I 

NASNTM-2005-2 1391 6Nersion 1 .O 

NESC-RP-04-08/04-050-E 

J 

Technical Consultation of the Hubble Space 

Telescope (HST) Nickel Hydrogen (NiH2) 

Battery Charge Capacity Prediction 

Steven J. Gentz 

NASA Langley Research Center, Hampton, Virginia 

Radha “Chetty ” Pandipati 

NASA Goddard Space Flight Center, Greenbelt, Maryland 

Jerri Ling and Thomas Miller 

NASA Glenn Research Center, Cleveland, Ohio 

Judith Jeevarajan 

NASA Johnson Space Center, Houston, Texas 

Gerald Halpert 

Jet Propulsion Laboratory, Pasadena, California 

Albert Zimmerman 

The Aerospace Corporation, El Segundo, California 

I 

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NASNTM-2005-2 139 1 G/Version 1 .O 

NESC-R.P-O4-08/04-05O-E 

Technical Consultation of the Hubble Space 

Telescope (HST) Nickel Hydrogen (NiH2) 

Battery Charge Capacity Prediction 

Steven J. Gentz 

NASA Langley Research Center, Hampton, Virginia 

Radha “Chetty ” Pandipati 

NASA Goddard Space Flight Center, Green belt, Maryland 

Jerri Ling and Thomas Miller 

NASA Glenn Research Center, Cleveland, Ohio 

Judith Jeevarajan 

NASA Johnson Space Center, Houston, Texas 

Gerald Halpert 

Jet Propulsion Laboratory, Pasadena, California 

Albert Zimmerman 

The Aerospace Corporation, El Segundo, California 

National Aeronautics and 

Space Administration 

Langley Research Center 

Hampton, Virginia 2368 1-2 199 

September 2005 
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Technical Consultation of the Hubble Space Telescope (HST) 

Nickel Hydrogen (NiH2) Battery Charge Capacity Prediction 

1 of49 

Technical Consultation 

of the 

Hubble Space Telescope (HST) 

Nickel Hydrogen (NiH2) Battery Charge Capacity Prediction 

June 17,2004 

Prepared by: 

Steven J. Gentz 

NESC Principle Engineer 
th NASA Engineering and Safety Center 

Consultation Position Paper 

W 

Document AI Version # 

RP-04-08 0.01 

Signature Page 

~ 

Title: 

Technical Consultation of the Hubble Space Telescope (HST) 

Nickel Hydrogen (NiH2) Battery Charge Capacity Prediction 

NESC Independent Technical Evaluation Team 

Original Signu fures on j?ie 

Page #: 

2 of49 

Steven Gentz, NESC Lead, Principle Engineer 

Jerri Ling, S&MA Representative, GRC 

Gerald Halpert, JPL/QSS 

Chetty R. K. Pandipati, GSFC/QSS 

Albert Zimmerman, Aerospace Corp. 

Judith Jeevarajan, JSC 

Thomas Miller, GRC 
EXECUTIVE SUMMARY 

A multi-Center and industry review team of knowledgeable Nickel Hydrogen (NiH2) battery 

specialists was convened to assess the Goddard Space Flight Center (GSFC) position paper 

entitled, “The State of the HST Batteries as it Relates to HST Health, Safety, and Ability to 

Support an On-going Science Operation”, dated May 21,2004 (provided in Appendix D) [ref. 11. 

This document was prepared by Dr. J. Keith Kalinowski, Deputy Manager, Hubble Space 

Telescope (HST) Operations Project, with planned approval by Preston M. Burch, Manager, 

HST Program Office. The document review was supplemented with extensive technical 

discussions with the position paper author, pertinent reports from literature, NASA-sponsored 

Aerospace Battery Workshops, and consultations with team peers. 

In the course of the assessment, it was recognized that the HST battery thermal control system 

has an average heat dissipation limitation of 30 W per bay per orbit cycle. This thermal 

constraint will continue to govern options for battery capacity maintenance. In addition, the HST 

usage represents the longest exposure of NiH2 batteries to Low Earth Orbit (LEO) at the current 

level of Depth of Discharge (DOD). Finally, the current battery life is at the limit predicted by 

the manufacturer, Eaglepicher. Therefore, given these factors, the potential exists that the HST 

battery capacities could radically degrade at any point. 

Given this caveat on any life extrapolations, the conservative model proposed in the GSFC 

position paper was viewed by the NASA Engineering and Safety Center (NESC) assessment 

teams as having several technical assumptions such as limited utilization of flight battery 

capacity data, the susceptibility of the proposed prediction method to large variations when 

supplemented with additional information, and the failure to qualitatively or quantitatively assess 

life prediction sensitivities. Additional life prediction models to those cited in the GSFC position 

paper were reviewed with the Hollandsworthl Armantrout model being viewed as having the 

greatest likelihood of estimating fhture HST battery capacities. 

It was also observed in the course of the assessment that the battery charge control strategy might 

not have been optimized to allow the batteries to reach their maximum charge state. Options 

appear to exist to improve or maintain overall battery capacity by maximizing the charge current 

and raising the temperature compensated voltage level (V/T) levels to improve the charge 

efficiency without exceeding the indicated thermal dissipation limit. Enhancements for 

decreasing cell divergences also appear possible by alterations to the capacity test procedures. 

In summary, it is recommended the identified conservative assumptions of the GSFC proposed 

model be addressed and the HollandswortWArmantrout model be enhanced to augment the life 

predictions for the HST batteries. It is also recommended the current HST battery charge control 

management strategy be revised to maximize both the charge efficiency and the capacity test 

depth and rate of discharge. 
Title: Page #: 

Technical Consultation of the Hubble Space Telescope (HST) 4 of49 

Nickel Hydrogen (NiH2) Battery Charge Capacity Prediction 

Request Number #: 04-050-E 

Requestor Name: William F. Townsend Requestor Contact Info: William.F.Townsend@nasa.gov 

301-286-5066 

Short Title: HST NiH2 Battery Charge Capacity Prediction 

Description: The paper examines the viability of the HST with respect to the NiH2 continued battery charge 

Date Received: May 25,2004 

Lead Assigned: Steven Gentz 

Date Consultation Initiated: May 25,2004 

Lead Contact Info: Steven.J.gentz@,nasa.gov 

Consultation Team Members 

Last 

Name First Name Center Office Number Email 

Gentz Steven LaRC 757-864-9263 Steven. J.GentzGiJnasa.go\r 

Ling' Jerri GRC 2 1 6-43 3-284 1 Jerri.S.Ling@nasa.gov 

Bugga Ratnakumar JPL 818-354-01 10 Ratnakumar.V.Bug,Irra(al,iDl.nasa.gov 

Gonzalez Guillermo 

Halpert Gerald 

Jeevarajan Judith 

Manzo 

Miller 

Pandipati 

Michelle 

Thomas 

Chetty 

Zimmerman Albert 

Fullen Pamela 

Lodding Deborah 

LaRC 

JPL/QSS 

JSC 

GRC 

757-864-7107 

8 18-354-5474 

281-483-4528 

216-433-5261 

GRC 2 16-433-6300 

GSFC/QSS 301-286-6338 

3 10-336-741 5 Aerospace 

Corporation 

LaRC 

LaRC 

757-864-4403 

757-864-2203 

Gui llcrcmo. A.Gonzalezb,nasa. gov 

Gcrald.HalpcrtQipl.nasa.gov 

Judith .A. Jcevarajan(2nasa.gov 

Michcllc.A.Manzo(ianasa.,Irrov 

Th0mas.B. MillerG$nasa.gov 

Radha.K.Pandinati@,mfc.nasa.gov 

Albert.H.Zimmerman(uZaero.org 

Pamcla.B.Fullcn@nnasa.gov 

D. S.Lodding(24Iarc.nasa.gov 

GSFC 30 -286-3369 J .Kcith.Kalinowski~~nasa. gov 

Kalinowski2 Keith 

Kichak3 Robert GSFC 301-286-1 199 Robcrt. A.Kkhak@,nasa.gov 

'S&MA Representative 

HST Liaison 

3NESC Discipline Expert - Power & Avionics 

2 
Technical Consultation of the Hubble Space Telescope (HST) 

Nickel Hydrogen (NQ) Battery Charge Capacity Prediction 

In addition to outstanding support provided by the assessment team and the technical exchange 

with Dr. Keith Kalinowski, the rapid response to this request could not have been accomplished 

without the exemplary services of the NESC Management and Technical Support Office 

(specifically Ms. Pam Fullen and Ms. Debby Lodding) and Mr. Hugh Baker, JSC Contracting 

Officer Technical Representative for Aerospace Corporation contract. 

5 of49 

Consultation Approach 

The purpose of the GSFC position paper is to identifl critical HST milestone dates for continued 

science studies followed by the attachment of a re-entry module or a robotic servicing mission. 

The paper examines the viability of the HST with respect to the NiH2 continued battery charge 

capacity. 

The NESC conducted an independent evaluation of the supporting information and assumptions 

to generate the predictions for battery capacity loss and practicality of on-orbit battery 

conditioning. The following are the major activities of the NESC assessment: 

Milestone 

Initial Teleconference 

Review Team Identification 

Position Paper and Support Documentation Review 

Date 

May 27,2004 

June 1,2004 

June 4,2004 

Technical Interchange Meeting June 6-7,2004 

Preliminary Findings Presentation to NESC Review Board (NEU3) June 10,2004 

Final Report Submission June 17,2004 

Data Reviewed 

Mr. William F. Townsend, GSFC Deputy Director, contacted Mr. Ralph Roe, NESC Director, 

concerning an independent assessment of a review copy entitled, “The State of the HST Batteries 

as it Relates to HST Health, Safety, and Ability to Support an On-going Science Operation”, 

dated May 21,2004. This document was prepared by Dr. J. Keith Kalinowski, Deputy Manager, 

HST Operations Project, with planned approval by Preston M. Burch, Manager, HST Program 

Office. 
FindingdObservationdRecommendations 

Following are three main areas pertaining to System Aspects, Life Prediction MethodoZogy, and 

Battery Charge Control Management. The information is intended to address potential system 

issues, the most accurate life prediction models, and methods to improve the capacity and life of 

the HST batteries. The recommendations are sequentially numbered to aid discussion. More 

detailed descriptions of the recommendations are provided in Appendix A. Note that these 

recommendations are suggested actions to the program and do not constitute NESC 

requirements. 

1. SYSTEM ASPECTS 

ObservationdFindings 

0 Battery thermal control system has an orbital average heat dissipation limitation of 30 W 

per bay per orbit cycle. 

Structural integrity life prediction (cell pressure vessel) is not addressed. 

Voltage plateau depression is not addressed. 

Recommendations 

1.1. Assess energy storage system safe life analysis based on fracture mechanics. 

1.2. Specify assumptions and limitations of analytical Minimum Reserve Margins: 

14 Ah uncertainty with respect to observed strain gauge drift and cell-to-cell 

divergences 

1.3. Clarify depressed voltage plateau and substantiate its retardation. 

1.4. Develop a power management strategy to minimize orbital night load: 

0 

0 

Verify safe mode requirements being maintained 

Perform trade studies and determine strategy that minimizes instrument cycling 

stress and maximizes science 

Communicate strategy to the science community 
2. LIFE PREDICTION METHODOLOGY 

ObservationdFindings 

0 Rationale for use of limited data sample (post Service Missions 3B) for projecting life of 

HST batteries is not properly provided. 

Proposed life prediction method is subject to large variations when supplemented with 

additional measurements. 

0 Additional NiH2 battery life prediction models are available: 

0 Meet indicated criteria 

0 HollandswortWArmantrout, “Hubble Space Telescope Battery Capacity Trend 

Studies”, 2003 NASA Aerospace Battery Workshop [ref. 21 

Listed life prediction models are not properly referenced. 

Recommendations 

2.1. Select prediction method that includes a statistical determination of its probability that 

allows its inherent uncertainties to be evaluated. 

2.2. Update HollandsworWArmantrout, “Hubble Space Telescope Battery Capacity Trend 

Studies”, 2003 NASA Aerospace Battery Workshop prediction: 

0 Incorporate additional data (2004 data) 

0 Correct for 9 A discharge offset 

0 Apply statistical bounding analysis 

Supplement results into Table 111, Total System Charge Capacity and Margins vs. 

Time, of the GSFC position paper 

2.3. Determine if a proportional correlation exists between the voltages in the test battery 

discharge at the 9 Ah point and the measured capacity to 1.2 V for each cell (Figure 2.3- 

1 ). 

2.4. Attempt to reproduce HST Battery 5 overcharge to the SBT batteries without exceeding 

thermal limitation to determine duration of improved capacity. 
Consultation Position Paper 

Title: 

Technical Consultation of the Hubble Space Telescope (HST) 

Nickel Hydrogen (NiH2) Battery Charge Capacity Prediction 

8 of49 

m c rn 

IO 

n 

a 

E 

h 

a 

s 

m 

f 

s3 

Voltage (Volts) at 9 Amp-hrs Discharged 

Figure 2.3-1. Typical Plot of Correlation between Battery Voltage (at 9 Ah load) and Cell 

Capacity at 1.2 V 

3. BATTERY CHARGE CONTROL MANAGEMENT* 

* Battery charge control consists of charge and discharge segments. Battery discharge occurs by “nominal” or 

“capacity testing”. Nominal discharge is the capacity used during the night periods. Capacity testing occurs 

periodically where an artificial load is induced to partially deplete the battery. Charging following a capacity test 

involves an initial charge cycle at a lower current for the first cycle. 

ObservationdFindings 

Charge management strategy (thermal constraints plus operational cycling) might not 

have been optimized to allow the batteries to reach their maximum state of charge. 
Recommendations 

Charge 

3.1. Maximize charge current and raise V/T levels to improve charge efficiency during 

cycling operations as heat rejection capabilities will allow. 

3.2. Explore increased temperature implications (3 to 5OC) in conjunction with alterations to 

charge modifications. 

3.3. Investigate ratchet charging: 

After the first ratchet to the voltage cutoff, then trickle charge mode and wait for recovery 

(1 0 to 20 minutes) and ratchet again. 

0 

On-orbital cycling operations (Figure 3.3- 1) 

Ratchet charging following capacity check (Figure 3.3-2) 

Discharge (Capacity Test) 

3.4. Revert to heritage method of continuous discharge using 5.1 ohm load. 

3.5. Investigate fiequency of capacity testing. 

Lower discharge level to below 15 V (3 to 5 V) 

Substantiate the maximum duration (hours) of reversal allowed for individual cells 
V 

1 

t 

a 

g 

e 

0 

On-orbit Cycling Operations 

(Same orbit) 

DISCHARGE 

4 Increased Charge Efficiency 

4 Increased Capacity Over 

Time 

RECHARGE 

Time 

Figure 3.3-1. Ratchet Charging for Nominal On-orbit Cycling Operations 
. VASA Engineering and Safety Center 

Title: 

Technical Consultation of the Hubble Space Telescope (HST) 

Nickel Hydrogen (NiH2) Battery Charge Capacity Prediction 

Consultation Position Paper 

Page #: 

11 of49 

Document #: 

RP-04-08 

Version #: 

0.01 

Capacity Check Cycle Operations 

V 

0 

1 

t 

8 

g 

e 

Second orbit after 

c8p8Cie Ckk 

3bl 

Volt 

First orbit after 

c8p8Cie Ckk 

Time 

Reconditioning = Discharge (Part A) + Recb8rge (Part B) 

Figure 3.3-2. Ratchet Charging following Capacity Test 
1. 

APPENDIX A 

Detailed Findings/Observations/Recommendations 

SYSTEM ASPECTS 

Observations/Findings 

Battery thermal control system has an orbital average heat dissipation limitation of 30 W per 

bay per orbit cycle. 

0 Structural integrity life prediction (cell pressure vessel) is not addressed. 

0 Voltage plateau depression is not addressed. 

Recommendations 

1. Assess energy storage system safe life analysis based upon fracture mechanics. 

The cells as well as other EPS components were designed with specific design life 

parameters. A reassessment needs to be performed in order to identi& components 

(pressure vessels, cell internal components, wire insulation, etc.) which may have limited 

on-orbit life remaining. Failure of any of these items could result in loss of HST or its 

functions. A particular item of concern is the pressure vessels, which have associated 

with them a certain number of pressure cycles. An updated fracture/fatigue analysis 

should be objectively performed by structural analysis experts using the proper modeling 

analysis tools for the purpose of projecting when a failure of this type might occur. 

Limited life items such as switches, sensors, etc., should be reassessed in accordance 

with reliability predictions. 

1.2. Specify assumptions and limitations of analytical Minimum Reserve Margins: 

7 14 Ah uncertainty with respect to observed strain gauge drift and cell-to-cell 

divergences 

Provide breakdown of various safe-mode Ah threshold requirements used in Table 111 of 

the GSFC position paper. Identifi any margins included and assumptions made in 

arriving at those thresholds. For example, if the battery capacity measurements are 

discontinued, then it is necessary to depend on the battery-cell strain (pressure) gauge to 

assess capacity of the batteries at any time. However, there is some uncertainty with 
Consultation Position P 

respect to observed strain gauge drift and the actual measured capacity. There is also 

uncertainty in translating cell capacity to battery capacity as cells diverge with life and 

cell voltages are not monitored. 

Claria if the strain gauge drift is a certain percentage of the total cell capacity at any 

time or is typically constant at - 2 Ah/battery irrespective of its capacity. Also, clariJL 

how typical annual drift in total system capacity (based on the pressure readings) 

correlates with the allotted 14 Ah uncertainty. 

1.3. Clarify depressed voltage plateau and substantiate its retardation. 

A depression of the upper discharge plateau (- 1.2 Vper cell) during capacity testing 

was observed beginning in 2002. This depression was not present in 1998. The cause of 

the depression is not clear, although it does not appear to be progressively getting worse 

with time since it appeared in 2002. Without understanding the origin of this depression, 

there is little assurance that it will not continue to lower or suddenly step the voltage 

downwards further, or perhaps cause failure to support the 26.4 V requirement. At the 

relatively low capacity test discharge rates for the HST batteries, this depression is not a 

typical aging behavior of NiH2 cells. It is possible that this is a new aging phenomenon 

related to the present - 15-year lifetime of the HST batteries. 

However, a number of system changes occurred between 1998 and 2002 that should be 

explored as possible contributors. A new solar array and new instruments were 

installed, a new PCU was installed, loads and DOD were increased, and temperatures 

were changed slightly. These changes may have system-level implications that could 

influence the capacity test discharge plateau level. It is uncertain whether the same 

voltage depression is seen in the battery voltages during orbital cycling. It is 

recommended that these issues be explored to better understand the voltage depression, 

and to develop confidence that it will not suddenly increase further in the future to 

potentially limit the useful battery lifetime. 

1.4. Develop a power management strategy to minimize orbital night load: 

Verify safe-mode requirements are being maintained 

Perform trade studies and determine strategy that minimizes instrument cycling stress 

and maximizes science 

Communicate strategy to science community 
Technical Consultation of the Hubble Space Telescope (HST) 

Nickel Hydrogen (NiH2) Battery Charge Capacity Prediction 

The science instruments are reported to be currently maintained in various levels of 

operate mode continuously regardless of orbital day or night. To reduce/minimize the 

orbital night load, consideration should be given to the following: 

14 of49 

- Turn offall or some of the science instruments during orbital night. 

- Switch all or some of the science instruments into low power or standby mode during 

orbital night. 

Operate some of the science instruments continuously, for a weeklmonth, and operate 

reminder of the science instruments the next weeklmonth and cycle them back and 

forth. 

- 

Although, it may be undesirable to duty cycle the science instruments in view of 

associated risks, various duty cycling options should be studied and presented to the 

science community. Trade studies should be performed to determine a strategy that 

minimizes cycling stress on the instruments while maximizing science productivity. 

Also, consider shifting some of the HST spacecraft bus operations to orbital daytime 

without affecting the science operations, like vehicle slews, etc. 

Tables I and 111 of the GSFC position paper present full science operations as a 

requirement. Tables I and 111 should be modified to include some of the options that 

might result from the power management strategy discussed above. Thus, the modfled 

Table III entries might lead to several more options for extended HST viability. For 

example, per current Conclusion 2, halting the science program may not be required and 

can be modified to operate some of the science instruments as per power management 

strategy above. This option might be more favorable for the science community rather 

than halting the science program. 

Further, consider providing a breakdown of various safe-mode Ah threshold 

requirements used in Table III and identifi any margins included in arriving at those 

thresholds. 

2. LIFE PREDICTION METHODOLOGY 

Observations/Findings 

e Rationale for use of limited data sample (post SM3B) for projecting life of HST batteries 

is not properly provided. 
Title: 

Technical Consultation of the Hubble Space Telescope (HST) 

Nickel Hydrogen (NiH2) Battery Charge Capacity Prediction 

e 

Page #. 

15 of49 

e 

e 

Proposed life prediction method is subject to large variations when supplemented with 

additional measurements. 

Additional NiH2 battery life prediction models are available. 

Meet indicated criteria 

Hollandswortldhantrout, “Hubble Space Telescope Battery Capacity 

Trend Studies”, 2003 NASA Aerospace Battery Workshop 

Listed life prediction models are not properly referenced. 

Recommendations 

2.1. Select a prediction method that includes a statistical determination of its probability that 

allows its inherent uncertainties to be evaluated. 

While it is recommended that an updated version of the Hollandsworth model be used to 

predict battery capacity and life, it is also recommended that an appropriate statistical 

analysis of the regression results be performed for whatever model is employed. The 

statistical analysis should use the data to provide the changes in degradation-line slopes 

for diyerentprobability levels. The results should be used to provide the probability of 

reaching each of the capacity benchmarks indicated on a time line such as that provided 

in Table 111 of the GSFC position paper. Alternatively, a time line could be developed at 

a given probability level (i.e., 90% might be a worst case while 50% would be a nominal 

case). This would allow HST Program plans to be developed both for well-defined worst 

case as well as for nominal battery lifetime scenarios. The Program would also see the 

years of extended battery operation that would accompany acceptance of additional risk 

(addedprobability of battery failure). 

A more rigorous evaluation of models for predicting battery life is recommended and 

should include the appropriateness of each model with respect to the HST battery 

situation. The following models are known to be available, and their applicability should 

be addressed: 

- The assessment of the Hafen model seems appropriate in the GSFC position paper. 

A simple wear out model has been developed by Thaller et. al. (Lim and Thaller, 

Proceedings of the 22nd IECEC, Vol. 2, pp 751-757, 1987. Updated by Thaller in 

Proc. of the 1997 NASA Battery Workshop, pp. 411-427) [rex 51. This model 

predicts - 35 years of operation at 10% DOD for generic NiH2 cell capability. 
IL 1‘ 

Technical Consultation of the Hubble Space Telescope (HST) I 16 of49 I 

NASA Engineering and Safety Center Document #: Vmion #: 

Consultation Position Paper RP-04-08 0.01 

Nickel Hydrogen (NiH2) Battery Charge Capacity Prediction I I 

Because the 10% DOD situation is an extrapolation fiom the DOD range covered 

by the data used in the model, this model may overestimate the battery life. 

The wear model developed by Dr. Zimmerman [re$ 61 does specijkally take the 

DOD, recharge conditions, temperature, and other environments to which the HST 

batteries have been exposed into account. The Zimmerman model predicts the most 

probable time of failure (50% probability) for the first battery to be about year 

2011, and a 95% probability of battery failure at 10% DOD by about year 2015. 

However, it is also based on data obtained at higher DOD (30 to 60%), and thus 

assumes a specific dependence of wear rate on DOD that may not be accurate at 

10% DOD. It firthemore predicts failure to be much more likely as a result of 

continued loss of usable capacity, rather than failurefiom a cell short circuit. It 

also predicts that at about 76,000 cycles the usable battery capacity will fall to 50% 

of nameplate (45 Ah) capacity. This is actually relatively close to the current 

measured capacity of about 50 Ah for the lowest batteries. This correlation is 

within the expected IO to 20% accuracy expectedfiom this model, and suggests that 

the HST batteries are actually degrading at rates and by processes that are filly 

consistent with industry NiH2 battery life test experience. 

The Hollandsworth model appears to be the best for actual HST life predictions, 

since it is based on trend analysis of the measured HST battery capacity data. The 

Hollandsworth model does not allow for any type of accelerated battery capacity 

degradation later in life, at least in its present form. Accelerated or non-linear 

degradation late in the battery life is the only reason for excluding the early-life 

data fiom the Hollandsworth model trend analysis (except for several points that 

are influenced by known charge control changes, and which are statistically out of 

family by However, no evidence appears that such accelerated 

degradation is occurring in the most recent battery capacity measurement data. 

- 

- 

2 sigma). 

2.2. Update HollandsworWArmantrout, “Hubble Space Telescope Battery Capacity Trend 

Studies”, 2003 NASA Aerospace Battery Workshop prediction: 

Incorporate additional data (2004 data) 

Correct for 9 A discharge offset 

Apply statistical bounding analysis 

Supplement results into Table 111, Total System Charge Capacity and Margins VS. 

Time, of the GSFC position paper 

Various models for predicting battery life were discussed in the GSFC position paper. 

Admittedly, time seems to have invalidated the prediction of the Hafen model that was 

cited. The Zimmerman model, which neither includes nor requires any a priori 

knowledge of the HST on-orbit battery trends, does make some interesting predictions of 
Title: Page #. 

Technical Consultation of the Hubble Space Telescope (HST) 17 of49 

Nickel Hydrogen (NiH2) Battery Charge Capacity Prediction 

life and reliability out to the 201 1 to 2015 time fiame. These predictions are based on 

generic experience with NiHz cells, and should be recalled when estimating lifeporn HST 

battery trend data. The validity of the Zimmerman model will ultimately be determined 

by how well it agrees with the observed battery trends. 

As was noted, the Zimmerman model does make some assumptions. The issue of 

predicting capacity and life to a 1.0 V failure criterion rather than a 1.2 V criterion 

appears to be a limited issue, since there only appears to be a 2 Ah digerence in cell 

capacity between these two voltage levels for the HST discharge conditions (see Figure 1 

of GSFCposition paper). In fact, all such models for predicting HST cell and battery life 

should attempt to predict the step change in voltage fiom 1.2 to 0.8 V, which tends to be 

centered in the 1.0 to 1.1 V range. It is this voltage step that will trigger a significant 

load 08-share (and anomaly declaration) when it occurs in a cell within any of the HST 

batteries. 

Probably the greatest limitation with the Zimmerman model is its uncertain applicability 

to the 9 to 10% DOD operation of the HST. The data on which the Zimmerman model is 

based and validated are in the 30 to 60% DOD range. Extrapolation to 10% DOD may 

result in some systematic errors in wear rates, which could decrease the accuracy of the 

model. 

The best previous model for the HST battery life is the empirical trend analysis of the 

HST orbital data reported by Hollandsworth, et. al. (Hollandsworth model). This model 

does meet the criteria described as needed for an acceptable model (GSFC position 

paper, page 11, paragraph 2), but this work is neither cited nor used. The 

Hollandsworth model simply uses a linear regression analysis of the HST capacity test 

battery measurements as a function of time to predict when each battery will cross 

particular capacity thresholds of interest. The added data obtained since late 2003 do 

not appear to deviate significantly Ji-om the trend lines in the Hollandsworth model, and 

thus do not suggest the batteries are now dropping more rapidly, on average, than they 

have in the past. The model ofered in the GSFC position paper is essentially the same as 

the Hollandsworth model, except that it does not include any of the battery data prior to 

2002. 

It is recommended that the Hollandsworth model be updated to include the most recent 

battery capacity measurements, and that the capacities in the Hollandsworth model be 

adjusted to reflect the expected actual battery discharge rate (4 Ah ogset). The 

Hollandsworth model should be used to most accurately predict HST battery capacity 

and lve, unless future data show statistically signif cant deviation @om the historical 

downward trend. It is also recommended that the resultant timelines (such as those 

provided in Table III of the GSFC position paper) be updated to reflect the 
- 

lk 

(1 

NASA Engineering and Safety Center Document #: Version #: 

Consultation Position Paper RP-04-08 0.01 

Hollandsworth model predictions, and that the trends and predictions be periodically 

updated as signijkant new data becomes available. It is recognized that this approach 

will provide a nominal battery life that is much longer than that presented in the GSFC 

position paper (since the degradation slopes are IN as great). However, the 

Hollandsworth model is believed to provide a prediction that is better supported by the 

data, assuming a worst case corresponding to 90+% battery reliability rather than 

simply the mean battery lifetime (- 50% reliability). 

Title: 

Technical Consultation of the Hubble Space Telescope (HST) 

Nickel Hydrogen (NiH2) Battery Charge Capacity Prediction 

2.3. Determine if a proportional correlation exists between the voltages in the test battery 

discharge at the 9 Ah point and the measured capacity to 1.2 V for each cell (Figure 2.3- 

1). 

Page # 

18 of49 

Correlations between NiH2 cell or battery voltage and the amount of stored capacity 

have often been found to be confounded by all the other variables that influence voltage, 

but not capacity. Such variables include current, temperature, and the past history of 

charge, discharge, as well as the SOC. 

Discussions concerning the FSB testing, where the voltages of all cells are measured, 

have sometimes equated cell voltage divergence during cycling to capacity divergence. 

Establishing a correlation between the amount of stored capacity and the cell voltage at 

the end of the 9 to 10% DOD orbital cycle would provide a simple, real-time diagnostic 

tool to monitor the evolution of cell imbalance during cycling. This correlation could be 

checked (or recalibrated each time the test battery is subjected to a capacity test. Such a 

correlation would plot the voltage of each cell at the DOD corresponding to orbital 

cycling (-10% DOD) during the capacity discharge against the capacity measured for 

that cell to 1.2 V. Ifa good correlation is in fact found to exist, cell voltage dispersion at 

the end of each orbital cycle can be used to accurately indicate relative shifts in cell 

capacities, and thus capacity imbalance. If a good correlation is not established, cell 

voltage divergence should not necessarily be used as a meaningfir1 measure of capacity 

imbalance. 

For HST batteries, where individual cell voltages are not available, this tool may help 

quanti& how shijh in battery voltage during eclipse operation or capacity testing could 

translate into capacity loss or capacity divergence. 

2.4. Attempt to reproduce HST Battery 5 overcharge to the SBT batteries without exceeding 

thermal barrier to determine duration of improved capacity. 

The GSFC position paper makes reference that Battery 5 was most affected by 

overcharge while the PCU bus impedance fault existed. However, no details of 
Title: 

Technical Consultation of the Hubble Space Telescope (HST) 

Nickel Hydrogen (NiH2) Battery Charge Capacity Prediction 

overcharge were included. There was also no mention if the thermal limitation of 30 W 

was exceeded. Hence, Jirst compile the conditions, like charge current, V/T level, charge 

duration, battery temperature, etc., under which the Battery 5 was inadvertently 

overcharged. Then, select two batteries $-om the SBT with approximately matched 

characteristics. Operate one battery (hereafter termed as Battery-A) similar to current 

flight batteries on-board HST and treat this battery as the control battery. Now operate 

the second battery (hereafter termed as Battery-B) with the conditions of overcharge that 

occurred on Battery 5 when the PCU bus impedance fault existed. Ensure that thermal 

barrier is not exceeded. Now continue to operate these two batteries and monitor them to 

observe how long the improved performance can be retained. 

Page #: 

19 of49 

If the improvedperformance can be retained for more than 4 months, then the same can 

be implemented to the batteries on-board HST. 

3. BATTERY CHARGE CONTROL MANAGEMENT* 

* Battery charge control consists of charge and discharge segments. Battery dmharge occurs by “nominal” or 

“capacity testing”. Nominal discharge is the capacity used during the night periods. Capacity testing occurs 

periodically where an artificial load is induced to partially deplete the battery. Charging following a capacity test 

involves an initial charge cycle at a lower current for the first cycle. 

ObservationdFindings 

Charge management strategy (thermal constraints plus operational cycling) might not have been 

optimized to allow the batteries to reach their maximum state of charge. 

Recommendations 

Charge 

3.1. Maximize charge current and raise V/T levels to improve charge efficiency during 

cycling operations as heat rejection capabilities will allow. 

The data shows the HST batteries and, in particular, the cells within the batteries 

degrading with time. This in itself is not surprising considering the length of time and 

number of orbiting cycles. Fortunately, the depth of discharge is only 9 to 10% of the 

original 90 Ah and the temperature is being maintained near 0°C. This operational 

methodology is known to be consistent with long LEO operating cycle life. There is 

concern, however, that even with these optimum operating conditions, the capacities of 

the batteries as measured during the near annual capacity test process have dropped to 

50.8 Ah on Battery 2 and 51.7 Ah on Battery 4. What is considered as important is the 

rate at which the measured capacity is decreasing (by 7 to 8 Ahbear) with a maximum of 

16.6 Ahbear for Battery 5. 
The complexities in the charge control system were known when the decision was made 

to use the 90 Ah NiH2 batteries in place of the 55 Ah nickel cadmium batteries. The 

replacement proved to be fortuitous by providing sign@ant margin for HST life in orbit. 

However, with 6 batteries, each charged from its dedicated 3 solar panels while at the 

same time tied in parallel to the power bus, required a complex method for controlling 

charge, minimizing overcharge, and assuring all 6 batteries were fully charged during 

the sunlight period without exceeding the thermal design of 30 W average. HST was 

therefore operated to limit the charge current and voltage to minimize heat and extend 

life. However, this method is counter to optimizing long-term cycling capacity. 

To counteract the decrease in capacity with cycling, those on this review committee 

considered methods for increasing battery capacity. The preferred method is to charge 

the batteries more eficiently during the sunlight period, thus attaining a higher capacity 

level. The purpose is to restore some of the lost energy storage on the charge cycle, to 

increase discharge capacity, or as a minimum maintain the present capacity. If the 

batteries had higher available capacity, then potentially the degradation could be 

slowed. 

One of the ways of increasing the Ah stored in the battery during charge is to use high 

rate (C/5 to C/2) as a minimum. Cells operated in this manner accept charge at high 

rates at nearly 100% eflciency before reaching an overcharge condition wherein heat is 

generated. 

HST is one of the first LEO missions to use NiH2 batteries. However, comparing HST 

with the Solar Max Mission, the charge current is presently only at 13.5 A (U6.7). It is 

interesting to note that the lower charge rate is less efficient and generates more heat 

than the higher charge current. At high current, the charge eficiency is near loo%, 

which means there is no accompanying heating. This has been verijied by numerous tests 

at Comsat and Crane as well as reported in the “NASA Handbook for NiH2 Batteries” 

[re$ 31. This implies that the effect on thermal limitation would be lowered. 

Secondly, the present V/T level cutoff of 33 V (1.50 V / cell) is too low for a NiH2 battery 

at 0°C. The original V/T levels for the HST batteries are shown on page 3-45 in the 

“NASA Handbook for NiH2 Batteries”. At OOC, the Kl-3 voltage initially used was 34.3V 

(1.56 V/cell). The chart shows that a V/T of 33V now in use is appropriate for 20°C, 

clearly too low for 0°C. The V/T values in report “On-Orbit Health Check of HST NiH2 

Batteries ’’ by Rao, Wajsgras and Krol, presented November 15, 2002, shows a dark blue 

line on the V/T curve page at V/T of 33.2 V (1.51 V/ Cell), which is also too low. 

From the conversations with HSTpersonnel and the data provided, it seems that to limit 

the orbital battery dissipation to about 30 W, the charge current was reduced from - 14 
to - 8 A after the first battery reached its software V/T limit (- 32.3 v) and to trickle 

charge current of - 2 A when the first battery reached its hardware V/T limit (- 33 v). 

Obviously, this scheme reduces the trickle charge duration and, in turn, reduces the 

battery dissipation during trickle charge period, but at the cost of lowering the charge 

eficiency as mentioned in the previous paragraphs. The lower charge eficiency results 

in a) increased battery heat dissipation during charging, and b) preventing battery @om 

reaching a high state of charge. 

Therefore, the following are recommendations for the HST project consideration to 

enhance the capacity of the HST batteries in orbit. 

a. 

b. 

C. 

d. 

e. 

f: 

Increase the V/T level to appropriate levels to provide improved charge eflciency 

and energy. Raise the V/T level to a minimum of 34 V and allow the high rate charge 

to continue until 65 to 70% of the Ah, removed on the previous discharge, are 

returned when the V/T level is reached. Increasing the end of charge voltage is 

accomplished by raising to a higher V/T level consistent with operating at 0°C. 

Concurrently, with paragraph a. above, increase the charge current so that it is at the 

maximum when entering into the sun acquisition portion of the orbit. As a minimum, 

the high current can be implemented by leaving the solar panels connected to the 

battery at night so when the s/c comes into the sunlight and the solar panels are cold 

they can provide the maximum charge current. Additional current may be available if 

some of the loads are not operating at that time. 

Develop a charge operation scenario in combination with paragraph a. above to 

utilize the lower current of trickle charge more advantageous by removal of solar 

panels or by ratchet charging described below in 3.3. 

Enter into discussions with the experimenters on how to balance their needs with the 

importance of maintaining or increasing the charge state of the batteries fully in 

order to extend the mission. 

Although the 30 W average heat load looms as a tall figure, it is not certain as to why 

this limit is so rigid. It is known that the long-term operation lower temperature 

aflords longer cycle life, but can the present temperature limit be opened somewhat to 

accommodate more eficient charging associated with higher current and voltage 

operation? 

Consider a method for interrupting charge with a trickle or lower level charge, then 

resuming the charge to increase the energy storage into the battery (see the 

discussion on ratchet charging below). When both the V/T level and charge current 
Title: Page # 

Technical Consultation of the Hubble Space Telescope (HST) 22 of49 

Nickel Hydrogen (NiH2) Battery Charge Capacity Prediction 

3.2 

3.3 

are increased, the trickle charge duration will be longer. Thus, a reduction of heat 

dissipation can occur if the trickle charge current can be reduced to below 2 A 

(perhaps to 1.5 or 1.0 A). 

It is understood the HST charge control system is complex and subject to limits and 

operations not fully understood by the committee members in the time allotted for the 

independent assessment. It is apparent that the members of the Project team have made 

a signijkant effort to continue the successful HST operation. The committee 

recommendations, made without the direct hands-on experience, are aimed at increasing 

the energy storage of the NiH2 batteries to meet the present life requirements. 

Explore increased temperature implications (3 to 5°C) in conjunction with alterations to 

charge modifications. 

Cell performance is linked to environmental temperature. There is an optimum 

temperature range for the performance of older cells. Providing the cells with a colder 

environment can favorably affect cell performance to a certain point. However, more 

capacity may be gained by raising the temperature 3 to 5 “c along with the proposed 

changes in charging procedures. Increasing the battery temperature by 3 to 5 “c should 

be investigated and tested using ground-based testbed cells. 

Investigate ratchet charging: 

After the first ratchet to the voltage cutoff, then trickle charge mode and wait for recovery 

(1 0 to 20 minutes) and ratchet again. 

On-orbital cycling operations (Figure 3.3- 1) 

Ratchet charging following capacity check (Figure 3.3-2) 

Ratchet charging involves recharge to a V/T level or other charge cutoff criterion at a 

high charge rate, followed by a period of rest (typically trickle charge or open circuit), 

then a repeated recharge at the high rate to the same V/T cutoff level. If time permits, this 

high rate recharge and rest period can be repeated multiple times to ratchet the state of 

charge to higher levels (in LEO there may only be time for one ratchet). The overall 

RCF is intended to be similar to that with no ratchet recharge. However, three factors 

provide higher charge efJiciency with the ratchet pulse-recharge sequence when the peak 

recharge rate is maximized. First, the lower recharge voltage that results from the rest 

period provides improved charge eflciency. Second, the trickle charge is more eficient 

when performed at a lower average state of charge. Third, charge eficiency always 

tends to be greater at higher recharge current. 
This charge control method is particularly advantageous when recharge to the V/T cutoff 

at high rate occurs in a short period of time. Application of much of the trickle charge 

during the middle portion of the recharge period takes maximum advantage of the higher 

charge eficiency for trickle charge at lower SOC. The second high rate recharge pulse 

will provide an SOC boost due to the improved charge efficiency at the high recharge 

rate. Repeated recharge pulses with adequate rest between can ratchet the SOC up 

signijicantly. 

This type of recharge profile should be evaluated to determine whether it is possible with 

the HST charge control harhuare/software. This recharge method is expected to 

particularly benefit the recharge of a newly capacity tested battery, since it should allow 

a higherpaction of the returned charge to occur at the high rate. This charge method is 

not expected to have deleterious thermal eflects if the total RCF is maintained to be 

similar to the RCF without ratchet charging. The average heat produced by the batteries 

can be limited to 30 W by appropriate adjustment of the software V7T levels for each 

battery. 

Discharge (Capacity Test) 

3.4. Revert to heritage method of continuous discharge using 5.1 ohm load. 

The battery capacity test method used by HST up to 2002/2003 involved discharge 

through a 5-ohm load to a cutoff limit of 15 V. In 2003 this was changed to 5-ohm to a 

cutoff voltage just below 26.4, then 50-ohm to 15 V. This change in procedure, which 

was instituted to reduce the discharge current during the cell reversals, extended the 

capacity test timeline without improving the cell balancing effects. 

It is recommended a return to the pre-2003 heritage capacity test procedure of applying 

a 5-ohm load until the battery reaches its cutoff level (whether this level remains at 15 V 

or is reduced to lower voltage levels in the future). The 5-ohm load will not allow 

reversal currents greater than 5 A, and the reversal current will be much less than 5 A 

for all reversals seen to date. The 5 A (- C/18) is well below the acceptable gas evolution 

rate limit of C/10 (8.8 A) in the HST cells. Such reversals (- 3 A rate) have been 

repeatedly demonstrated as not causing degradation every time the HST flight batteries 

have been capacity tested prior to 2003. Similarly, ground tests have not shown any 

evidence of degradation resulting @om low rate (<C/1 0) reversals, where the duration of 

the reversal was limited to less than 48 hours. 

The 5-ohm load during capacity testing will maximize the rebalancing benefit by driving 

the lowest capacity cells into depletion (reversal) sooner, particularly when com bined 

with a lower cutoffvoltage (i.e., - 3 V). 
3.5. Investigate frequency and depth of capacity testing. 

Lower discharge level to below 15 V (3 to 5 V) 

Substantiate the maximum duration (hours) of reversal allowed for individual 

cells 

In general, the capacity test has been found to serve several purposes for NiH2 batteries: 

- Provides a check of the battery capacity to various voltage levels, as well as the 

correlation between the pressure monitor readings and the capacity level. This 

allows correction for slow pressure drift and maintains pressure as a good 

indication of the available capacity over the life of the batteries. The capacity test 

procedure also provides long-term trends of capacity degradation that can be 

used to predict battery life or the need to modi& battery management methods. 

- Provides rebalancing of the battery individual cell. If individual cells spread in 

capacity (which they typically do after signijicant cycling), they can all be 

brought back to a common, fully discharged state by the capacity test discharge. 

This rebalancing occurs as a result of the lowest capacity cells going into 

reversal when fully depleted, thus allowing the SOC of the higher cells to be 

brought down closer to the common zero-point capacity. The rebalancing @om 

capacity test discharge only occurs during cell reversal, since this is the only time 

during discharge when no stored cell capacity is being depleted. The rebalancing 

can provide a sign#cant improvement in the state of health of the overall battery 

by restoring cell capacity balance. 

There is very little evidence that capacity testing provides any long-term improvement of 

the health of individual cells (it does improve overall battery capacity by rebalancing cell 

capacities, thus allowing lower capacity cells to achieve a higher SOC). There is, 

however, a short-term improvement of the performance of individual cells. After capacity 

testing, each cell will have a higher discharge voltage and will produce less heat during 

discharge. Unfortunately, the cells also have a higher recharge voltage behavior, 

making them somewhat more dif$cuIt to pump up to high SOC following the capacity test 

discharge. The higher voltage behavior constitutes a short-term change in performance, 

typically lasting through several months of LEO cycling before the cells settle back to 

their pre-capacity test behavior. 

Because cell rebalancing in a batteiy occurs when the lowest cells are in reversal, it is 

advantageous to maximize the reversal Ah to which these cells are exposed, within limits 

that are defined below. This will maximize the rebalancing effect. This can be 

accomplished by continuing the capacity test discharge to as low a battery voltage cutoff 

aspossible. This allows even the highest capacity cells to be brought down to near-zero 
Title: 

Technical Consultation of the Hubble Space Telescope (HST) 

Nickel Hydrogen (NiHt) Battery Charge Capacity Prediction 

SOC, and thus be capacity-matched to all the other cells in the battery. Capacity test 

cutoff voltage levels down to 2 to 3 V have been successfully used for 22-cell batteries to 

achieve satisfactory rebalancing even when cell voltage monitoring is unavailable. 

Page #: 

25 of49 

It is recommended the voltage cutoff levels for the capacity test that are significantly 

lower than 15 V be explored as a method for the HST to achieve more complete cell 

balancing. This is likely to become more important late in life when cell imbalances tend 

to become greater. 

Recharge following capacity testing is also a key portion of the conditioning exercise. 

More eflcient methods are needed to restore full battery capacity in newly conditioned 

HST batteries. Tests are ongoing to evaluate several approaches and we recommend that 

the recharge methods described in Recommendations 1.1 and 1.2 be included in these 

evaluations. 

Recommended Limits on Cell Reversal During Capacity Testing: 

- Cell reversal (in aged HST batteries) involves the evolution of hydrogen gas from the nickel 

electrodes and the recombination of this gas at the hydrogen electrode at -0.01 to -0.05 V, 

a process that involves no stored cell capacity and limited heat generation. Excessive rates 

of gas evolution are to be avoided during reversal, since the gas evolution pressure 

differentials can mechanically degrade the nickel electrode during reversal much as it does 

during overcharge (without the deleterious thermal effects of overcharge). Long-term 

overcharge at rates as high as C/I 0 are commonly applied to NiH2 cells during acceptance 

testing and capacity verijication tests. This same limitation should apply to the maximum 

acceptable reversal rate, which is 8.8 A (C/lO) for the HST batteries. 

- Cell reversal involves holding the nickel electrode at the highly reducing chemical 

potential of hydrogen gas (below zero cell voltage with hydrogen present). This voltage 

level initiates slow reactions that chemically reduce the active material in the nickel 

electrode. If allowed to proceed for a long period of time, these reactions can degrade 

electrode performance significantly. Controlling the duration of the low voltage exposure is 

thus paramount. Cell and battery testing (acceptance tests, capacity tests, and integration 

& test tests) typically put a limit of 16 hours on the duration of letdown periods to a low 

voltage. No performance degradation has been documented as occurring @om up to I6 

hours of low-voltage exposure. Low voltage cell storage tests with hydrogen gas present in 

the cells have indicated signs of undesirable degradation after 2 weeks of low voltage 

exposure. Thus, between 16 hours and 2 weeks there is a range of low voltage exposure 

time that can cause detectable nickel electrode degradation. Typically, a 48-hour 

limitation on low voltage (or reversal) exposure with hydrogen gas present has been 

recommended as a reasonable and safe threshold between 16 hours and 2 weeks. Many 
satellite programs have used the 48-hour limit over the past 10 to 12 years without 

experiencing deleterious eflects on cell performance. 
APPENDIX B 

List of Acronyms 

Ah 

DOD 

EPS 

FSB 

GSFC 

HST 

JSC 

LEO 

NiH2 

NRB 

PCU 

RCF 

S&MA 

SBT 

SOC 

V 

V/T 

w 

Amp How 

Depth of Discharge 

Electrical Power System 

Flight Spare Battery 

Goddard Space Flight Center 

Hubble Space Telescope 

Johnson Space Center 

Low Earth Orbit 

Nickel Hydrogen 

NESC Review Board 

Power Control Unit 

Recharge Fraction 

Safety and Mission Assurance 

Six Battery Testbed 

State of Charge 

Volt 

Temperature Compensated Voltage Level 

Watt 
APPENDIX C 

References 

1. 

2. 

3. 

4. 

5. 

6. 

Kalinowski, Dr. J. Keith. The State of the HST Batteries as it Relates to HST Health, Safety, 

and Ability to Support an On-going Science Operation. GSFC Position Paper, May 2 1,2004. 

HollandswortWArmantrout. Hubble Space Telescope Battery Capacity Trend Studies. 2003 

NASA Aerospace Battery Workshop. 

Dunlop, J.D., Rao, Gopalakrishna, and Yi, T. “NASA Handbook for NiH2 Batteries”. 

Rao, Wajsgras and Krol. On-Orbit Health Check of HST NiH2 Batteries. Presented 

November 15,2002. 

Thaller et. al. (Lim and Thaller). Proceedings of the 22ndIECEC, Vol. 2, pp 751-757, 1987. 

Updated by Thaller in Proc. of the 1997 NASA Battery Workshop, pp. 41 1-427. 

Zimmerman, Dr. Albert H., Model for Predicting the Eflects of Long-Term Storage and 

Cycling on the Life of NiH2 Cells, Proceedings of the 2003 NASA Aerospace Battery 

Workshop. Updated by Zimmerman for HST batteries in Life Projection for the Hubble 

Space Telescope Nickel-Hydrogen Batteries, ATR-2004(8 180)-3, The Aerospace 

Corporation, 15 March 2004. 
Technical Consultation of Hubble Space Telescope (HST) 

Nickel Hydrogen (NiHz) Battery Charge Capacity Prediction 

29 of 49 

APPENDIX D 

Review Copy 

Ihe State of the HST Blaterka 

As it 

Retate?e to HST 

Health, Safety 

aad Ability b Support an &-going Scknw Operation 

May 21,2004 

Approved by: 
Title: 

Technical Consultation of the Hubble Space Telescope (HST) 

Nickel Hydrogen (NiH2) Battery Charge Capacity Prediction 

Important Note 

Page #: 

30 of49 

This, the review copy of this paper, incorporates comments received on wo previous 

dmRc mid is now arbmitied for teview by GSFC’s Applied Engineering and Technology 

Directorate. 

2 
Technical Consultation of the Hubble Space Telescope (HST) 31 of49 

Nickel Hydrogen (NiH2) Battery Charge Capacity Prediction 

Contents 

Executive Summary 

Bottn?; and Eiecrical Power Subsystem C)verviw 

Battery Testbeds and Flight Batkry Diagnostics 

Cell Failures Mechanisms and Results fmm Destructive 

Physical .haIysis of Flight Spare Battery Cells 

Charge Capacity History and Trends 

Farecasts Based an Charge Capacity 'bnds 

Relationski of Pointing Perfmce to Battery Capacity 

Requirements in safemade 

The On-going Brdtery Test Program 

Appendix 

Page 

4 

5 

6 

6 

10 

12 

13 

16 

16 

16 

17 
Title: Page # 

Technical Consultation of the Hubble Space Telescope (HST) 32 of49 

Nickel Hydrogen (NiH2) Battery Charge Capacity Prediction 

Executive Summary 

Fmcmts of WST viability vs. time hmed on hatlery charge capacity trends and 

errtrapolations are context dependent. httery capacity requirements for an on-going 

science opi?ralion are significantly higher than tho.. for spacemait maintenance in a stmpoint safesno&. .Lu the present rate of capacity loss (48 ampere-hoursiyear at the 

system-level). a hiatus in HST science operatioils will more likely be caused by loss of 

gyw. In a ralatively law-electrical-lad safemade, WST will need less capacity margin 

to ensure mission health and sdety than it requires for science operations. If ftST is 

being maintained in safemode it is reasonable to assume that it can, ifnecessary, remain 

viable while awaiting a re-entry module attachment or a robotic servicing mission that 

reaches the spacecraft as by the middle of calendar year 2009. However, HST will have 

to be placed into sdmode sometime in 2008 ifh present capacity trend continues. 

The vfobiEity 41 risk “milestone” moves about a year earlier (into 2008) and the sujiemods 

reqwred “milestone” moves into 2007 ifthe present system-level trend worsens by about 

one-third. Both milestones move over two yem later ifthe present trend is reduced by a 

similar amount. 

At present, it is not possible to predict a significant change in the system-level battery 

capacity loss me. For now, prudence appears to didate that a re-entry module 

attachment or a robotic servicing mission to I IST is best scheduled for 2008, and if 

delayed, should occur before the fall of 2009. 

*Ntm-tctm, projections will be affected by the results of battery capacity measurements 

scheduled fm now through October 2004. Batlay 5 results available at the end of 

June will bu of prvticulur intarost. They will hdicatct whether Battory 5 haa; oonthued to 

loso capticity at a mtwh higher rate than the othar five batteries, or is now aging similarly 

to them. Available data indicate that the latter is more likely true. 

4 
Technical Consultation of the Hubble Space Telescope (HST) 33 of49 

Nickel Hydrogen (NiH2) Battery Charge Capacity Prediction - 

Battery and Elt?ctricnl Power Subsystem Overview 

The the primary elements of the NST Electrical Power Systmi (FPS) are the solar army 

(S.43). the Power Coiml Unit Replacenlent (PCU-R) atid the six Nickel-Hydrogen 

(NiH2) batteries. All the electrical energy used by IIST is either provided or distributed 

by these elentents. Neirhwthe Sa3 nor XU-R (nor for that tnatta, any downstream 

EPS element) is presently subject to any liens that preclude continuation of the HST 

opm~ion we11 beyond 2010. Among the three subsystems, only the batteries are original 

equipment. 

Each of the six batteries clontains 22 individual cells cme&d in series. The nameplate 

when new) capacity of each ofthe babies is 88 Ampere-Hours (A&) when 

discharged by a 15-ampere laad. Prior to hunch in 1990, however, the batteries were 

“super-charged so that each had over 90 A& of initial capacity. More specifically, the 

nameplate capacity Originally defined the number of AHr the battery was required to 

provide to a 15-ampere load befare the voltage at the battery terminals dropped below 

26.4 voks (VI. The voltage requirement itselfdates to the minimum spaced bus 

vol%qge that, &r diode drops and hss lases, is needed to keep each HST subsystem 

powered and reliably functional. Put diffemtly, individual battery cells, on avmge, had 

to provide -88 AHr Between their fully dwged voltage of--1.5 V, and an average 

dtscharged cell voltage of (26~22 =) 1.2 V. At launch there was very little dispersion in 

cell capacity or performance. The pmsmt-day safemode load comaponds to 

approximately 9 amperes per battery. At this discharge rate, each battery’s usable charge 

capacity is about 4 AHr smaller, to 26.4 V, than the value measured under the -5 amp 

load through the on-board 5.1 ohm discharge resistor. lhe make-up of an HST batteq 

and ba#q cell is further described in the Appendix. 

The batleries themselves ate connected in parallel to the main spacecraft bus. Each orbit 

day the SA3 powers the spacecrafl and provides the energy to =-charge the batteries. 

Each orbit night, the batkries pawet the spacecraft and are discharged appmxcrmately 

equally by MSTs aaitity- and configuration-dependent electrical load. ‘fie duration of 

ohit night ranges hm -27 to -36 minutes (biased to the longer time), and the 

operatiand load is pscntly -80-90 amperes (13-15 amperes per battery). For the longer 

nights as many as -34 AHr are removed from the bartm systsrn total (above 26.4 V) of 

-312 AHr (for a 9-ampere per battery load).* At the end of orbit night the system depthof-discharge (px)D) (relative to nameplate capacity) is about 9-10%. The €301) is -16- 

IWO ofthe psent total system charge capacity. Fundamentally, the system charge 

capacity remaining at the end of orbit night must support a worst-case safemode mtry 

from the spacecrrrft configuration and spacecraft attitude existing at the end of orbit ni&t. 

*For simplicity, usable battery capaciv is not debited for the higher load of science 

operations, since rcmahing capacity under a %amp load is the relevant measm of 

survivability safemode entry. At present, fie nuwgin in NST’s barteq capcity 

sotfmocie trijgers is more thm adequate to compensate fbr thc d2Tewnce. If thcsc 

mafgins are scrubbed m the future, the operational rate of capacity use will helve to he 

accounted for. 

5 
lh 

I 

NASA Engineering and Safety Center Document rl: Version # 

Consultation Position Paper RP-04-08 0.01 

The EPS system design assured WST survival following total failure of one battery. Prior 

ro SM3A’s instdlation of PCU-R, the possibility of simuhaneow loss of two batteries 

(caused by an open circuit in the PCU) was examined and found to be survivable with the 

then-availabie capacities of the nmaining batteries. Future survivability if total loss of a 

bw occurs will depend on the charge capcity of the remaining five batteries. 

Fortmiati?ly, tile probability ofNifl2 battery failure due to an open circuit within the 

battery is universalfry considered to be emmely low. Short circuits within cells are far 

mor* likely (to occur eventually). These lower, but do not zm-out available battery 

capcity. The probability of battery cell short-circuits wilt be discussed later. 

Title: 

Technical Consultation of the Hubble Space Telescope (HST) 

Nickel Hydrogen (NiH2) Battery Charge Capacity Prediction 

Spacecraft Electrical Lord and Orbit Night Battery Jkpktion 

v& Spacecraft Confi~rntion 

Page # 

34 of49 

Table I. enumerates the spacecraft load and battery capacity uti1b.a of‘ various HS’T 

configurations. It gives values for the present science operation, for Software Sun-Point 

(SWSP), Zero-Gyro Sun-Point (xC;SP), Spin-Stabilized Sun-Point (SSSP), and Eiardware 

Sun-Point (HWSP) Safiemodes, and for hiravity-Chdient Mode (WM). 

The table illustrates that forecasts of HST viability vs. time based on battery charge 

capacity trends and extrapolations are context dependent. In two significant respects 

battery capacity requirements fa an on-going science operation are significantly higher 

than those for spacecd maintenance in a sun-point safemode. First, the load 

requirements of the modes differ, and deplete the batteries differently each orbit night. 

Second, the margin needed to assure safemode survival in a sdemode cd is different 

&om the margin required for assured safemode entry from the science operation. 

Primarily this difference arises because the safemodes are already sun-pointed or nearsun-pointed and because the battery capacity niargin need not protect for a failed initial 

entry into a software safemode that must be fbllowed by a successful hardware safemode 

entry from an arbitmy attitude. Later, this difference will be illustrated by comparing 

mission need dates for which the batteries 1) are assumed to support continued science 

operations until the mission, and 2) support HST maintenance in ZOSP until the mission. 

Battorg Testbeds and Flight Bpttcry Mpgnasties 

The HST Program resources available for diagnosis and evaluation of battery-related 

anomalies, EPS opdonal and hardware configuration changes, battery degradation and 

performance maintenanceienhancement options reside at the Marshall Space Flight 

Center and have been in continuous use since approximately 1989. These resources are 

the Six-Battery Tatbed (SBT) and the Flight Spw~ Battery (FSB). For the majority of 

the HST mission, the SBT and FSB have been cycled to mirror the cycling of the fight 

batreties under similar conditions. (For e~~~~plc, the pre-SMJB PCXI bus fmh was 

shulalad UI the SKI’ so as to exposc its Battkuv 5 and 6 to the anomalous ovtrchatging 

and underutilization of oii-orbit Batteries 5 and 6.) Additional SRI’ and FSEI dascriptions 

are provided in the Appendix. 

6 
I 'I 

Title: 

Technical Consultation of the Hubble Space Telescope (HST) 

Nickel Hydrogen (NiH2) Battery Charge Capacity Prediction 

nsiltation Position Pa 

Page #: 

35 of49 

TaMe 1. Eiectricat Power Needs and Bat 

Requited (watts) 

-2400 

SWSPlZGSP 

w/psyfoad load shed 

(dpc4Eon) 

-1780 

-1450 

HWSP (Wiur oomm.) 1 -1440 

OaM * 

I Usage of Various WS? 

Minus a load 

:quivdc?nt to NICMQS 

& NCS (watts) 

[An one example of a 

laad reduction1 

-1900 

-1680 

~ 

-1350 

-1 340 

18hr-1500 

onfiguratons 

Max AIIr used 

-Mi42 

-4o/-37 

-32f-30 

-32i-30 

-191-16 

-221-20 

-3a-33 

* GGM dll under assesment. capability in standanl configuration is questionable. 

Recently, these test articles have begun to be used to explore approaches for improving 

the condition and performance ofthe flight batteries. Figures 1 and 2 document a recent 

dzep discharge of SBT battery 6. Typical features of battery aghg and degradation are 

redly m. First, originally similar battery cells now have rnarbdiy diRerent 

chofacteru. This is swn (Figure 1 in the way successive cells stop supporting voltages 

above X .2 V at different time; in the way their voltages decline differently to "second 

piatmu-' voltages around 1.0 V, and by where? exhausted of stored charge, each drop to 

7 
Consultation Position Paper RP-04-08 0.01 

a reversed voltage condition. Figure 2 shows the wide variance in the charge capacity 

available above 1.2 V, fiom 1.2 V to 1.0 V, and fiom 1 .O to 0.5 V. While many of the 

cells still store a large hction charge they could o@y store, much of this charge is 

no longer available above 1.2 V. In addition, individual cells charge to slightly different 

peak voltages and discharge to shghtly mkrent minimum voltagea during the testbed 

equivalent of orbit night. Note also (Figure 2) that the capacity (above 1.2 V) of the 

battery as a whole, 53 AHr, is only 4 AHr greater than the capacity of the weakest battery 

cell (49 AHr). This battery’s ability to provide ~26.4 V ww lost once four of its 22 cells 

dropped below 1.2 V . The present challenges include reducing the divergence in 

capacity (charge balance) among cells, and either stemming continued loss of capacity or 

increasing the percentage of cell capacity above 1.2 V. 

- 7 

Title: 

Technical Consultation of the Hubble Space Telescope (HST) 

Nickel Hydrogen (NiH2) Battery Charge Capacity Prediction 

1 .e 

Qischarge Load Reduction (Bdtefy at 26.4 v) 

Page # 

36 of49 

Fig. 1. Monitoring of individual cell volt es during disrhqe meals awide range of u s below l?V each cell 

plateau ’ of -1 V b$d%e%fy plummetd and the cePfOgoes into revmd Xhm 4 cells have 

dro ped to thew 2 plateaus the aery vol e s below 26.4 V. Cell voltages La nbouod as 8, test’s 5.1-ohm dischsrge resistor is c%an%for P 31-ohm resistor. The subltuboa 

E.. go mto revmal) he lh-smaller ZLhe cumnt markedly extends test duration. 

capacity. Once its volt -des some capac on aY4%g1 

vcnts.the test from subjecting cells to a undesirable mvenral arrmts after they “drop out” 

Monitoring of SBT Battery 6 since the March test has revealed that post-test battery 

performance improvements persisted only temporarily. The initial conclusion drawn 

fiom this is that HST battery degradation rates will be halted, and improved performance 

sustained, only if amore effective technique for recharging the batteries after discharge is 

found. It appears to be very important that all of a battery’s cells be initially recharged 

above the usual on-orbit charge cut-off voltage. 

8 
NASA Engineering and Safety Center 

H Consultation Position Paper 

Document # Version # 

RP-04-08 0.01 

Title: Page - #: 

Technical Consultation of the Hubble Space Telescope (HST) 37 of49 

Nickel Hydrogen (NiH2) Battery Charge Capacity Prediction 

Using the FSB, attention is now being fmed on how to optimize recharge. The 

subsequent, and more difficult question will be how to transfer this process to the 

batteries in the flight system so as to reetore charge balance among their cells and 

maintain that balance for usefully longs periods of time. The challenge exists because the 

six €hght batteries (three batteries m each of two modules, with one module in each of 

two adjacent battery bays) are electrically in parallel (and non-isolatable), and because 

the bay design limits the tolerable thermal dissipation of the batteries . 

The voltage of evq cell in the SBT and FSB can be individually monitored. On orbit, 

each flight battery’s aggregate terrninal voltage can be monitored, but individual cell 

voltages are not telemetaed. Moreover, each battery contains only two cells whose 

pressures can bemeaswed by strain gauges mounted on them, and only one of these is 

visible on the HST’s “active-side.” (Switching to redundant-electronics for the additional 

measurements has its own risks and is highly undesirable.) 

Nonetheless, when a €hght battery is discharged to 15 V during a periodic reconditioning 

exercise, the aggregate battery voltage vs. AHrs out signature permits determination of 

the usem battery capacity and also yields indicators of individual cell performance. 

Result8 &om the March 29,2004 discharge of flight battery 2 are shown in Figure 3. 

This test found the Battery 2 charge capacity above 26.4 V to be 50.8 AHr for a 9-amp 

load. It also provides indications that the battery voltage fell below 26.4 V after five cells 

9 
I NASA Engineering and Safety Center I Vmion # 

Consultation Position Paper 

Title: Page # 

Technical Consultation of the Hubble Space Telescope (HST) 38 of49 

Nickel Hydrogen (NiH2) Battery Charge Capacity Prediction 

dropped to a second plateau of -1 V, and shows the dropout (reversal) of six cells before 

the battery voltage reached the test cutoff value of 15 V. 

24d 

f f 

4~m Q.W ssm eo.m es.m mm 7s.m wm OS~ 

.a*r).lAhkor. 

Fig. 3. In the aggregate Merth 2004 discharge plot ofBPttay 2 voltage w. AHrremovcd (rcd 

line), the signatures of cells falling -0.2 V to their 2"plateaua, dischac resistor substitution, and 

cell dmpout (0.8-1V decreases) we analogous to the cell-level sign- seen in Figure 1. 

As is to be expected, the HST fl@t batteries and the test assets at MSFC exhibit similar 

charactexistics. All of the flight batteries are losing usell charge capacity, and 

experiencing increasing cell divergence with time. Because cell divergence is increasing, 

the battery capacity re@ derived from the pressure of the de, actively monitored 

cell in each battery is less reliable as an overall capacity indicator than it once was. 

Cell Failures Mechanisms and Results from Destructive Physical Analysis of Flight 

Spare Battery Cells 

Cell failures within NiH2 batteries are usually attributed to two principle mechanisms: 

gradual loss of capacity red- in inability to maintain a required voltage for a required 

time while supporting a required load, and cell loss caused by an internal 'bd" short 

circuit Although different shorting paths can develop, the usual mechanism within an 

HST-type NiH2 cell is the development of a conductive path though the insulation 

separating successive plates. In HST's NiH2 cells, the insulation is provided by a 

double-layer Zircar separator. 

10 
Technical Consultation of the Hubble Space Telescope (HST) 39 of49 

Nickel Hydrogen (NiH2) Battery Charge Capacity Prediction 

Overthe yeme various models have been used to predict the probability ofeqmhcing 

8 f&Im in one or more af WSTs battery cells, The definition offhilure used in these 

models is itwitwive of hard yhort circuils within dls. hut much m broadly and 

predornlnaniy atbcompasscs all 0th ehang;r?ti that deprive a cell of the capacity above 

some minimum voltage thal is required by the load the eel1 must support. The models 

produce €dum probabilities as a hction of battery DQD and cumulative dkwharge 

cycles. The madel publiebed by D. Hdm in 1998 predicted the likelihood of one WST 

call failure after 13-14 years of operatian. A difTerent model, by A Zhmman in 2004, 

predicts that HST is Sess likety to have a cell fdrlure before 201 1 thorn it is to have one. 

&ah aersessments, hwewr, defm failwe as the inability of a cell to provide a certain 

capacity down to 1 V. In Zimmennan's model, a cell is failed when it dossnY femain 

above 1 V down to a MdD ab' 10% (about 9 M). 

Fat he masons, the abovedad models canna be used to ddy answer 

questions about HWs future viability on-orbit. First, neither model dinxtly addresses 

cell performance (or totad battery eapa~ity) down to 1.2 V; nor do the databases used to 

develop the models contah specific informdon on discharge cycle life to 1.2 V. 

SaCoMt, as ia itlu9tmtsd &ow, a significant portion of HST battery cap=@ is no ionger 

gvailsblc above 1.3 V. Third, as is 

26.4 V is no longer tamtaid when as few &9 four cella (for now al least) fall to "mnd 

plateau" &hr@ vaitagm of4 V. Cells do not med toofoil per the encornpssmg 

dejiniiim ofcelJfailwe for a baaery to lack adequate capac~y above 1.W- a small 

mbm 0fth.m OH& nd to drop to their r"'pXolem wiltage. 

In anticipldion ofthis papar's Table III, another illustration of this point (with a simple 

om of many patentiah examples) is, useful: Iferch battery is to provide an equal sbared 

the 1 10 AHr total usefkl capacity needed to comply with the criterion in the table's last 

row, then, per Figures 1 wd 3, no mate than four ar dive of its 22 cdls can fall to their fd 

plateaus before the battery has provided -18 AHr of charge. Neither presently observed 

HST cell trends nor available models can predict the SUCIXSE or failure ef a battery to 

meetthiscriterim 

m Figures 1,2 and 3, a battery VO~~E above 

Returning to the topic of cell short circuits, ~~an~S model also predicts that the 

probability of a hard cell short is four orders @magnitude smaller for the HST operating 

condition than the probability of failure within his broader definition There ere, at 

present, no reported instancr?s afa hard short in my NiM2 cell posesing double-layer 

Zircar sqwldots artd cycled per HST Operating conditions. 

At &&rent times owr the last sevd yeam, four cells have been removed fiom the 

original 22-(xsll FSB and subjected to Destructive Physical An&& (DPA). DPA results, 

including thorough chemical and physical dym, have been documented elsewhere, 

and will not k ovedwd herein. %e single point to be made hemin concern the 

absence of evidence af incipient cell failure resulting from migratiadpnetra&m of 

condudiva material thrairgh the dual Zim sparators in the cells. In fmk no 

pmetdons spanning more than thc first Zircar layer were observed. If dl the Right cells 

are behaving similarly to this smdl sample from the flight spa battery, it may be some 

11 
Title: Page #: 

Technical Consultation of the Hubble Space Telescope (HST) 40 of49 

Nickel Hydrogen (NiH2) Battery Charge Capacity Prediction 

time before any of them sqm-ience a hard short. Qn balance, no firm conclusion can be 

dmwn. On the one hand it seems vexy reasonable to assume the absence of‘a hard cell 

short in the HST flight batteries during the next live or six yem. On the other hand, the 

DPA sample is mall, aid the batteries met their lifetime requirement long ago. The 

ocoup~ence of a cell short would hardly he termed anomalous. 

Loss of charge capacity, Irowever, is clearly a phenomenon occurring in the HST 

batteries. The key question is whether tho total system"^ recent capacity loss rate will 

persist, increase. lessen, or even be reversed by improvements that might be made to the 

manner m which the batteries am discharged during reconditioning and suhsequesltly 

recharged. ”he answers to these questions await fiuther testbed experimentation and 

additional tests of the on-orbit batteries. 

Battery Chnrpe Capadty History and Trends 

Primarily since 1994, periodic discharpbased measurements of the capacity of ench 

battery have been made -partly to update the calibration of the strain gauges on the 

pressure-monitored cells to correct for anticipated beginningsf-life ddt, and partly to 

obtain direct measurements ofthe total battery capacity. Over the last few years, gauge 

drifl has become negligible. and the inacwmcy of capacity estimates from single-cell 

measurements has become a function of the increasing voltage and capacity divergence 

of each battery’s 22 cells. Figure 4 shows the historical word of capacity measurements. 

E3y 19% decreasing battery capacity had hecame enough of a concern that a decision wam 

made to turn off the primary battery heaters and allow the batteries to drop several 

degrees C in temperature. As expected, this change increased the capacity of the batteries 

and succeeded in maintaining the 1995 total system capacity through the end of 1997. 

There were no measurements in 1999 as SM3A approached, and only a single 

measurement (for Battery 3) in 2000. Following the appearance of the impedance fault io 

PCU &Is C, the next battery reconditioning was halted when it resulted in instability of 

the fault impedance. This phenomenon caused a hiatus in capacity measurements until 

PCU-R installation in March 2002. Measurements resumed in 2002 and yielded a modest 

loss of battery capacity between 1998 and 2002. The annual average capacity loss was - 

2.0 iU-ftlbaaeryl’y~i~ across this time interval. 

As shown in Table If, concerns abwt battery longevity arose during the reconditioning 

“season*’ in 2003. Second-epoch measurements of three batteries yielded a much steeper 

annualized average capacity decline aver the preceding year: -6.3 AHr/battery/year, or 

triple the average over the preceding 4 years. This average excluded the mult for 

Battery 5 (-16.6 AHr!year), which wm most Hected by overcharge while the PCU bus 

12 
YASA Engineering and Safety Center 

Title: 

Technical Consultation of the Hubble Space Telescope (HST) 

Nickel Hydrogen (NiH2) Battery Charge Capacity Prediction 

Consultation Position Paper 

page R: 

41 of49 

Document #: 

RP-04-08 

Version #: 

0.01 

i *I 

li 

"k 

t *"f 

0 

r 

impedance 

a 3 

Ut existed. A key question to be answered in June 2004 is whether Battery 

5 continuea to lose capacity rapidly, or is now following the trend of the otha batteries. 

With the April 2004 second-epoch measurement for Battery 6 now included, the 

annualized capacity loss rate calculated fiom five batteries remains -6.3 

AJWbattery/year. 

Forecasts Based on Charge Capacity Trends 

Absent the answer to the previously posed question about future charge capacity trends, 

forecasts of future HST mission capability will now be made using the currently available 

data. It i assumed below that the annual capacity decline in Battery 5 is now equal to the 

average decline of the other five batteries with two capacity measurements since SM3B. 

Thie key Bgsumption will be confumed or refuted in late June 2004. For now, it appears 

reasonable because the Battery 5 capacity presently derived fiom its strain gauge reais little different fiom the elapsed-time-adjusted July 2003 discharge-derived capacity. 

With this assumption, the annual system-level capacity loss rate (rounded up) is presently 

-38 AHrbear. 

13 
Consultation Position Paper 

I I 

Title: I Page# I Technical Consultation of the Hubble Space Telescope (HST) I 42of49 

1 Nickel Hydrogen (NiH2) Battery Charge Capacity Prediction I 

Tdh L 

HST NiH, Battery Capadty Measurements Through April 27,2004 

With just a few measuremanta so far, each additional datum can affect the average 

significantly. The present spread in recent measurements suggests that it's not yet 

meaningll to talk of statistical significance. Moreover, as the battexies age fiuther, the 

trend line slope may become shallower (especially if testbed experimentation identifies 

useM capacity preservation techniques); or it may become steeper, or even non-linear. 

With thwe possibilities in mind, the present forecast reduces the last-determined capacity 

of each battery by the adopted average annual loss rate times the elapsed year hction 

since the measurement. The start-of-April 2004 system total is thus estimated to be 3 12 

AHr. Linear extrapolation into the future follows and permits construction of Table 111. 

Actual system performance may be better or worse, and move the negative-margin 

predictions for each row of the table backward or forward in time. 

Table 111's enties lead to several conclusions (in reverse chronological order of 

occurrence): 

1. Through mid-2009, HST will most likely survive continued battery capacity loss 

at the assumed, constant rate if it is being maintained in a low-load Sun-Point 

Safemode controlled by the HST486 computer. After that time the risk of mission 

Mure ifHWSP entry occurs will grow. 

2. Sometime in 2008, power limitations may require halting the science program and 

placing HST into, and maintaining it in safemode. 

3. In 2006, inftared science with NICMOS/NCS may have to be terminated, or a 

mix of other load reductions imposed. Alternatively, the HST Program may have 

to sigmficantly lower the SOC safemode triggera now in use and, earlier than 

otherwise, move decisively toward adoption of the "Minimum Reserve Capacity 

for Science Operations" defined in Row 3 of Table 111. 

4. Some downward adjustments to the current EPS SOC safemode triggers will be 

needed in 2005. 

14 
NASA Engineering and Safety Center 

Consultation Position Paper 

Document x: Version #: 

RP-04-08 0.01 

Title: Page #: 

Technical Consultation of the Hubble Space Telescope (HST) 43 of49 

Nickel Hydrogen (NiHz) Battery Charge Capacity Prediction 

23 

78 40152' 

lO!Yl17* 

164 

Reaerve Capacity for 

EWSP Warode (70 

AHr"") HEST is being 

*Minus a load equivalent to turning NICMOS 

April %-p+ AprU Aplil Y 

2/14" 

67/79" 

126 

cs off 

I 

~ 

- 

12 

**Suitability of a 70 AHr reserve capacity if entering HWSP fiom ZGSP is subject to 

vdication in near-term simulations. Based on present knowledge it includes 14 AHr 

uncertainty in the system-level capacity derived solely from pressure readings of 1 

cellibattery. 

-= Minimum Reserve Capacities no longer ensure survival ifcertain additional 

failures occur concwently with safaode entry (e+, fdwe of the active si& of the 

Hdware Safemode computer). 
Technical Consultation of the Hubble Space Telescope (HST) 44 of49 

Nickel Hydrogen (NiH2) Battery Charge Capacity Prediction 

Relationship oPPointing Performance to Battery Capcity Requimments in 

Safemode 

Coih&ms 1 and 2 ofthr: previous section poiiit to the cvcntual need to maintain EIST in 

n low-load Sun-Point Safemode controlled by the HST486 computer. Whether or not a 

One-Gp Science Mode is found feasible, tIST will likely be in, or have to be put into 

ZOSP in 2008. The Prognun's expFicnce with ZCSP (in late 1999 and since) is 

generally positive - especially since the introduction of several software fixes in 200. 

Nevertheless, a known characteristic ot'the current ZGSP implenienlal;ian is that the IfST 

will occasionally driR coiisiderably off its required orientation during orbit night. The 70 

ANr minimum reserve is believed to b adequate for both a large-angle sun-point 

recovery and WWSP entry should it be required. To verify this, computer runs simulating 

numerous starting conditions will be conducted, and will include the additional 

assumption that EIST has only three working Reaction Wheel Assemblies (RU"). 

Meanwhile, the Program's Two-Gyro Science Mode development experience suggests 

that a replacement ZGSP that provides good attitude stability regardless oForbit phase 

might be possible. This option will be carefully assessed in due course. 

The On-going Battery Test Program 

An exptnienced team of WST contractor and CiSE'C civil servant EPS and battery 

engineers are acti\dy engaged in ddining, executing and interpmthg results from a 

continuing series of SKI' and FSH hsts. In parallel, the 2004 series of on-orbit battery 

reconditioning tests is underway. MSFC tests are focusing on three objectives: reducing 

the degree of voltage, impendence, and capacity divergence that has developed within the 

cells of HST's NiR2 batteries; finding a flight-viable technique for improved battery 

charging that stems ~~11 divergence and increases the charge capacity available to support 

the minimum necessary bus vokagc; and demonstrating that candidate techniques can be 

successfully applied to the on-orbit batteries. These effortq will continue through 2005. 

On-orbit tests ofthe best approach will likely begin during Spring 2005, if not before. 

Summary 

Ihe principle concern regarding useful IfST battery life and mission survivability is the 

rate of loss of iota1 system charge capacity identified in 2003. While hard shorts in 

battery cells may OCLW eventually, there is no present evidence that they wiU occur soon 

enough to be relevant. ,bsurning the observed capacity loss rate reniains ~mstrtnt over 

the next five years, the HST science program will have to be halted, and the spaced 

will have to be placed in safemode in 2008. However, in safemode HST should surVive 

on-otbit into the second half of 2009. Prior to 2008, new operational constraints will bc 

required and the use ofsmnller, less robust EPS %demode margins will hnvc to be 

awepted. Eflom ltro underway to identi@ baltcr.ry wwnditionhg and recharge 

techniques that will stem the rate of capauity loss, improve cell charge balance, and 

prolong mission viability. Relevant tests using the MSFC battery testbeds have begun. 

16 
NASA Engineering and Safety Center Document K 

Consultation Position Paper RP-04-08 

Title: 

Version d 

0.01 

Page # 

Appendix 

Technical Consultation of the Hubble Space Telescope (HST) 

Nickel Hydrogen (NiH2) Battery Charge Capacity Prediction 

Flight nd Testbed Battery Dcscripth 

45 of49 

17 
HUBBLE SPACE TELESCOPE PROJECT 

1 HST Nick.1 Hydroaen (NiHJ Cell b 

1 Deaig~ Desk&pdon -. I 

- Eagk Picher Techndogka (Em RNH 90-3 

- A1r Force MAWECH Design 

- Rabbit Ear Terminals 

- Graduated Loa& 

- DwbIa Layer Zltcu S.$aratw 

- 8.Clr-tilFBack uscbpa ConAgurstlon, 48 Pt.tm 

- Dry Sinter Nidr.1 Plaque (84% Pmny) 

- EPT-Cdondo Springs Plaque 

- Ebctrochemicaf lmpngnatfon 

- EPtJoplin lmpnOnMlon a Assembly 

- Zirconia Wall WIck 

- 27% KOH (At Di-gclcl) 

. ,- , 
llUCi 

Technical Consultation of the Hubble Space Telescope (HST) 

Nickel Hydrogen (NiHz) Battery Charge Capacity Prediction 

47 of 49 

19 
Document #. 

HUBBLE SPACE TELESCOPE PROJECT 

I HST Resources C 
Technical Consultation of the Hubble Space Telescope (HST) 

Nickel Hydrogen (NU&) Battery Charge Capacity Prediction 

Approval and Document Revision History 

49 of49 

Approved: Original signature on file 7/6/04 

NESC Director Date 

Version Description of Revision 

0.01 Initial Release 

Author Effective 

Steven Gentz, NESC Lead 

Date 

July 17,2004 
REPORT DOCUMENTATION PAGE 

01- 09 - 2005 I Technical Memorandum 

4. TITLE AND SUBTITLE 

Form Approved 

OMB NO. 0704-0188 

Technical Consultation of the Hubble Space Telescope (HST) Nickel 

Hydrogen (NiH2) Battery Charge Capacity Prediction 

1. REPORT DATE (DD-MM-YYYY) 12. REPORT TYPE 

6. AUTHOR(S) 

Gentz, Steven J.; Pandipati, Radha "Chetty"; Ling, Jem; Miller, Thomas; 

Jeevarajan, Judith; Halpert, Gerald; and Zimmerman, Albert 

3. DATES COVERED (From - To) 

7. PERFORMING ORGANIZATION NAME@) AND ADDRESS(ES) 

9. SPONSORlNGlMONlTORlNG AGENCY NAME@) AND ADDRESS(ES) 

NASA Langley Research Center 

Hampton, VA 2368 1-2 199 

10. SPONSOWMONITOR'S ACRONYM(S) 

5a. CONTRACT NUMBER 

5b. GRANT NUMBER 

5c. PROGRAM ELEMENT NUMBER 

17. LIMITATION OF 

ABSTRACT 

16. SECURITY CLASSIFICATION OF: 

a. REPORT b. ABSTRACT c. THIS PAGE 

5d. PROJECT NUMBER 

5e. TASK NUMBER 

5f. WORK UNIT NUMBER 

18. NUMBER 

OF 

PAGES 

104-08-33 

8. PERFORMING ORGANIZATION 

REPORT NUMBER 

U I U 

I 

U I uu I 54 

L-19171 

National Aeronautics and Space Administration 

Washington, DC 20546-000 1 

NUMBER(S) 

I NASAITM-2005-2 139 16 

12. DISTRIBUTION/AVAILABlLlTY STATEMENT 

Unclassified - Unlimited 

Subject Category 17 

Availability: NASA CAS1 (301) 621-0390 

13. SUPPLEMENTARY NOTES 

An electronic version can be found at http:llntrs.nasa.gov 

14. ABSTRACT 

The purpose of the GSFC position paper is to identify critical HST milestone dates for continued science studies followed by the attachment of a re-entry 

module or a robotic servicing mission. The paper examines the viability of the HST with respect to the NiH2 continued battery charge capacity. In the course 

of the assessment, it was recognized that the HST battery thermal control system has an average heat dissipation limitation of 30 W per bay per orbit cycle. 

This thermal constraint will continue to govern options for battery capacity maintenance. In addition, the HST usage represents the longest exposure ofNiH2 

batteries to Low Earth Orbit (LEO) at the current level of Depth of Discharge (DOD). Finally, the current battery life is at the limit predicted by the 

manufacturer, Eaglepicher. Therefore, given these factors, the potential exists that the HST battery capacities could radically degrade at any point. Given this 

caveat on any life extrapolations, the conservative model proposed in the GSFC position paper was viewed by the NESC as having several technical 

assumptions such as limited utilization of flight battery capacity data, the susceptibility of the proposed prediction method to large variations when 

supplemented with additional information, and the failure to qualitatively or quantitatively assess life prediction sensitivities. The NESC conducted an 

independent evaluation of the supporting information and assumptions to generate the predictions for battery capacity loss and practicality of on-orbit battery 

conditioning. 

15. SUBJECT TERMS 

Battery charge control management; Hubble; NiH2; System aspects; VIT; Battery; Life prediction Methodology; Peak rates; 

Ratchet; Voltage 

19a. NAME OF RESPONSIBLE PERSON 

STI Help Desk (email: help@,sti.nasa.gov) 

19b. TELEPHONE NUMBER (/dude Bft?a code) 

(301) 621-0390 

Standard Form 298 (Rev. 8-9f 

Prescribed by ANSI Std. 239.18