Mathematical Modeling of Current-Interrupt and Pulse Operation of Valve-Regulated Lead Acid Cells

Abstract

Electric and hybrid electric vehicles use valve-regulated lead acid ͑VRLA͒ cells that are subjected to dynamic operation with charge, rest, and discharge periods in the order of seconds. Such operation requires more sophisticated models that incorporate the electrochemical double layer. While this effect has been incorporated in a handful of electrochemical systems, the lead-acid cell, with its sluggish reaction kinetics, is one of the few where it is significant. This significance is demonstrated with use of the current-interrupt technique, where the model is used to provide guidelines for the estimation of various resistances. The usefulness of the modeling approach is exemplified by its ability to explore the effect of changing electrochemical area and concentration with state of charge, and the role of parasitic side reactions in the voltage response of the cell. Simulations of pulse charging and dynamic stress test of VRLA cells, where considerable differences are shown when including the double layer, illustrate the need for modifying the presently used modeling approach. In addition, simulations are compared to current-interrupt experiments on commercial cells in order to evaluate the applicability of the model and to identify the differences.With renewed interest in the development of electric and hybrid electric vehicles over the last decade, advanced batteries have received much attention. At present, only the valve-regulated lead acid ͑VRLA͒ and the nickel-metal hydride ͑Ni-MH͒ battery are being actively considered for these applications, while Li-ion cells could become more popular in the future when all safety-related issues are resolved. Of the two candidate battery systems, the low cost and ease of operation of the VRLA battery assures it a prominent role in the years to come. This can be seen from the choice of the VRLA battery as the main power source in a number of electric vehicles ͑EVs͒ and as a high power source used in conjunction with an internal combustion engine in hybrid electric vehicles ͑HEVs͒.This renewed interest has spurred research in this area, with focus on improving the performance of the system and tailoring the manufacturing process to achieve the requisite power and energy demands. In particular, the ability of the battery to handle the simplified federal urban driving schedule ͑SFUDS͒ and the dynamic stress test ͑DST͒ 1 are being extensively studied, as they provide a suitable yardstick to characterize the performance of EV batteries. These schedules, which involve repeated discharge, rest, and charge periods, each lasting a few seconds, test the dynamic response of the system. In addition, recent evidence that the cycle life of the VRLA battery can be extended considerably by the use of smart charging algorithms 2 has spurred interest in current interrupt ͑CI͒, 3 pulse, and ''burp'' ͑i.e., where a short discharge pulse is incorporated during a pulse charge process͒ charging of the battery. 4 While considerable effort has gone into testing batteries under these schemes, a co...

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