Volume-based static model for nickel hydrogen cells

Abstract

A static Ni-H/sub 2/ secondary cell model has been developed to help explore the long-term performance projections for Ni-H/sub 2/ cells and batteries. The model consists of a spreadsheet arrangement of the volume, porosity, wettability and characteristics for each of the cell's wettable components. Following the development of the basic model, the growing databases related to plate expansion, plaque corrosion and electrolyte management considerations were reviewed as they impact changes in the electrolyte requirements of the cell. The basic model was modified to incorporate these effects. Taken together, the expanded model can be used to project cell performance over the useful life of a particular cell design. The model suggests that an important feature of a cell design is the percentage of the separator's pores that remain filled with electrolyte over the course of cell cycling. Factors that reduce the amount of electrolyte contained in the separator adversely impact the mass transport processes within the cell. This ultimately results in significant performance degradation. These factors include: (1) expansion of the positive plates; (2) corrosion of the nickel sinter substrate material; (3) incorporation of potassium hydroxide (KOH) into the structure of the gamma phase portion of the charged active material; and (4) condensation of water from the electrolyte onto the colder cell wall. Literature information related to these issues have suggested how materials can be selected and components can be designed to be better able to accommodate changes in electrolyte volume during the required life of the cell.