Determination of the Reliability of Aerospace Ni‐Cd Batteries from Survival Data on Cells Fabricated Between 1964 and 1977

Abstract

We have performed a reliability analysis on Ni-Cd cells manufactured between 1964 and 1977 and aged by NASA ("CRANE") in simulated low earth and geosynchronous orbits. Similar cells are used in the construction of Ni-Cd batteries that serve as the energy storage medium in communications satellites. The statistical treatment of cell aging data conforms to techniques applied to semiconductor devices. In low earth orbit, the failure distribution by shorts is lognormal with standard deviation (s) of 1.06, median life (tin) of 128,000h and activation energy of 0.29 eV. In geosynchronous orbit, the additional failure mode of capacity degradation leads to a bimodal lognormal distribution characterized by tm of 283,000 and 122,000h and s of 1.14 and 0.27 for short and degradation, respectively. We have also deduced the probability of battery-survival and battery-failure rate, respectively. The inclusion of cell redundancy is a key feature of the analysis. It is shown that enhanced redundancy is a major factor in assuring long-term battery performance.In view of the high initial cost and impossibility of repair at the present state of the art, no communication system requires a higher degree of long-term reliability than satellite communications. In geosynchronous orbit, the electronic system of transponders is powered by reliable solar cells at all times except during solar eclipses, when the energy storage system that frequently employs two rechargeable Ni-Cd batteries takes over. Each year, the satellite is exposed to two eclipse seasons, each lasting 44 days. During these seasons, the eclipse time varies from 4 to 70 min in a predictable manner. Then, unless the spacecraft's batteries deliver sufficient power above a specified voltage to the transponders, the operating loads must be reduced and the mission's objectives can only be partially met.In general, solid-state devices, such as solar cells and transponders comprising semiconductor components (1), enjoy a greater reputation for reliability than batteries. Indeed, conventional Ni-Cd batteries exhibit a time-dependent decrease in voltage due to cell degradation as well as cell shorting (2). Since at present the batteries on geosynchronous satellites may well be the life limiting component (2), a study of their reliability is a vital prerequisite in guaranteeing a successful mission of, say, 10 yr duration. The most exhaustive source for life-test data on aerospace Ni-Cd batteries originates from the Naval Weapons Support Center, Weapons Quality Engineering Center at Crane, Indiana (CRANE) (3). The major objective of the current investigation is to determine by means of a large-scale statistical study the reliability of Ni-Cd cells and batteries tested by CRANE. Although these conventional cells are of an early vintage with respect to the date of design and manufacturing, it is considered that their statistical treatment would provide a "benchmark" to compare with more modern cell designs. Moreover, since the battery fs but one component of the satellite's communic...