Selecting recharge protocols for deep-discharge nickel-hydrogen cells and batteries

Abstract

An ongoing study within The Aerospace Corporation has been the review of the nickel-hydrogen life cycle testing programs being conducted by several organizations. The overall objective of the study has been to review the database information that is available as it relates to missions requiring long cycle life at deep depths of discharge (DOD), and from this study to develop a knowledge base that can be applied to these advanced missions. In addition to this database review, earlier studies by other investigators will be examined as they relate to changes in the charging potential of the nickel electrode caused by changes in the level of cobalt additive in the active material. Coordinated posttest analyses carried out on selected cells in our laboratories will be presented as they relate to establishing conditions where the cycle life can be increased while at the same time increasing the DOD to which the cells are cycled.When setting conditions, under which cells or batteries are recharged, there are several stress factors that can be set at the discretion of the experimenter or the spacecraft designer. These include: (1) the recharge ratio, (2) the nominal temperature at which the cells are cycled, and (3) the maximum charge voltage that is seen by the cells as they are recharged. These charging conditions must be played off against several adverse impacts, including (1) capacity walk-down due to what is believed to be caused by changes in the potential of the nickel electrode; (2) corrosion of the nickel sinter material due to overcharge, which leads to loss of usable capacity; (3) deactivation of active material caused by high voltages during recharge; and (4) expansion of the nickel electrodes, which results in separator dryout and reduced electrode performance. Improved understanding of these factors has made it possible to design a protocol for battery operation that can optimize cycle life by minimizing the stress factors. This report will describe information that is available to guide the selection of an optimized management protocol to reach the goal of 60,000 cycle at 60% DOD.