Common aircraft use NiCd-or NiMH-batteries for auxiliary power supply, for starting the APU and as backup power sources. Since Li-Ion batteries offer advantages concerning power and energy density first airframers have started using this technology on civil aircraft. Nevertheless this technology requires a more sophisticated security concept than the traditionally used batteries. Recent safety issues on Li-Ion batteries in civil aircraft show, how much effort is needed to produce highly reliable and safe batteries for aircraft. This paper first describes the main differences on battery system level, arising from using Li-Ion battery cells in comparison to NiCd batteries. Based on this knowledge a battery concept is presented which satisfies reliability requests for aircraft equipment by consequently applying safety strategies and functions for Li-Ion batteries as well as using redundancy on component and system level. Special emphasis is put on the analysis of pressure dependent properties of Li-ion batteries, which are relevant for aircraft applications. Nomenclature FCE= Full Cycle Equivalents -defines, how often the battery would have been completely charged and discharged with the throughput energy (e.g. two times charged and discharged only half is one FCE) SoC = State of Charge -defines how much charge is in the battery. 0 % refers to a completely discharged battery, 100 % to a completely charged DoD = Depth of Discharge = (1 -SoC). Thus 100 % DoD refers to a completely discharged battery OCV = Open Circuit Voltage -is the voltage which sets in under no load conditions in steady state

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