Performance of 3rd electrode cells in OAO.

The OAO 2 satellite employed nickel-cadmium batteries with auxiliary (signal) electrodes for overcharge control. During the early life of the satellite, these third-electrode signals provided an orbit-by-orbit indication of the overcharge received by the batteries. By ground command, the battery charger was adjusted on a need basis to provide optimum recharge conditions as indicated by the third-electrode signals during all phases of spacecraft operation and orientation. After more than two years, an unusual degradation in third-electrode signals was observed, providing the first indication of battery degradation. The present paper examines the operation of the battery system and analyzes the battery degradation mechanism. The abnormal performance of the third electrode is related to cell-voltage divergence during periods of overcharge. Cells that had a higher cell voltage showed signs (third-electrode degradation) of small amounts of hydrogen gas generation.