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Testing High Power COTS Cells and Developing High Voltage Batteries for Space

Jeremy S. Neubauer*, Blake Cardwell†, and Jake Dembeck‡ ABSL Space Products, 2602 Clover Basin, Ste. D, Longmont, CO 80503

ABSL Space Products has been the leader in Lithium ion batteries for space applications since it delivered the first Lithium ion battery launched into Earth orbit. Since then, ABSL has received battery contracts for more than 100 spacecraft and launch vehicles, more than 50 of which have been launched. All have employed Commercial Off The Shelf (COTS) Lithium ion technology. In support of these applications, ABSL has qualified multiple high energy and high rate secondary and primary cells. It has also developed battery packages up to 150 Ah and 270 V using these cells. To stay at the forefront of the industry, ABSL is currently pursuing the space qualification of two new high power cells. In addition, ABSL is building upon its heritage high voltage battery designs to deliver larger and man rated high voltage systems. This paper will discuss ABSL’s ongoing testing to down-select and qualify its new high power cells for space applications, as well as the new high voltage space battery projects currently in progress.

I. Nomenclature Ah =Amp Hours BATS =Battery Analysis Thermal Software BEAST =Battery Electrical Analysis Simulation Tool BOC =Beginning of Charge BOD =Beginning of Discharge BOL =Beginning of Life C-rate =The value of current necessary to discharge a cell in one hour COTS =Commercial Off The Shelf DOD =Depth of Discharge EMF =Electro-motive Force (open circuit voltage) EOC =End of Charge EOD =End of Discharge EOL =End of Life IPmax =Current at Max Power LEO =Low Earth Orbit LIFE =Lithium ion Fade Evaluator Pmax =Max Power PTC =Positive Temperature Coefficient Polyswitch R =DC Resistance of a Cell SOC =State of Charge TVC =Thrust Vector Control

II. Introduction ATTERIES make up a significant portion of the dry mass of a spacecraft, and are mission critical items that provide electrical power to all systems during launch, early operation, and periods of solar array shadowing.

As space applications require lightweight technology due to the finite lift capacity of launch vehicles, all spacecraft

* Chief Engineer, 2602 Clover Basin, Ste D, AIAA Senior Member † Project Engineer, 2602 Clover Basin, Ste D ‡ Project Engineer, 2602 Clover Basin, Ste D, AIAA Member

B

7th International Energy Conversion Engineering Conference 2 - 5 August 2009, Denver, Colorado

AIAA 2009-4627

Copyright © 2009 by ABSL Space Products. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.

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components must be as light as possible. This has naturally driven the adoption of Lithium ion battery technology owing to its high specific energy.

Space applications are not only demanding of batteries with respect to performance levels, but also diverse in the nature of the requirements placed upon batteries. All missions typically expose the battery to extreme vibration and shock levels on launch and during spacecraft separation events, as well as harsh temperature cycling, radiation and vacuum conditions on orbit. However, with respect to performance, demands vary widely. In contrast to terrestrial applications, where batteries (and even the powered devices themselves) are replaced frequently, satellite applications typically must undergo thousands of charge/discharge cycles over mission durations on the order of five to ten years. Spacecraft battery voltages are typically 20 V to 100 V, with charge and discharge rates on the order of C/2 or less. On the other hand, launch vehicle applications can require extremely high voltage and high rate performance for much shorter durations, but still necessitating a high level of reliability. Different still, landers and other planetary applications can require operation at extremely cold or extremely hot temperatures after long dormant periods while in transit to their destination.

ABSL has historically been proactive in responding to these needs. Indeed, ABSL claims a host of “world’s firsts” when it comes to Lithium ion batteries in space, including having supplied the first Lithium ion batteries to orbit Earth, Mars, and Venus. This paper introduces recent and on-going efforts at ABSL to further advance the state-of-the-art in space batteries. This centers on building ABSL’s next generation cell suite, such that a broad range of chemistries are available to allow ABSL to provide an optimized battery for many applications. Specific examples of testing currently being performed for two high power cells under consideration are discussed at length. Finally, high voltage battery development is discussed with respect to past and current high voltage battery projects at ABSL, highlighting progress towards increased safety and modularity.

III. ABSL’s Approach to Building a Cell Suite The requirements for a battery can vary dramatically from mission to mission. A typical earth orbiting satellite

mission, for example, may demand operation from a battery for more than ten years and 60,000 charge-discharge cycles. On the other hand, a launch vehicle avionics battery may be operable for less than an hour, but demand a much higher rate of discharge. A planetary lander or rover may combine the need for operation at extreme temperatures with high mechanical robustness. To optimize battery performance for each of these different applications, a different cell is necessary. To this end, ABSL has established a program to continually identify, assess, and space qualify new high performance cells. Figure 1 shows the basic flow of this program.

Figure 1 ABSL’s Cell Identification, Assessment, and Qualification Process ABSL has an active R&D program to identify, assess and space qualify new COTS cell technologies that has

yielded five space qualified cells The first stage is the continually tracking available and upcoming commercial Lithium ion cells. Data is

collected and databased on these cells from multiple sources. Of particular advantage is ABSL’s relationships with select cell manufactures, providing access to data and samples of prototype commercial off the shelf (COTS) cells.

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From this database, currently containing details of over 140 cells, the most promising cells are down selected and procured for in-house Phase B testing. The testing regime implemented in Phase B is designed to address performance aspects critical to future space applications and identified risk areas, and, accordingly, varies from cell to cell. Once the in-house testing phase is complete, a decision to advance the cell to Phase C and fully qualify it for space use is made, based not only on cell performance, but also on the future needs of the space market as envisioned by ABSL and on the long term production plans of the cell’s manufacturer.

A cell’s Phase C space qualification program is built around ABSL’s proven standard qualification campaign. Five key areas are addressed in Phase C testing: basic parameters, safety, environmental robustness, cell characterization, and life testing. The tests contained therein are critical to ensuring safe, reliable, and predictable operation in the launch and space environment as part of an ABSL battery. For instance, results from such testing are employed to develop proprietary cell screening and matching algorithms used to identify and place cells in s-p batteries (where cells are first connected in series to form strings and build voltage, then multiple strings are connected in parallel to form a battery and build capacity), as well as to create detailed performance and degradation models used by ABSL’s in-house developed software (BEAST for short term performance modeling, LIFE for long term degradation predictions, and BATS for detailed thermal modeling) to accurately predict beginning of life (BOL) and end of life (EOL) performance.

To date this cell assessment and qualification program has yielded five space qualified COTS cells, three of which have flown in space, with others being actively marketed for space use (Table 1). The current cell suite is capable of meeting a broad range of space vehicle requirements, from the high reliability, long cycle requirements of many satellites, to the high power, short cycle life demands of many launch vehicle batteries, to extremely high energy, one use only needs of highly specialized programs. It is important to note, however, that the percentage of cells chosen by ABSL for space qualification is quite small relative to the number of available COTS cells. This is not only because of the high levels of performance and robustness demanded by space applications, but also because of the variability that exists between manufacturers in terms of build quality and consistency. However, investigating so many COTS cells for space applications has allowed ABSL to amass a library of ‘lessons learned’ that allow it to efficiently identify the best cell models. The qualified status of the ABSL cells, together with an increasing amount of space heritage being accrued, demonstrates that ABSL’s COTS approach works and encourages further expansion of the cell suite as the performance of commercial Lithium ion cell technology continues to progress.

Table 1 ABSL Cell Suite

Cell Chemistry Energy Density

Attributes Applications Status

ABSL 26650HC Cobalt Oxide 120 Wh/kg Excellent Cycle Life

All Space Programs

Space Qualified

Flight Proven

ABSL 18650HC Cobalt Oxide 130 Wh/kg Excellent & Well

Characterized Cycle Life

All Space Programs

Space Qualified

Flight Proven

ABSL 18650HR Mixed Metal Oxide

100 Wh/kg High Power Capability

High Power Programs

Space Qualified

Flight in 2009

ABSL 33111PR Sulfuryl Chloride

(non-rechargeable)

450 Wh/kg Extremely High Energy,

Excellent Structural Integrity

One Use Only Programs

Space Qualified

Flight Proven

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Figure 2 A Sampling of ABSL’s Space Qualified Cells

ABSL has space qualified several different COTS Lithium ion cells, including three different size formats (18650, 26650, and 33111), high rate and high energy

secondary chemistries, and a high energy primary chemistry.

IV. Testing Phase-B High Power Cells Currently ABSL has several cells undergoing in-house Phase-B testing, two of which are discussed herein.

These cells, both high power chemistries, are summarized in Table 2 below. The Phase-B testing of these cells has been prescribed to address electrical performance (most specifically at high rates), environmental tolerance (vibration, temperature, etc.), and cell safety, and is selectively discussed below.

Table 2 Phase-B High Power Cells

Cell Chemistry Anticipated Attributes Possible Applications

ABSL 18650NP Iron Phosphate High power, long cycle life, improved safety

High power programs; long life programs; manned missions

ABSL 18650BC Manganese Spinel High power, long cycle life High power programs; long life programs

A. Consistency Initial cell screening has shown significantly different consistency characteristics between these two cells, best

illustrated by comparing the low rate discharge capacity plots in Figures 3 and 4. The ABSL 18650BC cell shows two primary capacity groupings – one centered around 1.355 Ah and the other around 1.380 Ah. As all of the cells came from the same lot, it is not believed that this behavior is due to difference in manufacturing practices or changes in materials. Rather, it is thought that the appearance of these variations can be attributed to the fact that the entire batch has not been sampled (the set explored here is less than 20% of the manufacturer’s complete batch). For instance, if the nature of the manufacturing process is to produce cells with slowly increasing capacity throughout the production run, the witnessed trends could be explained by the inclusion of ~900 cells from the beginning of the run and ~100 cells from the end of the run. However, despite this behavior the standard deviation for the entire set is extremely low at only 8.7 mAh – approximately 0.6% of the average 1.357 Ah capacity value. Because of this, it is possible to accurately match the capacities of cells used in a string, as visualized by the right hand plot of Figure 3. For example, if one were to appropriate the cells of this batch for use in 28 V batteries based on matching discharge capacity alone, enforcement of a maximum per-string capacity spread of 5 mAh (<0.4% of average discharge capacity) could be implemented while still allowing 100% batch utilization.

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Figure 3 Discharge Capacity Data for the ABSL 18650BC Cell

ABSL 18650BC screening shows excellent overall consistency. This allows cells to be capacity matched to very

tight tolerances without waste.

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Figure 4 Discharge Capacity Data for the ABSL 18650NP Cell

The ABSL 18650NP shows similar consistency to that of the ABSL 18650BC cell, again allowing for close

matching within strings and little to no waste Results from the ABSL 18650NP screening also show excellent consistency, without the dual distribution

noticed for the ABSL 18650BC cell but with slightly larger overall spread. The standard deviation for this cell comes in at 11 mAh, 1.2% of the average 0.955 Ah discharge. Note that based on the percentages, the standard deviation is nearly double that of the ABSL 18650BC, but the overall value is so small that this difference is nearly inconsequential. Again, when sorted on discharge capacity the achievable per-string capacity variance can be minimized to nearly imperceptible levels while allowing near complete batch utilization.

B. High Rate Performance Rate characterization testing has been performed to assess the high power capabilities of each of these cells.

This has included both continuous and pulse discharges at temperatures ranging from -20° C to +40° C. Results for both cells are presented in Figures 5 and 6, where the max delivered power and total delivered capacity for each C-rate and temperature are plotted.

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Figure 5 Rate and Temperature Performance for the ABSL 18650BC Cell

The ABSL 18650BC has been proven capable of extremely high power discharges, but with available capacity decreasing significantly with increased rate and decreased temperature

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Figure 6 Rate and Temperature Performance for the ABSL 18650NP Cell

The ABSL 18650NP shows similar rate and temperature trends as does the ABSL 18650BC cell, but with lower overall power levels.

Note that for both cells the witnessed trends of power and capacity with temperature are similar. Performance at

20 and 40°C are nearly identical; however, at 0°C delivered power and capacity are much reduced, and at -20°C nearly no capacity is usable. This implies that the sensitivity of resistance to temperature of these two cells (of notably different chemistry) are similar, as supported by direct resistance calculations from both the continuous and pulse discharge tests. Trends with C-rate are also comparable between these two high power cells. Rate has little effect on each cell up to nearly 15C, after which delivered capacity falls steeply. Above 20C delivered capacity quickly approaches zero, as cell voltage drops to the cell minimum almost immediately upon initiation of the discharge.

Although the sensitivity of these cells to temperature and rate are similar, the total delivered power values are not. As can be seen in the above plots, the ABSL 18650BC cell delivered a maximum power pulse of over 100 W, while the ABSL 18650NP’s strongest delivered pulse was less than 60 W. These values represent the maximum achievable power while respecting the cell’s minimum recommended voltages.

If, however, it is acceptable to temporarily drop below these limits under moments of extreme power demand, higher outputs could be had. To quickly assess these capabilities, the amperage for the peak power point was forecasted using the previous data and two additional test cases for each cell was run – one at the predicted peak

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power point, and one at significantly higher amperage. The forecast was based on a simple Randles cell model, in which a Lithium ion cell can be reasonably modeled as a perfect voltage source in series with a resistor and a parallel resistor/capacitor circuit. For short pulses, the model can be reduced by removing the resistor/capacitor circuit, as the capacitor will effectively behave as a short circuit path. This leaves a simple series voltage source and resistor representation of the cell, as shown in Figure 7.

Figure 7 Simple Cell Models for Peak Power Prediction

Starting from a Randles cell model on the left, this can be reduced to a simple resistor / voltage source model of

a cell for short pulse operation The power delivered from the cell can then be calculated from Eq. (1):

(1) Then by setting the derivative of Eq. (1) with respect to current (I) to zero, the current at which peak power

occurs can be calculated via Eq. (2):

(2) Finally the max deliverable power from the cell can be calculated by combining Eqs. (1) and (2) to yield Eq. (3):

(3) The rate data up to 25C accrued above was processed to provide several points of delivered pulse power and

current where EMF and R were constant. Plotting these points as power versus current and fitting them with a second order polynomial yields Eq. (1) for each cell. This forecasts peak power levels of 136 W and 74 W for the ABSL 18650BC and ABSL 18650NP, respectively, both occurring at a rate of approximately 50C. It was therefore decided to test each cell at ~50C and ~70C. The ~50C cases produced peaks of 127.0 W and 75.5 W as shown below in Figure 8, corresponding to approximately 3000 W/kg for the ABSL 18650BC and 1900 W/kg for the ABSL 18650NP. Note that the correlation of the power prediction and curve fit for the 18650BC cell is excellent, illustrating the accuracy achievable in predicting the power curve of a cell from a few lower rate pulse data points. The same comparison for the 18650NP cell indicates a less accurate prediction, but the predicted peak power (74 W) and measure peak power (75.5 W) are still within 3% of each other.

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Figure 8 Power Comparison of the ABSL 18650BC and ABSL 18650NP Cells

Although both cells have shown extremely similar resistance characteristics, the ABSL 18650BC is capable of

much higher power deliveries thanks to its higher voltage level. The test points at ~70C additionally illustrated the current carrying limitations of each cell. After less than 30 seconds of operation at these rates, the cells failed open circuit. In both cases, no venting of high pressure gases, flame, or explosion occurred. In both cases it is suspected that the responsible failure mode is melting of internal electrical connections.

C. Cycle Life Cycle life is a critical aspect of battery performance for satellites. Not only is it necessary to characterize the rate

of capacity fade and resistance growth, but when using a passively balanced architecture it is further necessary to show that active cell management electronics can be omitted without negative effects on performance or safety.

To this end ABSL has initiated a life test program employing an 8s2p battery employing the high power Iron Phosphate chemistry. This battery is equipped with individual cell voltage monitoring and employs a different cell matching algorithm on each string – the first primarily favoring closely matched self discharge rates, while the other primarily favors closely matched capacities. Testing different matching algorithms is an important aspect of investigating new cells, as it not only provides evidence to how well passive balancing works, but how necessary it is and to what degree matching must be performed.

In the interest of exploring an exceptionally stressful set of conditions and accumulating information quickly, a high rate, high DOD cycling regime was selected. This entails charge and discharge rates substantially greater than 1C, which draws and replaces nearly 100% of the total battery capacity each cycle. A standard capacity

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measurement, in which the rates are reduced to C/10 to allow for an accurate assessment of total remaining battery capacity, is included at cycle 100, 1100, and every 1000 cycles thereafter.

The to-date capacity fade and resistance growth for this test are presented in Figure 8. It was anticipated that this cell would perform well in life cycle testing, as has certainly been proven the case. After 3100 cycles at high rate and near 100% DOD, capacity fade is approximately 12% and resistance growth has yet been negligible. Given the high rate and DOD of this test, as well as the off-nominal behavior being observed at the cell level discussed below, these results are beginning to evidence the excellent capacity retention capabilities of this chemistry.

Figure 8 Capacity and Resistance Histories for the 8s2p Life Test

Dispersion trends amongst the cells have also been monitored. Dispersion data for each string is plotted below,

Figure 9 showing dispersion for each string individually at EOC and Figure 10 at EOD. In each case, dispersion has been calculated as the difference in voltage between the highest and lowest voltage cells within each string. Several notable points are immediately obvious from these figures:

• For both strings, dispersion levels are low following extended stressful cycling. After 3,100 cycles,

Strings 1 and 2 show less than 10 and 25 mV of dispersion at EOC, respectively. No negative effects of dispersion are evident in the battery level performance metrics.

• String 1 performs better than string 2. In both EOC and EOD cases, and with respect to both magnitude and rate of dispersion growth, string 1 is showing better cell balance in the cycling performed to date. This might suggest that the different matching algorithms employed on the cells within each string are responsible for differences in performance.

• For both strings, EOC dispersion is lower than EOD dispersion. This is due primarily to the fact that

the cells employed have a steep drop in EMF at EOD. Therefore slight variations in SOC of a few percent or less can result in large voltage differences between cells when at or approaching 0% SOC. This point is evidenced by taking a closer look at dispersion characteristics throughout a full discharge (Figure 11), showing that dispersion is on the order of the EOC results for the majority of the cycle.

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Figure 9 Dispersion at EOC for the first 3100 cycles of the life test

After 3100 cycles of extremely stressful cycling, cell level EOC voltages show extremely low dispersion and a

slow rate of dispersion growth.

Figure 10 Dispersion at EOD for the first 3100 cycles of the life test

Dispersion is higher at EOD due largely to the steepness of the EMF vs. SOC curve at low voltages, causing

small differences in SOC to appear as large voltage differences.

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Figure 11 Dispersion throughout a standard capacity measurement

Data from an individual cycle shows that dispersion is typically only a few millivolts, but increases significantly at EOD

A closer look at the dispersion data in Figure 12, however, shows that there is more to the story. A review of

individual cell voltages shows that the high level of dispersion present in string 2 is predominantly due to one individual cell operating at lower than average voltage, both at EOC and at EOD. This would suggest that the low voltage cell possesses a higher than average self discharge rate, a point confirmed by review of the cell screening data. The effect is significant enough in fact to drive the cell to negative voltage during cycling. This data is valuable in multiple respects. First it has highlighted the need to modify the first generation screening and matching algorithms employed for this test. The review of cell level screening data and comparison with the above results has allowed ABSL to improve its screening and matching criteria for this cell to ensure that this particular phenomena will be prevented in the future. Second, as cycling continues this test case will provide data on the response of these cells to abusive overdischarge and overcharge conditions. Clearly one cell is already undergoing reversal; as this continues it is expected that this abuse will reduce the cell to little more than a resistor, operating at near zero volts. This will overcharge the remaining seven cells in the string near EOC, increasing degradation rates and likely dispersion rates, too. The response of this battery to such conditions will be reported in future publications.

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Figure 12 Cell voltages throughout a standard capacity measurement (after 3100 cycles)

This data shows that the high dispersion of string 2 can be attributed to one particular cell that actually reaches

negative voltages at EOD.

D. Continued Testing The testing completed to date on both cells is encouraging, but is only a small fraction of the total testing that

must be completed to confidently select the best cell for a given application. Furthermore, once the decision is made to qualify one of these cells for space use, a thorough qualification program must be conducted. This will include a broad range of tests assessing basic parameters, safety, environmental robustness, cell characterization, and life testing, each covering a comprehensive set of conditions. As discussed earlier, these efforts are critical to ensuring safe, reliable, and predictable operation in the launch and space environment.

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V. Employing Phase-B High Power Cells Historically at ABSL, such high power cells have been in demand primarily for high voltage applications. High

voltage batteries can benefit significantly from ABSL’s approach to building batteries, in which a large number of highly uniform small cells incorporating protection devices at cell-level are connected into an s-p cell topology. Note that this architecture is passive in nature – complex external cell protection and charge-balancing electronics, which can become quite an obstacle to meeting performance requirements when string length grows to 80 cells and beyond, are not included. This is made possible by the demonstrated ability of ABSL’s batteries to passively maintain balance, thanks to the quality of its cells, the employed cell matching algorithms, and battery level engineering techniques.

At the time of writing, ABSL has qualified more than thirty space battery designs using this approach. The benefits discussed above have been proven by an extensive flight heritage, currently encompassing the launch of 34 vehicles powered by ABSL and more than 10,000 cell years in space without failure. This includes both the first Lithium ion battery launched into Earth orbit, the longest operating Lithium ion powered spacecraft (Proba, which has accrued almost 7 years and more than 35,000 cycles in LEO), and multiple high voltage batteries as discussed below.

E. 344V High Power TVC Battery In the summer of 2005, ABSL was awarded the contract

to provide the suite of batteries for the South Korea Aerospace Research Institute’s (KARI) KSLV-1 launch vehicle. Over the following year and a half, ABSL not only supplied a set of ABSL 18650HC based 28 V batteries for use in avionics, activation, and flight termination systems, but also designed and qualified a 344V Lithium ion battery for the vehicle’s electro-hydraulic TVC system. This system, essential to the rocket’s in-flight stability and trajectory tracking, necessitated a high power, high voltage battery. As such, these requirements presented a challenging design problem – the combination of high required power and little available volume omitted the use of typical Cobalt oxide chemistries, the location of the battery demanded tolerance to a broad range of operating temperatures and harsh vibration levels, and the need to operate the battery during launch at such high voltages presented a high risk of corona discharge occurring during flight with possibly catastrophic results. Furthermore, ensuring safety during manufacture, test, and integration was critical due to the high voltage and power capabilities of the system. To solve this problem, ABSL qualified its first high rate Lithium ion cell (the ABSL 18650HR), developed proprietary high voltage space battery performance and safety technologies, and conducted a rigorous qualification program.

On early analysis of the battery requirements, it became clear that KARI’s TVC battery performance was driven by power demands. This was markedly different from the majority of ABSL’s previous space applications, in which energy and cycle life were the driving requirements. It quickly became clear that ABSL’s heritage cell, the ABSL 18650HC, was not the optimal choice – preliminary battery sizing exercises using alternative high power cells from ABSL’s COTS cell database showed the potential to reduce relative battery size by more than 30%. Therefore, to

Figure 13 ABSL 18650HR Cell Qualification

Test Flow

ABSL conducted a comprehensive space qualification program for its first high power Lithium ion cell.

Figure 14 KSLV-1 High Voltage Battery Qualification Testing

Individual components were verified free of corona

discharge prior to testing the final assembly.

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maximize the performance of the final product, ABSL selected the best suited high power cell from its database and initiated a space qualification program for it, later branding it the ABSL 18650HR. As with all of ABSL’s cells, a rigorous qualification program proved its quality and performance, incorporating cell characterization, environmental, safety, and endurance testing. The process strategically targeted the key risk areas ABSL identified for the TVC battery and similar space applications – vibration, thermal vacuum performance, and safety. Importantly, the cell’s ability to perform without active balancing electronics to the requirements of the mission was verified.

With the detailed cell knowledge acquired from the cell qualification program in hand, and ABSL’s standard simulation tools at the ready, achieving the optimal battery configuration was straightforward. The majority of the development effort therefore focused on the high voltage nature of the battery; specifically, ensuring safe ground handling of the product and protecting against corona discharge in-flight. Several innovative design features were developed and employed, including an arming mechanism that limits maximum voltage to less than 90V pre-flight for safe ground handling. Multiple methods were used to eliminate the risk of corona discharge, including special string and wiring routings, proprietary coatings, and careful adaptation of ABSL’s heritage battery architecture.

Of course, it was not merely assumed that these features were sufficient to provide complete corona protection. Rather, a comprehensive testing program was carried out to monitor for discharge events throughout a wide range of temperatures, pressures, and voltage levels up to 400 V. First, individual components were incrementally qualified, progressing from connectors and coated metal walls, to small battery assemblies. Finally, full scale performance tests were conducted on a flight like battery at multiple pressures and temperatures (from 0° C to 40° C), showing both that the battery could conduct the complete mission free of corona discharge events and maintain an adequate margin over the program’s minimum voltage requirement during the mission mandated high power operations.

Since qualifying the design and delivering the product to KARI in 2006, the first two qualified KSLV-1 modules have passed extensive system level testing in a flight like configuration. Subsequent sets of KSLV-1 TVC batteries (4 modules in total) have been built, acceptance tested, and delivered by ABSL as well. These will fly on the first and second KSLV-1 launches in late 2009, and have already paved the way for future ABSL high voltage Lithium ion space battery systems.

F. 328V MARES Battery for Manned Missions ABSL has built upon its TVC battery heritage to provide a man-rated

high voltage Lithium ion battery to power the Muscular Atrophy Research and Exercise System (MARES) aboard the International Space Station (ISS). MARES is essentially an exercise machine for astronauts on the international space station with electrically controlled resistance. Due to the high forces and torques present, a high power, high voltage (270V nominal) battery was deemed necessary to condition the ISS power supply and meet peak load requirements. ABSL became involved in MARES late in 2007 when the need to replace the baselined NiCd solution became evident. Due to the design and build status of the system, it was necessary that the physical and electrical interface of the battery be maintained. Providing such drop-in replacement solutions can be quite challenging, especially when changing cell chemistries, the safety requirements for inhabited ISS flight are present, and the battery is of the high voltage variety.

Although the combination of these requirements was daunting, ABSL was able to leverage its high voltage TVC battery design and delivered a solution based on its high power HR cell. Again utilizing the test data accumulated from the cell’s qualification program, the feasibility of the design to meet the project’s performance and safety requirements was proven to the customer. The safety of the final design – namely its compliance to NASA’s manned mission requirement of two fault tolerance to catastrophic failure – is currently under final review by NASA safety experts at the Johnson Space Center. This design incorporates many key features, some derived from past ABSL projects and others unique to the MARES battery solution:

• ABSL’s space qualified 18650HR high power Lithium ion cell • ABSL’s heritage cell topology, mounting, and interconnections

Figure 15 Muscular Atrophy Research and Exercise System

(MARES)

The 328 V battery provided for MARES by ABSL will be the first

high voltage Lithium ion battery on the International Space Station

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• Arming mechanisms derived from ABSL’s high voltage TVC battery, capable of reducing max voltages present during integration, launch, and return to maximize safety and protect against corona

• Unique safety features including redundant manual battery deactivation, autonomous thermally activated battery deactivation capability, a battery status indicator, and fused charge and discharge circuits.

Once operational, this battery will be the first high voltage Lithium ion battery aboard the International Space

Station. Having met the rigorous demands of providing a high voltage battery for such a safety conscious application, ABSL will be well positioned to employ the techniques and design features of MARES to future man rated high voltage batteries.

G. 340V / 680V Modular Ground Test Batteries In support of an exploratory study of a high power, high voltage system under consideration for the upcoming ARES launch vehicle, ABSL again grew its high voltage battery experience to provide a modular high power, high voltage ground test battery system. This application demanded a power source capable of delivering more power in a smaller package than was achievable with the ABSL 18650HR, along with a reconfigurable architecture that was capable of operating at both 340 V and 680 V. Although this project did not entail the requirement to space qualify the hardware, it did involve a challenging schedule necessitating the design, build, and test of eight high voltage batteries in approximately four months. The project was further complicated by the need to use two new high power cells. Ultimately, ABSL was able to meet all of these challenges by applying the many lessons learned from years of testing and employing COTS cells, extending its modular architecture used for dozens of space battery designs, and building upon its space qualified high voltage designs and safety procedures. Identifying and working with new cells was facilitated by ABSL’s already ongoing cell investigations. This not only allowed the selection of the ABSL 18650NP and ABSL 18650BC cells for this program, but also provided a starting point for expanded cell characterization testing. The efficient implementation of effective screening and matching processes for the ABSL 18650NP and ABSL 18650BC cells was enabled by ABSL’s experience of working with large quantities of COTS cells using derivatives of ABSL’s standard procedures for similar cells. The final battery design consisted of single string 340V modules, providing maximum flexibility to the end user who could configure multiple modules in series for increased voltage and/or parallel for increased capacity. The design of the individual modules focused on two critical aspects, the first of which was safety. Fuses and other safety features, much like those of the MARES and TVC batteries, were incorporated to enhance end user safety, while ABSL’s standard practices were refined and followed during build and test to ensure technician safety. The second focus area was design for manufacture, which was necessary to streamline the build process to meet the ambitious schedule targets. To this end increased modularity relative to a typical ABSL battery was utilized within each module to enable production line style build processes.

The timely delivery of these batteries allowed the customer to complete their testing on schedule, while representing the first hardware to be delivered from ABSL’s new Colorado facility. The construction techniques employed on this project that successfully enabled meeting such a demanding schedule while maintaining safety and performance levels are currently under consideration for use in ABSL’s space grade batteries.

Figure 16 340V / 680V Modular Ground Test

Batteries

ABSL’s high power ground test batteries utilized both the ABSL 18650NP and ABSL 18650BC cells along with a

highly modular architecture

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H. 330V High Energy Chariot Rover Battery The Chariot Rover is NASA’s next generation lunar rover concept vehicle (Figure 17). This manned, electric

powered, six-wheel-drive, six-wheel-steer vehicle employs an active suspension system to handle any lunar terrain, including 40 degree slopes (1). In route to meeting NASA’s performance targets, ABSL was contracted to develop and supply a high voltage battery for their ground test rover that could demonstrate an energy density of 150 Wh/kg or more.

To meet this energy density target, ABSL has called upon its top high energy cell from its cell watch program, the ABSL 18650JN. At the cell level, the ABSL 18650JN has demonstrated an energy density greater than 190 Wh/kg. Extensive testing has been performed on this cell over the past several years to characterize rate and temperatures sensitivity, cycle and storage life, safety, and environmental robustness. All test results have been encouraging. In addition, this cell is currently being employed for NASA’s Extravehicular Mobility Unit Long Life Battery (EMU LLB) at ABSL. Between these two concurrent programs, a large amount of additional testing at the cell, string, and battery levels is being performed – with a heavy focus on safety and fault tolerance for these two manned applications.

The test data accrued has been utilized to maximize the safety of the battery design. A preliminary BEAST model, assembled for the ABSL 18650JN cell from this data and capable of detailed electrical performance simulation, was initially employed to size the battery. To

meet the 18 kWh and 330V requirements, it was determined that an 80s28p system was necessary. As it was required to replace the older generation battery while using the same distributed envelope, this battery had to be divided into seven 80s4p modules. The design of each module utilizes features similar to that of the high voltage ground test batteries discussed above to stream line the manufacturing process.

With respect to safety, ABSL’s standard insulation and safety features have been included, but further considerations had to be made. For example, unlike the high power cells previously employed for high voltage applications, this high energy cell incorporates a Positive Temperature Coefficient Polyswitch (PTC) for short circuit protection, which has been known to fail when installed in high voltage batteries. Based on extensive PTC characterization testing of the ABSL 18650JN cell, a PTC protection system has been designed and incorporated which eliminates the risk of a PTC failing due to an overvoltage condition. In addition, due to the high expected environmental temperature and limited available thermal interface, the thermal performance of the battery has been addressed in detail. This has involved utilizing the preliminary BEAST model to predict cell level thermal dissipation, combined with a modified BATS finite element thermal model to assess cell temperature distributions within each module. Example results of this analysis are illustrated in Figure 15. Note that BEAST and BATS are both in-house developed ABSL software for high fidelity battery modeling and analysis. Both software packages have been written to readily allow the assessment of new cells as evidenced by this project.

Figure 17 NASA’s Chariot Rover

ABSL is currently supplying a set of high voltage, high energy density Lithium ion batteries to power future ground

testing of NASA’s Chariot Rover

American Institute of Aeronautics and Astronautics

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Figure 18 Chariot Rover Battery BEAST and BATS Analyses

ABSL has employed its BEAST and BATS software tools to complete a performance and safety analysis driven

design of the high voltage, high energy Chariot Rover battery .

VI. Summary This paper has briefly introduced ABSL’s efforts to assess and qualify new Lithium ion cells for space, while

detailing some of the testing currently ongoing for two high power cells currently under consideration. Results for both of the cells with respect to performance, safety, and environmental tolerance are encouraging. Further testing will continue to explore these areas; however, initiation of a full qualification program is pending an application in need of the unique performance that can be had from these cells.

ABSL’s experience with high voltage batteries has also been discussed, which has included ABSL’s first high voltage space qualified TVC battery, the man rated MARES battery due for the ISS, a reconfigurable systems of high voltage ground test batteries utilizing new high power cells, and the high energy Chariot Rover batteries. The progression of features and technologies included in each battery has been highlighted, showing increasing safety and modularity within each design. These characteristics have been pursued to broaden the applicability of such batteries to more manned applications while decreasing cost and cutting lead time.

VII. Works Cited 1. NASA's New Lunar Electric Rover (LER). NASA.gov. [Online] [Cited: May 5, 2009.] http://www.nasa.gov/exploration/home/LER.html.