Bipolar Nickel Metal Hydride Batteries for Aircraft Applications 2002-01-3215
Electro Energy Inc. has been developing their wafer cell bipolar nickel-metal hydride battery for aircraft applications. The design consists of individual sealed wafer cells that are stacked in series to make a multi-cell, multi voltage battery pack. Each individual wafer cell consists of one positive electrode, a separator material and a negative electrode contained in an outer envelope so that the faces of the wafer cell represent the positive and negative contact of the cell. The perimeter of the cell is sealed to contain battery gases and electrolyte, making a fully sealed cell. To construct a multi-cell battery, identical cells are stacked one on top of the other, such that, the positive face of one cell makes contact with the negative face of an adjacent cell to make a series connection. Multi-cell stacks are held in compression for physical integrity and to ensure good cell-to-cell contact. In a cell stack, the current is conducted perpendicular from cell to cell as opposed to conventional mono-polar cells, where current is conducted to the edges of the electrodes and through terminals. This design concept eliminates the need for separate terminals, tabs, current collectors for the electrodes, and improves active material utilization through uniform current densities. Cells can be designed to fit the full cylindrical or prismatic geometry of a housing to maximize the use of available space in a battery container. The nickel-metal hydride battery is considered to be environmentally friendly and may become a replacement chemistry for lead-acid and nickel-cadmium batteries in aircraft applications. In addition to environmental concerns, the EEI bipolar nickel-metal hydride battery design offers the aircraft designer a battery that is 30-50% lower in volume for a given capacity, with the ability to develop high voltage systems simply. Worldwide efforts have been directed at improving the low temperature performance, reliability and packaging of the nickel-metal hydride system. The sealed bipolar battery design also provides increased power and capacity with lower weight for replacement of the valve-regulated, 28-Volt nickel-cadmium systems. This paper describes the effort of a dual–use program, sponsored by NAVAIR Research and Engineering and the US Air Force. Efforts encompass design, packaging and performance of the bipolar wafer cell nickel-metal hydride system as a replacement to the sealed lead-acid presently used on the F-18 and potential application on the USAF tactical aircraft using a common cell design. Results will be discussed from laboratory safety certification testing in preparation for flight evaluation on an F/A-18A-D model aircraft. These tests were conducted in accordance with MIL-PRF-8565K and MIL-PRF-8565/4D, which defines the battery requirements for the F/A-18A-D battery. This paper also discusses the planned flight evaluation effort.