Richard Ewell

LITHIUM BATTERIES FOR AEROSPACE APPLICATIONS: 2003 MARS EXPLORATION ROVER

Future NASA\ planetary exploration missions require batteries that can operate at extreme temperatures and with high specific energy and energy densities. Conventional aerospace rechargeable battery systems, such as Ni-Cd, Ni-H;! and Ag-Zn, are inadequate to meet these demands. Lithium ion rechargeable batteries are therefore being chosen as the baseline for these missions. The 2003 Mars Exploration Rover mission plans to deploy twin rovers onto Mars, with the objectives of understanding its geology, climate conditions and possibility of life on Mars. The spacecraft contain various batteries, i.e., primary batteries on the lander, thermal batteries on the back shell and rechargeable batteries on the Rovers. Significant among them are the Li ion rechargeable batteries, which are being utilized for the first time in a major NASA mission. The selection of the Li ion battery has been dictated by various factors, including mass and volume constraints, cycle life, and its ability to operate well at sub-zero temperatures (down to -30°C), at moderate rates. This paper describes the selection criteria, design and performance of the three battery systems on 2003 MER mission.

New materials and devices for thermoelectric applications

New high efficiency thermoelectric materials with ZT values substantially larger than 1.0 have been recently developed. The successful development of a segmented thermoelectric generator utilizing a combination of state-of-the-art thermoelectric materials and these novel thermoelectric materials could result in a high thermal-to-electric materials conversion efficiency over 19%. This is because the new thermoelectric materials have a higher average ZT value and the generator can be operated in a temperature range wider than possible with state-of-the-art (SOA) materials. Such improved power generators could be used for a variety of applications including waste heat recovery. This could also lead to the development of high performance thermoelectric generators operating in a relatively narrow temperature range for specific applications. However, there are also new low power and thermal management applications for small thermoelectric devices using SOA materials. If more efficient thermoelectric materials are incorporated, the performance of these new devices will be even higher.

Design and performance of the MER (Mars Exploration Rovers) solar arrays

The Mars Exploration Rovers (MER) program posed a significant engineering and technology challenge. Now that the Rovers have operated beyond their original design life of three months by nearly a factor of four it is clear that the challenge was met and far exceeded. A key to the success of MER has been the enhanced power provided by the cruise and Rover solar arrays. Benefiting from a nearly 50% improvement in cell efficiency compared to the single junction GaAs cells used on Pathfinder, the MER designs were subject to many constraints both in design and in operation. These constraints included limited available panel area, changing illumination levels and temperatures, and variable shadowing, atmospheric conditions and dust accumulation for the rovers. This paper will discuss those constraints and their impact on the design. In addition, flight data will be provided to assess the performance achieved during the mission.

An Update on the Performance of Li -Ion Rechargeable Batteries on Mars Rovers

NASA’s Mars Rovers, Spirit and Opportunity have been exploring the surface of Mars for the last thirty months, far exceeding the primary mission life of three months, performing astounding geological studies to examine the habitability of Mars. Such an extended mission life may be attributed to impressive performances of several subsystems, including power subsystem components, i.e., solar array and batteries. The novelty and challenge for this mission in terms of energy storage is the use of lithium-ion batteries, for the first time in a major NASA mission, for keeping the rover electronics warm, and supporting nighttime experimentation and communications. The use of Li-ion batteries has considerably enhanced or even enabled these rovers, by providing greater mass and volume allocations for the payload and wider range of operating temperatures for the power subsystem and thus reduced thermal management. After about 800 days of exploration, there is only marginal change in the end-of discharge (EOD) voltages of the batteries or in their capacities, as estimated from in-flight voltage data and corroborated by ground testing of prototype batteries. Enabled by such impressive durability from the Li-ion batteries, both from a cycling and calendar life stand point, these rovers are poised to extend their exploration well beyond 1000 sols, though other components have started showing signs of decay. In this paper, we will update the performance characteristics of these batteries on both Spirit and Opportunity.

Potentiostatic Depassivation of Lithium-Sulfur Dioxide Batteries on Mars Exploration Rovers

NASAs 2003 Mars Exploration Rovers, Spirit and Opportunity, have been performing exciting surface exploration studies for the past 3 years, providing conclusive evidence for the presence of past water on Mars. Although the rovers are being powered by Li-ion batteries and solar arrays, their critical entry, descent, and landing (EDL) maneuvers were successfully supported by primary lithium-sulfur dioxide batteries. These batteries exhibited voltage delay at the end of cruise, which necessitated a depassivation of these batteries prior to EDL. In the absence of conventional depassivation across a specified load, a new method of depassivation via potentiostatic discharge was employed for the mission. Several simulation tests were performed on cells, cell strings, and battery assemblies, at different potentiostatic voltages and durations to characterize the depassivation process. Effects of repassivation of the lithium anode subsequent to depassivation were also studied, mainly to establish the timeline necessary for depassivation prior to use during the EDL process. Finally, a phenomenological model was developed for the potentiostatic depassivation of Li-SO 2 cells, based on dielectric properties of the surface film on Li, which gave voltage predictions in quantitative agreement with the experimental data. The laboratory results obtained were subsequently corroborated by the in-flight data received from the spacecraft.

Solar array model corrections from Mars Pathfinder Lander data

The MESUR solar array power model for the Mars Pathfinder spacecraft initially assumed values for input variables. After landing, early surface variables such as array tilt and azimuth or early environmental variables such as array temperature can be corrected. Correction of later environmental variables such as tau versus time, spectral shift, dust deposition and UV darkening is dependent upon time, on-board science instruments and the ability to separate effects of variables. Engineering estimates had to be made for additional shadow losses and Voc sensor temperature corrections. Some variations had not been expected such as tau versus time of day and spectral shift versus time of day. Additions needed to the model are thermal mass of lander petal and correction between Voc sensor and temperature sensor. Conclusions are: the model works well; good battery predictions are difficult; inclusion of Isc and Voc sensors was valuable; and the IMP and MAE science experiments greatly assisted the data analysis and model correction.

Performance characteristics of lithium-ion cells for Mars sample return Athena Rover

Future planetary exploration missions, especially Landers and Rovers, will utilize lithium ion rechargeable batteries, due to their advantages of reduced mass and volume compared with nonlithium systems. In addition to the usual requirements of high specific energy and energy density, some applications, e.g., Mars Landers and Rovers, require the batteries to be functional over wide range of temperatures, i.e., -20 to +40/spl deg/C. Several prototypes with modified chemistries commensurate with these mission needs were built by US Battery manufacturers, in conjunction with a NASA-DoD Interagency developmental effort and are being tested at JPL in the last couple of years. The proposed Mars Rover will have three lithium ion batteries (with one serving the purpose of redundancy) connected in parallel, each with four 6-9 Ah cells in series, to augment the solar array. The charger for these Rover batteries is being designed and built in-house. In this paper, we present several performance characterization tests, including cycle life at different temperatures, rate capability at various charge/discharge rates and temperatures and real time and accelerated storage, which were carried out on these prototype cells in support of the Mars exploration missions.

Progress on the SP‐100 Power Conversion Subsystem

The SP‐100 Space Reactor Power System converts reactor heat to electricity by thermoelectric conversion, a static method proven in many long‐lived spacecraft such as Voyager. For SP‐100, the specific power of the thermoelectric converter must be greatly increased over existing technology. Non‐useful temperature drops need to be minimized to reduce the mass of the heat rejection subsystem. Both these objectives can be achieved from thermoelectric converters by conductively coupling the active material to the heat sources and sinks. This coupling introduces very high stresses within the thermoelectric elements due to the very difficult thermal environment. The design of the SP‐100 power conversion subsystem was largely determined by the need to accommodate these stresses. This paper summarizes the critical technologies needed to achieve the needed increase, and how these technologies were developed. The remaining technical issues are also summarized.

Thermoelectric Devices and Diamond Films for Temperature Control of High Density Electronic Circuits

The increased speeds of integrated circuits is accompanied by increased power levels and the need to package the IC chips very close together. Combined, these spell very high power densities and severe thermal problems at the package level. Conventional packaging materials have difficulty dealing with these thermal management problems. However, it is possible to combine both active and passive cooling by using thin film bismuth‐telluride thermoelectric coolers (microcoolers) and diamond substrates for the temperature control of these high density electronic circuits. The highest power components would be mounted directly onto thin film thermoelectric elements, which would maintain the temperature of these components from a few degrees to tens of degrees below that of the diamond substrate. This allows these components to operate within their required temperature range, effectively manage temperature spikes and junction temperatures, and increase clockspeed. To optimize the design of the thermoelectric coo...

Performance Testing of Lithium Li-ion Cells and Batteries in Support of JPL's 2003 Mars Exploration Rover Mission

In early 2004, JPL successfully landed two Rovers, named Spirit and Opportunity, on the surface of Mars after traveling > 300 million miles over a 6-7 month period. In order to operate for extended duration on the surface of Mars, both Rovers are equipped with rechargeable Lithium-ion batteries, which were designed to aid in the launch, correct anomalies during cruise, and support surface operations in conjunction with a triple-junction deployable solar arrays. The requirements of the Lithium-ion battery include the ability to provide power at least 90 sols on the surface of Mars, operate over a wide temperature range (-20(super 0)C to +40(super 0)C), withstand long storage periods (e.g., including pre-launch and cruise period), operate in an inverted position, and support high currents (e.g., firing pyro events). In order to determine the inability of meeting these requirements, ground testing was performed on a Rover Battery Assembly Unit RBAU), consisting of two 8-cell 8 Ah lithium-ion batteries connected in parallel. The RBAU upon which the performance testing was performed is nearly identical to the batteries incorporated into the two Rovers currently on Mars. The primary focus of this paper is to communicate the latest results regarding Mars surface operation mission simulation testing, as well as, the corresponding performance capacity loss and impedance characteristics as a function of temperature and life. As will be discussed, the lithium-ion batteries (fabricated by Yardney Technical Products, Inc.) have been demonstrated to far exceed the requirements defined by the mission, being able to support the operation of the rovers for over three years, and are projected to support an even further extended mission.