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The AFRL, Electrochemistry and Thermal Sciences Branch has evaluated numerous aircraft battery designs and chemistries since the 1960s. Recent experiments on advanced battery chemistries have shown poor performance at ultra low temperatures below −20° C. Aircraft battery designs stress low weight and volume and maximum capacity. One design concept uses lower capacity cells in a series parallel configuration to reduce overall battery resistance and should also improve ultra low temperature performance. Our organization has begun experiments with series-parallel cell designs to evaluate the concept and to solve low temperature performance issues. Progress, observations on the effect of different chemistries, and the impact on aircraft battery characteristics are discussed.

The Air Force Research Laboratory, Energy Storage and Thermal Sciences Branch conducted performance tests of 22 Ah nickel-metal hydride (Ni-MH) cell packs containing four prismatic sealed design cells over the operational temperature range for a modern fighter aircraft at the C/2, C and 2C rates. Capacity tests indicated satisfactory performance at the C/2 and C rates down to -30 °C and at the 2C rate down to -20 °C. Self discharge tests at elevated temperatures up to +60 °C were also conducted. Test results will be presented and implications for further development of an environmental main aircraft battery will be discussed.

The Air Force Research Laboratory (AFRL), Energy Storage and Thermal Sciences (PRPS) Branch has been developing nickel-metal hydride (Ni-MH) rechargeable batteries as an environmental replacement for existing valve regulated lead-acid (VRLA) and vented/sealed nickel-cadmium (VNC/SNC) batteries since 1995 and has evaluated cylindrical, prismatic and bipolar designs for this application. Recent advances in cell chemistry and design have resulted in a significant improvement in ultra low temperature performance indicating the suitability of these batteries for military aircraft applications over the temperature range from -40 °C to +65 °C. Results of the latest in-house tests of developments in bipolar and prismatic cell and battery designs indicate the current prismatic cell formulations are limited to temperatures above -25 °C while those used in bipolar designs operate over the full military aircraft temperature regime.

As part of the DoD/NASA Lithium-Ion and More-Electric Aircraft (MEA) development programs, in-house life-testing and performance characterization of lithium-ion batteries of sizes 1-20 amp-hours (Ah) were performed. Using AC impedance spectroscopy, the impedance behavior of lithium-ion cells with respect to temperature, cycle number, electrode, and state-of-charge was determined. Cell impedance is dominated by the positive (cathode) electrode, increases linearly with cycle number, and exponentially increases with decreasing temperature. From cell performance testing, we have seen the cell behavior is extremely sensitive to the ambient temperature. Preliminary battery performance results as well as AC impedance and life cycle test results are presented below.

As the maintenance and disposal costs of aircraft batteries have risen, it has become critical to increase battery lifetime and to reduce maintenance cycles. This has led to the development of charging techniques designed to increase battery life while continuing to satisfy battery performance requirements. However, the cost of battery chargers accounts for 60% to 80% of the battery/charger system cost. AFRL/PRPB has initiated an in-house project to evaluate F-16 batteries using the existing F-16 charger. The objective is to determine which batteries can pass all F-16 performance and lifetime requirements using this charger. Several batteries were procured from several sources and two F-16 chargers are on loan to us from Sacramento/ALC. Depending on the outcome of this phase the project may be extended to include other aircraft and other chemistries such as Nickel-Metal Hydride and Lithium-Ion. Results to date and future plans will be discussed in this paper.