102 103 104 105 Cycle Life (Cycles)

Fig. 11 -11. Depth-of-Dlscharge vs. Cycle Life for Secondary Batteries. Increased cycle life reduces the amount of energy available from the batteries during each cycle—DOD decreases with cycle life.

terminal voltage of 1.22 to 1.25 Vdc depending upon discharge loads. The typical individual pressure vessel battery design consists of multiple cells connected in series to obtain the desired battery voltage. Cell diameters are typically 9 to 12 cm, with capacity ranges from 20 to over 300 Amp-hr. The common pressure vessel NiH2 technology is very similar to individual pressure vessel, with the primary difference in the wiring connection of the internal electrode stacks. In the individual pressure vessel, the electrode stacks are all connected in parallel. In a common pressure vessel, there are two sets of electrode stacks within the pressure vessel that are series connected, yielding a working terminal voltage of 2.44 to 2.50 Vdc. This design has a higher specific-energy at the battery level since there are half as many pressure vessels and a significant reduction in cell piece-parts. Common pressure vessel NiH2 technology has been space qualified in the 6 cm and 9 cm cell diameter configuration for capacities in the 12 to 20 Amp-hr range. Batteries with larger Amp-hr capacities should be qualified for aerospace application in the near future. The single pressure vessel N1H2 battery is designed such that a common hydrogen supply is used by three or more series connected cells with a single pressure vessel. Each cell stack contains its own electrolyte supply which is isolated within individual cell stack containers. The key operating characteristic of this design is to allow the free movement of hydrogen within the cell stacks while maintaining cell stack electrolyte isolation. These batteries are presently available in a 12.5 cm or 25 cm diameter design.

Lithium Ion battery technology offers a significant energy density advantage and a much wider operating temperature range over NiCd and NLH2 battery types. Typical cell constituents are lithium thionyl chloride, lithium sulfur dioxide, and lithium carbon monofluoride. The nominal operating voltage for a lithium ion cell is 3.6 to 3.9 Vdc, which allows us to reduce die number of cells by approximately one-third when compared to NiCd or NiH2 cells. The lithium ion secondary battery system offers a 65% volume advantage and a 50% mass advantage for most present day aerospace battery applications. Lithium ion battery technology should be qualified for

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