What a change from a few short years ago! Nickel-hydrogen was king, and a few Nickel-cadmium power supplies were still going up into orbit because of their long and trustworthy performance (and the price was right when they were freebie leftovers from previous program inventories). Lithium-ion? That was the stuff for toys, laptops and cell phones. It had been thought that there was not enough power...not enough cycle life for the really tough space flight demands. Now, the picture has changed. It is November 2001 at the NASA Battery Workshop in friendly Huntsville, Alabama. Lithium-ion is no longer the poor cousin, but the new kid on the block, and it is getting its electrodes in everything from orbital to planetary missions. Of the 29 NASA Workshop presentations, 20 were either directly or indirectly focusing on Lithium-ion.
|A presentation on the X-38 Crew Return Vehicle was given by Eric Darcy of NASA Houston. The vehicle will employ primary Lithium batteries in the de-orbit propulsion, Nickel-metal hydride for in-cabin power and Nickel-cadmium for nose equipment power. Each chemistry was chosen to optimize the function. NASA Houston, which has prime design responsibility for the X-38, anticipates full qualification and delivery of batteries in 2002. +|
Last year, AEA Technology established a precedent by using commercial Sony hard carbon 18650 cells in large arrays; Lithium-ion flew as a parallel power source in the STRV micro satellite. (See BD 57-3.)
This year the AEA/Sony 18650 based battery established enough credibility to find itself being the lone provider of power aboard the European Space Agency PROBA satellite which was launched on October 22nd, 2001. It is the first Lithium-ion only satellite in space. PROBA, begun in February of 1998, was originally intended to be powered by Nickel-cadmium, but the availability of the 18650 Sonys provided an opportunity to build experience in the two year mission. The 1.5 Ah cells are connected in a series string of six cells. Six strings are then connected in parallel. Charge control is at the battery level only. A cell failure causes its connector to open, isolating that string but leaving the other strings with sufficient power for the satellite. From the lots of Sony cells received, only about 1% were rejected. Other Lithium-ion batteries have flown with NASA, powering cameras and computers on the Shuttle.
Why convert to Lithium-ion at this time? Three major reasons include: power, energy density and cycle life. Cycle life, when considering limited depth of discharge, is turning in stellar performance for both low earth orbit (LEO) and geosynchronous (GEO) missions. Dr. Yannick Borthomieu of Saft gave an update on the large VES 140 S cells (see BD 57-3) which are expected to provide 27 years of GEO performance with 3% fade and 20,000 LEO cycles with 12% fade. Qualification will be complete next year as planned on the Stentor flight in March of 2002. The 139 Wh of each cell will be combined in a module with 12 cells in parallel, and the battery will consist of 6 modules.
The energy density of Lithium-ion is almost three times better than Nickel-hydrogen with a maximum of 140 Wh/kg for Lithium-ion and only 50 Wh/kg for Nickel-hydrogen. It remains to be seen how great this available energy advantage is because of the trade-off in depth of discharge (DOD) with cycle and calendar life. Terrestrial Lithium-ion is usually categorized below 1,000 cycles because users fully deplete the batteries before recharging. In high cycle requirements such as LEO applications, the battery people limit the depth of discharge to 25-30% of capacity because lifetime cycle demands may be in the realm of 40,000 cycles. GEO orbit demands are less stringent so that 70% DOD can be considered. It may be that calendar life has a greater impact on GEO applications although capacity loss in 10 years is projected to be just over 6% at 15 0C and 11.4% at 25 0C.
In the realm of power density, acceptable performance is being demonstrated in both high reliability specialty space cells and off-the-shelf Sony 18650s. With power and cycle life providing acceptable performance, the clincher is gravimetric energy density; Lithium-ion does better than any other chemistry. Lithium-ions gravimetric energy density at 140 Wh/kg is almost tree times the nominal 50 Wh/kg of Nickel-hydrogen. Packing more Watt hours per pound on the launch pad means larger payloads or lower launch costs, either of which is a delectable selection for space gurus.
Not without appreciation is the wonderfully low self discharge characteristic of Lithium-ion with a few tenths of a percent per day compared to the 10% per day for Nickel-hydrogen or even the 1% per day for Nickel-cadmium. In circuitous reasoning, the low self discharge produces very low thermal dissipation, and the recharge ratio approaches 1.0. Both of these characteristics lead to lower launch pad costs.
The Lithium-ion application story was not one sided. Large cells are competing with small cells. LEO and GEO are both candidates. Manned and unmanned flight is cycling Lithium-ion power. Safety concerns are being met aggressively, and charging methods for better performance are unfolding. NASA has flown Lithium-ion cameras and computers on the shuttle in past years. Yet this space community, which has become an international family, recognizes that multiple concerns must be understood to make the chemistry suitable. Cycle life must be separated from calendar life; temperature capabilities must fit the mission requirements, and a bold but conservative track record of reliability and fault tolerance must be established to grow the applications in space.
While Lithium-ion receives the attention of a new movie starlet, the old, rugged Nickel-hydrogen still musters Workshop interest with 20 year deep data from Eagle Picher; they have a record of proven performance and reliability so stellar that it calms the concerns of investors placing their fortunes in nose cone mounted gadgets which must include power for a decade (or two) in the cold confines of space. Each month new launches such as EOS and Space Station include hardware with very well mannered Nickel-hydrogen which has established a successful record of performance. Such a record will set the bar for Lithium-ion in order for it to qualify for replacement status.
The reader can get a flavor of the topics presented from the following listings. Future issues will present materials from this conference in, as Toby would say, greater de-tail.
NASA Aerospace Battery Workshop Presentation Titles/Presenters
Battery Safety Testing by EV-ARC Calorimetry. Thermal Hazard Technology
Performance of Li-S Cells Under LEO Test Regime and at Low Temperature (to -40C). Moltech Corp.
Development of Li/MN02, NiMH and NiCd Battery Systems for the X-38. NASA Johnson Space Center
AEA Technologies Battery Cell By-pass Device Activation: An Update. QSS Group Inc. NASA Goddard SFC
Performance of Small, Commercial, Primary, Cylindrical Alkaline and Lithium Cells. BAE Systems
Life Test Results with Adaptive Charge Control. The Aerospace Corporation
A Dual Mode Lithium-Ion Battery Charge Controller. Eagle-Picher Technologies, LLC
Impact of Charge Methodology Upon the Performance of Lithium-ion Cells. JPL
EOS Battery Cell Life Test Update. TRW.
Single Pressure Vessel Life Test Update. Eagle-Picher Technologies, LLC
Method Used to Prevent Capacity Fade in Nickel-Hydrogen Batteries. Eagle-Picher Technologies, LLC
Package Design Concepts for Use in Small Satellite Applications. Eagle-Picher Technologies, LLC.
Space Station Nickel Hydrogen Startup and Performance. Boeing
Li-Ion Module Design for GEO and LEO Satellites. SAFT Defense and Space Div. Spec. Batt. Gp.
Calendar and Cycle Life Prediction of 100 Ah Lithium-ion Cells for Sp. Apps. Japan Storage Battery Co. Ltd.
SAFT Li-ion Cells GEO and LEO Life Test Update. Saft Specialty Battery Group
Evaluation of Cycle Life and Characterization of YTP 45 Ah Li-Ion Batt. used for EMU. Lockheed Martin, NASA
Update of Lithium-Ion Cell Evaluation. Lockheed Martin, NASA Goddard
Li-Ion DD Cells Space Application Cycling Update. Saft America
Simulated LEO Cycling of AEA-STRV Lithium-ion Battery Modules-2001 Update. TRW/AEA Technology
PROBA, The First ESA Spacecraft Flying Lithium-Ion. ESA-Estec/AEA Technology
DPA of 1.6 Ah Li-Ion Pouch Cells Using Coin Cells. U.S. Government
Perf. & Safety Testing of Cylind. Moli Lithium-Ion Cells. Lockheed Martin/Appl. Pwr Tech./NASA Johnson
Pulse Performance of Small Li-Ion Cells. COMDEV/NASA Johnson
Low Temperature and High Rate Performance of Lithium-Ion Systems for Space Applications. Lithion Inc.
Study of the Effects of Overdischarge on SONY 18650H Cells. ESA-ESTEC/AEA Technology
Perf. and Safety Tests on Samsung 18650 Li-Ion Cells with Two Capacities. Lockheed Martin/NASA Johnson
Thermal Modeling of Prismatic Lithium-Ion Cells. Mine Safety Appliances
Battery System Studies in a Virtual Prototyping Environment. University of South Carolina