreases to zero in the end of the discharge. During the charge in both electrolytes, pseudo-inductive time constant was observed too. It was attributed to the phenomena of the dehydration of Pb(OH) 4, an intermediate in the reaction scheme of the PbSO 4 oxidation. The state of health influences mostly the ohmic resistance R Ω, the charge transfer resistance R ct and the parameters of the constant phase element accounting the diffusion in the pores (CPE diff), when the plate is well charged.