Evaluation of Maxwell Technology PowerCache Ultracapacitor

Henry Machado SAIC – JSC/NASA Introduction The Parts Analysis and Assurance (PA&A) group at the Johnson Space Center (JSC) evaluated the PowerCacheUltracapacitor, part number PC5, from Maxwell Technology. These capacitors are used in the ISS PEEK (Portable Electrical Equipment Kit). The PEEK hardware provides electrical power extension industrial cables and outlets as well as 120 to 28Vdc professional converter units to power portable electrical hardware on the ISS.Ultracapacitors, also known as double-layer capacitors, incorporate new technology and material not used inother capacitors. The ultracapacitor stores electrostatic energy by polarizing the electrolytic solution and can be charged and discharged hundreds of thousands of times. An electrode-electrolyte interface with activated carbon fibers allows obtaining extremely high capacitance per unit area. There are several methods of construction, but JSC tested one where an insulator separates two activated carbon electrodes. These electrodes consist of analuminum foil with a carbon binder mixture deposited in it. The set is tightly rolled with leads attached to thealuminum foils. An electrolyte is added and the component is sealed.This technology is suited for applications where there is a need of providing large bursts of power, for fractions ofseconds up to several minutes, with the ability of rapid recharge. During a power outage, power levels from the ultracapacitor can be held until power returns. Due to their large energy storage capability, ultracapacitors havefound applications in the electric vehicle industry, memory back-up devices in appliances such as videocassetterecorders, typewriters, wristwatches, and measuring devices. The advantage of ultracapacitors is that they are smaller in size than other capacitors of the same value. This allows for greater packaging efficiency. The typical voltage rating for a single ultracapacitor is 2.3Vdc. The components can be stacked in series to achieve the proper operating voltage for any specific application. Space Application The PEEK project had the JSC RITF (Receiving and Inspection Test Facility) evaluate these commercialcapacitors to determine their suitability in space flight hardware. The evaluation consisted of a series of tests todetermine their performance across a range of environmental stress conditions. The following tests wereconducted: • Visual Inspection • Fine Leak Test • Accelerated Life (Six tests) • Static Burn-in (five tests) • Reverse Bias • Self-Heat via Rapid Charge-Discharge (three tests) • Over Voltage Pulse (eleven tests) • Thermal Shock (one test) • Destructive Physical Analysis • Electrical Parameter testing at periodic intervalsThe RITF test results demonstrate that the ultracapacitor is suitable for use in the ISS PEEK hardware, as well asin other equipment that is used in ISS manned pressurized modules. Ultracapacitor pedigree in military or aerospace applications is presently not available and no specific standard has been developed for screening andtesting purposes. However, the PowerCache ultracapacitor survived the environmental tests in MIL-STD-810“Environmental Engineering Considerations and Laboratory Tests”. A series of tests were performed at JSC Receiving Inspection and Test Facility (RITF) to determine the reliability of the ultracapacitor under different conditions. The PC5 ultracapacitor is rated at 4 Farads at 2.7Vdc maximum and the operating temperature is between -20ºC to +70ºC.