On-Orbit NiH2 Battery Performance and Problem Solving on the APEX Spacecraft

The Advanced Photovoltaic and Electronics eXperiments (APEX) spacecraft, launched in August 1994, contained an Electrical Power Subsystem which included two 10 Common Pressure Vessel nickel hydrogen batteries manufactured by Eagle-Picher Industries, Inc. The spacecraft bus has fully supported over 18 months of on-orbit payload operations, well exceeding the mission requirement. Over the duration of the mission, three fundamentally different battery charging algorithms were used, as necessitated by two hardware failures. After each failure, engineers evaluated the available options for extending the mission and implemented the most desirable option. All three charging methods -- pressure-based, V/T-based, and recharge-ratio-based (with significant overcharging) -- have demonstrated acceptable battery performance for periods of at least several months each. The on-orbit phase of the APEX program demonstrated the flexibility of the APEX Electrical Power Subsystem and the robustness of the APEX batteries. This paper gives an overview of the APEX Common Pressure Vessel and battery designs, summarizes the different charging methods that have been used, and presents on-orbit performance data for each method. Data for an incident where the batteries got very cold and may have begun to freeze are also presented.