Meeting report: New batteries for spacecraft and mars-surface explorers - part I

Advanced Battery Technology,  Oct 2000  

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Earth-orbiting communication and scientific observing satellites get their electric power from solar-cell arrays. However, they also need power when the satellite is in the Earth's shadow, and this can occur many times every 24 hours in low-Earth-orbit. A battery can store solar energy for use when no sunlight is available, but it needs to be light in weight because the cost of boosting a satellite into low-Earth-orbit has been over $10,000 a pound. A nickelcadmium battery can store 25 watt-hours per kilogram. However, long satellite life is also required to avoid the cost of boosting a replacement satellite into orbit every few years. A decade of testing has proven that NiCd batteries can indeed support a long satellite lifetime if only 20% of their stored energy is extracted during each discharge. The resulting energy density is only 5Whr/kg. Today's lithiumion cells are storing 100Whr/kg.

This advance in battery performance has created an intense development effort. Manufacturers of proven batteries are developing higher energy-storage capacity in charge/discharge cycling service. New producers of batteries are testing design variations that deliver long charge/ discharge cycle life even in the cold environment of a geosynchronous satellite during its 1.5-hour linger in the Earth's shadow. Progress in the development of new batteries, even for Mars rovers and sample-and-return vehicles, were exciting topics at the 35th Intersociety Energy Conversion Engineering Conference (IECEC) in July. Each year one of six technical societies sponsors this conference. The 2000 sponsor was the American Institute of Aeronautics and Astronautics.

The IECEC brings together engineers who design and test batteries, and who design spacecraft and battery energy-management systems. Pertinent developments in battery-related fields were also reported. For example, a neural-logic-based battery manager can be commanded to determine a battery's state-of charge from a few measurements such as battery's temperature, voltage, and age. The manager can then be given the true coulomb-count history from a test, and then made to modify its parameters to reproduce test data precisely. Battery modeling has also progressed to the point where simple changes in battery chemistry can be analyzed, and their effect on battery performance can be predicted.

Nickel-Hydrogen Batteries with Longer Life

Satellites in Earth orbit are costly to build and launch, so long life in charge/discharge service is an important requirement of batteries that power the satellite's electronics whenever solar power is cut off by the Earth's shadow. NiCd batteries were adopted for this service in the early years of satellite development. Years of battery-life testing at the U.S. Navy's Surface Warfare Center at Crane, Indiana, showed that long lifetimes in low-Earth orbit required keeping the battery's depth of discharge under 20%.

In the 1970s Eagle-Picher Co. developed NiH batteries in which water is electrolyzed during charging, producing oxygen which combines with the nickel at the positive cathode, plus hydrogen which is stored as a high-pressure gas in the cell's case. The pressure-retaining case added weight, but the battery could store 65Wh/kg, and lifetimes up to 16,000 charge-discharge cycles were obtained with depths of discharge as high as 70%. As a result, the NiH battery became the choice for energy storage in low-Earth satellites where discharge could occur every few hours. Nickel-Hydrogen Battery Modeling

The NiH battery now has delivered

many years of service in low Earth-orbiting (LEO) spacecraft. Design improvements include substitution of double Zircar separators for the asbestos separators that were used in the earlier batteries. Many sophisticated performance and life tests have been conducted during qualification of these batteries for specific missions.

The engineer designing a satellite which needs a NiH battery for energy storage must determine the battery's operating parameters to assure that the required satellite lifetime will be achieved. These data can be obtained from life tests in which groups of cells are tested at various depths of discharge, and then at voltage and current limits during the phases of recharging. Such testing, even if accelerated, would consume a lot of time that may not be available during the design phase.

To aid the spacecraft designer, Robert A. Brown developed a model that accurately sizes a battery that meets given mission requirements or, alternatively, predicts the cycle life of a given battery size under the mission parameters ( 1 ). The designer can then specify system components, such as the battery-- charge controller that sets limits on battery voltage, based on battery temperature.

The model is based on data from life tests, such as the ongoing test of RNH 65-33 cells at the U.S. Navy Surface Warfare Center at Crane, Indiana. Cells in this test have reached 42,000 charge-discharge cycles at 60% depth of discharge (see Figure 1), and the test is continuing. Examples of other tests that provided Brown data for his model are listed in Figure 2.