Comparison of Nickel-Hydrogen and Nickel-Cadmium Reliability for Low-Earth-Orbit Batteries 929257
Nickel-Hydrogen (NiH2) realtime and accelerated low-earth-orbit (LEO) cycle test data at 10°C have been used to generate a model of reliability as a function of depth-of-discharge (DOD). The reliability model is specific to cells incorporating positive electrodes manufactured by the wet slurry process. A cycle life prediction has also been derived from the reliability equation and has been compared to two other predictions: one for NiH2 cells of generic design and one for Nickel-Cadmium (NiCd) hardware. The comparison clearly shows the superiority of NiH2 in LEO.
At each DOD for which test data existed in the 5°C to 10°C range, the realtime and accelerated LEO life cycle test data were algebraically fit to a Weibull distribution which passes through the current cycle count of the ongoing tests, none of which have experienced a failure. The analysis was performed for the 15%, 30% and 40% DOD test conditions for “wet slurry” NiH2. A best-fit to a Weibull distribution was performed for the comparison predictions, at DOD's of 40%, 60% and 80% for “generic” NiH2, and at DOD's from 15% to 50% for NiCd.
The end result of the cell reliability analysis is a plot of cell probability of success (PS) versus cycle time. The PS graph may be converted to a graph of expected cycle life for the case of a battery, given number of cells per battery and a value for required battery reliability. Low earth orbit life test data at 10°C shows that NiH2 has a superior cycle life to the established NiCd system. In addition, the life projections for “wet slurry” NiH2 are above those of “generic” NiH2. In conjunction with the energy density advantage, NiH2 is clearly the system of choice for both GEO and LEO missions requiring high reliability and longer life.