The Effects of Separator Design on the Discharge Performance of a Starved Lead‐Acid Cell

Abstract

A mathematical model of a starved lead-acid cell has been developed to study the dynamic behavior of the cell during discharge. Concentrated binary electrolyte theory and a volume-averaging technique were used to model the transport of electrolyte. The model can be used to predict cell voltage and profile of: acid concentration, overpotential, porosity, reaction rate, and electrode capacity, as functions of time. The effects of separator thickness and its porosity were examined with respect to cold-cranking amperage and reserve capacity of the battery. The separator was found to be a significant factor governing performance.Starved lead-acid batteries are becoming increasingly popular in the secondary battery market due to many of their potential advantages: zero maintenance, operation in any position, internal gas pressure potentially usable as a built-in charge indicator, and low battery profile and weight, which is especially attractive in automotive SLI (starting-lighting-ignition) applications. A traditional flooded-type lead-acid battery contains excess acid to compensate for the water loss due to oxygen evolution on overcharge. A unit ceil of the battery is composed of a positive electrode, an acid reservoir, a separator, and a negative electrode. A starved lead-acid cell does not have an acid reservoir. Instead, a thicker and more porous separator is used to prevent physical contact of the positive and negative electrodes, serve as an electrolyte reservoir (this gives rise to the concept of an immobilized electrolyte), and promote oxygen gas transport from the positive electrode to the negative electrode for recombination on charging.The separator has always been thought to play a critical role in the operation of a starved lead-acid battery. Atlung and Fastrup (1), extending the work of Turner and Moseley (2), developed a mathematical model to study the effects of separator design on discharge rate and cell capacity. Their model, however, is of the separator alone, with constant reaction fluxes used as boundary conditions. We present here a detailed mathematical model of the whole starved lead-acid cell for discharge, which will allow the analysis of the total system and interactions between the components-positive electrode, separator, and negative electrode. This model is a modification of an earlier model by Gu et al. (3) for the flooded-type cell. With this model, the effects of porosity, tortuosity, thickness, and the level of electrolyte saturation of the separator, on the discharge performance of a starved lead-acid cell, can be studied, including any interactions between the separator and the electrodes.This paper is organized in the following manner. In the "Model Description" section, detailed formulation of the model is given. In the "Model Output" section, the model results are illustrated graphically using an example of a CCA calculation. Each plot is briefly discussed regarding its significance. In the latter section, the effects due to the separator on cold-cranking amperage (CCA) and res...

Study of Charge Kinetics in Valve-Regulated Lead-Acid Cells

“…The resulting large activation overpotential in this region of the positive electrode causes the rapid decline of cell voltage that ends discharge. Not only does migration thwart the transport of HSO 4 Ϫ into the positive electrode, but decreasing c A and ⑀ l result in increased diffusion resistance.…”

“…We also use A PbO 2 ϭ A PbO 2 max (Q PbO 2 /Q PbO 2 max ) PbO 2 for j PbO 2 . 4 Null exchange current densities were used for Reactions 3 and 4 due to their decreased importance during discharge and uncertainty in kinetic constants. Since the expressions could not simulate the experimentally observed charge behavior, 8 we presumed that the mechanisms of the forward and reverse directions of Reactions 1 and 2 were different; hence, our use of different kinetic constants for charge and discharge.…”

“…27͒, cm ␦ k exponent for the p O 2 dependence of exchange current density in Reaction k ⑀ i volume fraction of species or phase i ␥ k exponent for the c A dependence of exchange current density in Reaction k k surface overpotential for Reaction k, V electrolyte conductivity, mho/cm l electric potential in the liquid solution phase, V s electric potential in the solid phase, V k PbSO 4 morphology parameter associated with Reaction k i mass density of species i, g/cm 3 i gas-phase mole fraction of species i conductivity of the solid matrix, mho/cm k morphology parameter in active area expression for Reaction k Subscripts and Superscripts ͑*͒ adsorbed species A H 2 SO 4 species c charge d discharge ext external to the cell stack, but inside the cell case eq equilibrated concentration g gas phase i ionic or molecular species i,ii steps in electrochemical mechanism k Reaction 1, 2, Pb, or PbO 2 l liquid phase max fully charged condition net amount produced during a given charge only o initial condition for charge ͑discharged state͒ ppt PbSO 4 or at the PbSO 4 …”

“…Since the earliest published models of the lead-acid cell, 2,3 extensions have been proposed, [4][5][6][7][8] with a more recent advancement being a two-dimensional model including free convection. 9 Still lacking, however, is an accurate description of charge.…”

Role of electrical resistance of electrodes in modeling of discharging and charging of flooded lead-acid batteries

“…Equations of Nguyen et al [21] are used to convert concentration into molality. Diffusion coefficients and conductivity in the electrolyte are calculated from the equations of Tiedemann and Newman [22].…”