Battery Testing and Assessment to Promote Mission Assurance

Valerie Ang, Boyd Carter, Warren Hwang, Margot Wasz, and Albert Zimmerman

Aerospace expertise in diverse cell chemistries and technologies has helped resolve numerous issues with launch and space system batteries.

Batteries are used on all space and launch vehicles. These batteries consist of a number of cells, usually connected in series, and sensors to monitor conditions such as voltage and temperature. Space systems impose stringent requirements concerning mass, stability, reliability, and performance that typically necessitate batteries that are specifically designed for space applications. For example, requirements for high energy density favor cell chemistries with highly reactive electrochemical electrodes that can provide tremendous electrical power on demand; however, these cells degrade over time, and their highly reactive, high-surface-area electrodes can participate in a number of side reactions and processes. Performance requirements, such as 20 years of service life and 50,000 charge/discharge cycles, stretch the capabilities of the cell technologies and designs. Cells and batteries also need to meet high safety standards, remain stable through long storage periods, and withstand the stressful launch environment. Batteries for space applications are specially designed and tested to meet these multiple requirements. For space vehicles, nickel hydrogen (NiH₂), nickel cadmium (NiCd), and lithium ion (Li-ion) are the technologies now in use or under consideration. For launch vehicles, silver-oxide–zinc (AgZn) batteries have traditionally been used, and Li-ion technology is being considered.

Battery tests and test assessments at Aerospace have supported mission assurance by helping to define requirements and verify that selected batteries can meet them. These tests—at the battery, cell, and cell-component levels—have involved multiple programs and suppliers and provided critical information for both generic and specific battery types. They have shown that large decreases in battery performance can often be traced to issues with cell components, such as the positive electrode, the negative electrode, the separator between the electrodes, and the electrolyte that transports charge during cell operation.

Nickel Hydrogen

In the 1980s, the Air Force began its Nickel-Hydrogen Cell-Test Program, an extensive effort to life-test several new cell designs. Aerospace initiated the program concept and provided the planning, technical oversight, analysis, and assessments for the tests. Still active, the program has provided long-term low-Earth-orbit life-test data for NiH₂ cells, with variations in cell vendors, cell designs per vendor, temperature, depth of discharge, and storage. Results have shown how these variables affect battery life and have provided the basis for mission life verification for several programs. The present testing is providing more extensive life data for specific variables and is helping improve the fidelity of NiH₂ life performance models.

Early research and testing at Aerospace helped clarify the cause of permanent loss of capacity in NiH₂ batteries during prelaunch storage. At that time, the batteries were made from cells that maintained an excess of hydrogen gas upon complete discharge (a condition known as hydrogen precharge). The capacity loss was caused by slow reactions of the hydrogen gas that predominate at low cell voltages. This understanding led to changes in cell designs, and all NiH₂ cells are now made with an excess of nickel rather than an excess of hydrogen upon complete discharge. This change has dramatically reduced the number of batteries in space programs exhibiting loss of capacity as a result of hydrogen precharge.

Still, the excess nickel in NiH₂ can decrease with time, and cells originally made with nickel precharge can acquire a hydrogen precharge, with its associated capacity loss. Aerospace characterized the different voltage signatures for cells with the two types of precharge and used this information to develop a simple, nondestructive test that is widely used to monitor battery cells prior to launch. The test entails completely discharging the battery at the launch site and monitoring the recovery of the open-circuit cell voltage. The voltage signature indicates whether the cells in the battery still have nickel precharge.

The first NiH₂ cells used in space were designed with separators made of asbestos, a material used in fuel cells. Aerospace tests, using a unique gas-sampling apparatus, detected chemical contamination from the asbestos, which had been stored under uncontrolled conditions. This contamination reduced usable capacity over time. This finding helped to promote better cell designs that used a separator made from zirconium oxide, a material that does not exhibit the contamination problem and that retains electrolyte more readily; this material is now used by all space vehicles that use NiH₂.

Liquid electrolyte in the central electrolyte stack in NiH₂ cells can condense on the colder cell walls during normal operation. In ground tests, this liquid would collect at the cell bottom, but in space, it would remain along the wall. If the electrolyte freezes on the wall, or if the average condensation rate away from the stack is greater than average electrolyte return rate along the wall wick (a structure designed to return liquid from the wall to the stack), then the cell can suffer from insufficient electrolyte. Aerospace developed a unique, nondestructive acoustic method to detect liquid levels at the bottom of NiH₂ cells during ground testing. The method was used by a program that had experienced a battery failure during testing. The method showed that the failure was the result of improper test design and that modifications in thermal control would avoid any problem in space. The method is now being used on internal tests of other programs.

A nondestructive acoustic method was developed to detect liquid levels at the bottom of NiH₂ cells during ground testing. In one case, the method showed that the cause of an external test failure was improper test design and that modifications in thermal control would avoid any problem in space.

Nickel Cadmium

A life-test program for NiCd batteries—similar to the Nickel-Hydrogen Cell-Test Program—was initiated and overseen by Aerospace for design variations in NiCd technology. Assessments of results identified designs with sufficient performance life and aided the selection of cells for several space-vehicle programs. In cases where the mission durations were short and program managers wanted to use commercial NiCd cells, Aerospace tests were used to establish qualification and acceptance tests and to verify life capability. Results from internal tests and assessments of external tests were also used to help Air Force Range Safety develop guidelines for using commercial NiCd batteries on space and launch vehicles.

Aerospace has also developed a unique destructive physical analysis to assess the condition of stored sample cells from a flight lot. The process entails disassembling the cells in oxygen-free conditions; the amount of active material in various oxidation states of the electrodes can then be electrochemically quantified. Results from this type of test have been used to help assess shelf-life extensions for space vehicle programs that use NiCd batteries.

In one test program, latent leaks were discovered in the fill tubes of NiCd cells. These flight units had been stored for several years and had passed periodic leak tests. Aerospace guided and closely coordinated the root cause investigations, which involved three programs and two prime contractors. X rays and destructive physical analyses of a large number of cells indicated that the leaks were caused by the propagation of cracks in the fill tube used to introduce electrolyte into the cell during manufacturing. The cracks were reaching large internal voids in the weld region and connecting them to the outside. The voids were caused by electrolyte left in the fill tubes before their pinch-off closure and weld. Based on these tests results, Aerospace helped develop an x-ray procedure for screening cells for potential leaks; thanks to this procedure, all three programs were able to use the batteries successfully.

Latent leaks were discovered in the fill tubes of NiCd cells in flight batteries that had been stored for several years. X rays and destructive physical analyses revealed that the leaks were caused by the propagation of cracks in the fill tube. These cracks were reaching large internal voids in the weld region, caused by electrolyte left in the fill tubes prior to their pinch-off closure and weld. Based on these tests, Aerospace developed a way to screen for potential cell leaks.

Silver-Oxide Zinc

The AgZn battery has been used in all launch vehicle programs since the 1950s because it is robust, energy dense, and capable of providing high currents at a low system weight. Its common failure modes, such as internal shorting and electrolyte spewing, are well known and understood—as are the more unusual failure modes, such as the sensitivity of the cellophane separator to sugar crystals or the relationship between current capability and cell compression. Aerospace has captured many of these "lessons learned" in technical reports and has also used them to develop a menu-driven software program that can diagnose the most likely causes of abnormal battery behavior. Developed for NASA, this expert system has proven highly effective in responding to battery failures close to launch.

For example, a week before launch, a low capacity was found in a coupon cell (a sample cell from the production lot) associated with the flight hardware. The expert system was used to identify a likely cause: direct exposure to air after electrolyte was added to the battery cells. The contractor then found that the coupon cells for a follow-on flight were being built without the necessary valves to prevent exposure to air. Aerospace conducted a quick physical analysis of the low-capacity cells and produced evidence consistent with the scenario. The flight batteries were approved for use once it was verified that processing procedures would not permit a similar mistake on the flight hardware, and the mission was successfully flown on schedule.

Similarly, low performance of another battery being processed for flight was also evaluated and found to be caused by a current collector that was thinner than in previous builds of the same design. The current collector in these batteries was a highly conductive grid used inside the electrode plates to reduce resistance and provide mechanical strength. The relationship between the thickness of the grid and conductivity was confirmed using a four-point probe, and the impact on voltage during discharge was found to be consistent with the performance losses seen on the flight batteries. That mission also flew on time and without anomaly with batteries produced at the same time as the suspect hardware because Aerospace's evaluation confirmed that the condition was screenable by test.

The open-circuit voltage of AgZn batteries in launch vehicles is closely monitored through the umbilical connection to the ground support equipment to detect problems prior to launch. In one instance, unexpected voltage trends for a new battery design and new vehicle caused concern from range safety about possible leakage currents and paths caused by an unknown mechanism with unpredictable consequences. Internal tests and analyses demonstrated that the phenomenon was the result of chemical phase realignment at the current collectors, caused by the connection loads from other active electronic devices on the electrical bus. Because these loads were so small—on the order of microamps—they were not simulated during qualification testing. Using precision test equipment capable of applying loads in the nanoamp range, the connection loads were applied to a variety of test cells under different states of charge to reproduce the effect. Then, mission loads were simulated to demonstrate that cell performance was still acceptable. These results provided confidence in the new flight designs and reduced concerns about insufficient remaining battery capacity.

Lithium Ion

Safety of space Li-ion cells and batteries is a concern that needs to be addressed for mission assurance. The safety issues can vary according to the design of the cell, battery, and control electronics. When the lack of a safety demonstration caused concern for one of the first U.S. missions to use Li-ion batteries, testing of the cells under conditions that enveloped worst-case safety conditions was performed. The tests were quickly designed and carried out to assess worst-case discharge, overcharge, and temperature. The results, which demonstrated safety under worst-case conditions, were presented to range personnel in time to avoid any launch schedule slip. Those results, along with results from other safety tests of performance limits, are providing an expanding database for mission assurance questions of this new space technology.

Conclusion

Battery tests and test assessments at Aerospace have supported mission assurance for all space battery technologies used in high-reliability space and launch vehicle applications. Results have led to improvements in technologies, procedures, testing, and reliability. In a number of cases, results have allowed the maintenance of schedule in high-reliability programs.

Acknowledgment

The support from space and launch vehicle programs, Mission Oriented Investigation and Experimentation, and Independent Research and Development is very gratefully acknowledged.