State-of-the-Art All launches up to 1989 have used the COMSAT design cell. The first commercial launch of a spacecraft using the Hughes/USAF design was accomplished in March, 1989. There are several more launches of spacecraft which will use this cell design (or a modification of this design) scheduled for the near term, including a LEO mission (Hubble Space Telescope) which is scheduled for this year. More launches are expected as the LEO database is expanded. A concerted effort is underway, with USAF and USN sponsorship, to expand this database [12,13]. Limited databases are available from Hughes Aircraft [14], Martin Marietta [15], RCA [16] Eagle-Picher [17] and other US aerospace contractors. Data ranges from 6500 to 10000 cycles at an 80% depth of discharge [14] to in excess of 32 900 cycles at a 15% depth of discharge [17]. The temperature range is 10 to 25°C. NASA is also sponsoring LEO testing in support of space station [18]. There have been several modifications/improvements to the Hughes/USAF cell during the past several years. These were made under USAF [19], NASA [20] and private sponsorship [21]. As originally designed, the 3.5 in. (8.9 cm) diameter pressure vessel had a capacity limitation of about 60 ampere-hours (Ah). By lengthening the pressure vessel and including another series stack, the capacity can be increased to about 90 Ah; however, additional pressure vessel design is needed to keep the burst/operation pressure ratio in excess of safety margins required (>3). Another approach is to increase the pressure vessel diameter to 4.5 in. (11.4 cm). Use of the 4.5 in. diameter individual pressure cell (IPV) is considered a better alternative to the risk of development and successful demonstration of a common pressure vessel cell (CPV) [11], A 4.5 in. IPV cell with a single stack has a capacity range from 90 to 160 Ah. Recently capacities up to 220 Ah have been achieved using a tandem stack arrangement [22]. Recent Advances in the Technology Even though the capacity of nickel-hydrogen cells has been improved by the above innovations, the more significant issues such as improvement of cycle life and specific energy remain. In the case of cycle life, it has long been established that the life limiting component in a nickel-hydrogen cell is the nickel oxide electrode [23] in spite of advances that have been made at Bell Telephone Laboratories [24], USAF laboratories [25] and in private industry [26]. Probably the most significant breakthrough in cycle life improvement of nickel-hydrogen cells has been reported by Lim & Verzwyvelt in an effort at Hughes Aircraft that has been ongoing under NASA LeRC sponsorship for the past few years [27]. They were able to increase the cycle life to over five times that of state-of-the-art with a simple adjustment of electrolyte concentration (26% vs 31% KOH). A summary of their data from tests using boiler plate cells is shown in Fig. 2. The results are currently being re-evaluated using flight design hardware. Results to date are shown in Fig. 3 [28]. It is hoped that sufficient data from these tests will be available prior to finalization of the space station cell design so that this technology can be utilized. Nickel-hydrogen battery design and development for the space station has been initiated by Ford Aerospace [18]. Present requirements for the battery are 41000 cycles (five years) at a depth of discharge of 35%. To support the 75 kilowatt (kW) average user load (100 kW peak) modules with five batteries, 90 cells at 81 Ah (3.5 in. cell diameter) are the baseline, with 30 cell orbital replacement units. Studies are
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