The battery performance of the IUE spacecraft launched in 1977 is updated. Changes made in the battery operational procedures are detailed. The aging effects after several eclipse seasons and the effects of 8 C temperature was anticipated. The batteries met their design goals and should meet the 10 year goal

Tiller, S. E.

English

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PERFORMANCE OF THE IUE SPACECRAFT BATTERIES 
AFTER 70 MONTHS 
Smith E. Tiller 
Goddard Space Flight Center 
ABSTRACT 
The IUE spacecraft was launched on January 26, 1978 into an eccentric 
synchronous orbit with inclinations of 28.940 to the equator. The IUE power 
system contains two 6 ampere hour nickel-cadmium batteries designed to provide 
a beginning life of 145 watts during the eclipse seasons. In addition, 
battery power is required when the main buss exceeds the solar array output. 
The spacecraft design life was for three years with a design goal for 5 
years. As of November 1983 the batteries have supported the spacecraft 
requirements for 70 months with excellent performance. 
Papers presented at the 1978 and 1979 battery workshops describe the 
properties of the IUE power system including battery and cell characteristics. 
The objective of this paper is to update the battery performance since 1979. 
CONFIGURATION 
The battery in figure 1 is typical of the two flight batteries aboard the 
IUE spacecraft. Each battery contains 16 regular 6 ampere hour nickel-cadmium 
cells and 1 signal electrode cell. The batteries are diode coupled to the 
main buss via a boost regulator that provides 28 volts of regulated power to 
the spacecraft. Battery telemetry data includes battery voltage, current, 
signal electrode voltage and battery temperature. 
INTRODUCTION 
The eccentric synchronous orbit requires the spacecraft to experience 2 
eclipse seasons each year, in coincidence with the solar equinox. Shadow 
period range from 14 to 77 minutes with each eclipse season lasting from 23 to 
25 days. During periods when the solar array (spacecraft beta angles less 
than 00 or greater than 1300) is sufficient to supply the spacecraft power 
requirements, the batteries are required to augment the spacecraft needs. 
FLIGHT PERFORMANCE 
Each battery charger was designed to provide charge control as illustrated 
in figure 2a. This was accomplished by a C/10 (0.6 amp) charge current limit 
followed by a taper charge to be controlled by the signal electrode. Details 
of this charging scheme was presented during the 1978 battery workshop. 
During the early part of the eclipse season 1 it became apparent that the 
https://ntrs.nasa.gov/search.jsp?R=19840025621 2020-03-22T10:18:28+00:00Z
s igna l  electrodes were not performing i n  accordance with design. Figure 2b 
represents  a t yp i ca l  charge p ro f i l e  as observed i n  o rb i t .  Because of t he  
operational  anomaly, procedures were modified f o r  manual switching of the  
ba t te ry  chargers t o  a t r i c k l e  charge mode once t he  s i gna l  electrode had 
exceeded 0 . 1 v o l t s  f o r  3 hours. The t r i c k l e  charge r a t e  i s  0.1 amps. The IUE 
c e l l s  on l i f e  t e s t  a t  t h e  Naval Weapons Support Center (NSWC) a t  Crane, 
Indiana did  not i n i t i a l l y  show the  anomalous behavior. However a f t e r  several  
ec l ipses  the re  w a s  a progressive l o s s  of s i gna l  e lect rode s e n s i t i v i t y  t h a t  
resul ted i n  the  t e s t  c e l l s  experiencing an increase  i n  overcharge. A s  a 
r e su l t ,  t he  t e s t  a t  Crane was  modified t o  be ind ica t ive  of the  spacecraft  
recharge scheme f o r  the  f l i g h t  ba t te r ies .  
During the  s o l s t i c e  seasons the  spacecraft  b a t t e r i e s  a r e  maintained i n  t he  
low r a t e  t r i c k l e  charge mode except during periods when science a c t i v i t i e s  
require  ba t te ry  power t o  augment spacecraft  operations. As  a result of normal 
so l a r  a r r ay  degradation, the  beta angles a t  which the  power system buss 
remains power posi t ive  has decreased. Consequently addi t ional  demands have 
been placed on t he  b a t t e r i e s  t o  support an  increase  i n  the  demand f o r  
s c i e n t i f i c  data. These a c t i v i t i e s  have required t h e  ba t t e r i e s  t o  be 
discharged a t  low r a t e s  while  augmenting so l a r  a r ray  power. Because of the  
addi t ional  demands being placed on t he  ba t t e r i e s  operational  const ra ints  were 
developed t o  minimize bat tery  degradation and/or damage. The constra ints  
included a lower l i m i t  of ba t te ry  voltage of 20.9 v o l t s  f o r  average spacecraft  
loads, and 20.5 v o l t s  f o r  peak loads. When e i t h e r  of these conditions occur 
t he  spacecraft  is maneuvered t o  a more favorable sun angle f o r  bat tery  
recharge. 
Since t he  exact re la t ionsh ip  between t he  above s t a t ed  bat tery  voltage, and 
bat tery  s t a t e  of charge was not c lea r ,  the  l i f e  t e s t  a t  Crane w a s  interrupted 
t o  evaluate these new conditions. Following an 8 week t r i c k l e  charge (0.1 
amp), t he  Crane c e l l s  were discharged a t  .05 amps t o  a 20.5 vo l t  l i m i t .  
Following t he  discharge, the  c e l l s  were recharged and placed on a t r i c k l e  
charge f o r  7 days pr ior  t o  repeating the  t e s t  a t  a .1 amp discharge ra te .  
Figure 3 depic ts  the  discharge p ro f i l e  f o r  both of these conditions and 
compares t he  data  with the  voltage l i m i t s  se lected f o r  spacecraft  operations. 
Typically the  amount of ampere hours experienced from the  spacecraft  ba t t e r i e s  
a t  the  normal discharge r a t e s  (3 t o  4 amps) i s  2.3 t o  3.8. This is  contrasted 
with the  4.5 t o  6 ampere hours obtained a t  the  0.5 amp and .1 amp discharge 
ra tes .  
Figure 4 i s  a summary of the  composite of the  summer s o l s t i c e  periods 
using data  of 100 day in te rva l s .  A devia t ion from t h e  normal trend is  
i l l u s t r a t e d  a t  approximately day 1150 (Eclipse season 7) when the  s igna l  
electrode of ba t te ry  SN 05 increased t o  an output l e v e l  observed pr ior  t o  
launch. The reason f o r  t h i s  peculiar  behavior of t h e  s igna l  electrodes a r e  
not  known a t  t h i s  time. The low point observed a t  around 1300 days i s  
a t t r i bu t ed  t o  t he  mode of operations of t he  spacecraft .  This f igure  a l s o  
shows t h e  ba t te ry  temperature h i s to ry  and i l l u s t r a t e s  the  approximate 8 degree 
bat tery  temperature d e l t a  t ha t  has been prevalent throughout the  mission. 
During t h e  e a r l y  s t ages  of e c l i p s e  season 12, t h e  f i r s t  ind ica t ion  of 
ba t t e ry  current  divergence an  discharge i s  apparent (Figure 5)* By day 5 of 
t h e  season t h e  ba t t e ry  current  divergence reached a maximum of .8 amps near 
end of discharge. By day 16 load sharing between t h e  two b a t t e r i e s  was back 
t o  near normal. The exact  cause of t h i s  divergence i s  not  known. However, i t  
i s  thought t o  be associa ted  with v a r i a t i o n  i n  t h e  p la teau voltage between the  
2 ba t t e r i e s .  Battery cur ren t  divergence w i l l  be f u r t h e r  analyzed during t h e  
f u t u r e  e c l i p s e  seasons, 
Figure 6 i l l u s t r a t e s  a summary of GSFC c e l l  t e s t ,  spacecraf t  ba t t e ry  
performance, and r e s u l t s  of c e l l  l i f e  t e s t  conducted a t  Crane as compared t o  
pre-launch predict ions.  The ba t t e ry  voltage degradation predic t ion curve 
shown i n  t h i s  f igure ,  w a s  obtained from d a t a  acquired during synchronous o r b i t  
t e s t i n g  of o lder  c e l l s  of s i m i l a r  design, The ava i l ab le  power degradation 
predic t ion curve was ca lcula ted  using t h e  above b a t t e r y  vol tage  predic t ion data ,  
so la r  a r ray  degradation and the  e f f i c iency  fac to r  of t h e  power system boost 
regula tor .  
The GSFC t e s t  r e s u l t s  shown i n  f i g u r e  6 was acquired from Pnhouse test on 
f l i g h t  c e l l s  t o  an  80% peak depth of discharge and using the  s i g n a l  e l ec t rode  
f o r  charge control .  The t e s t  was accelera ted  by reducing the  s o l s t i c e  periods 
from 5 months t o  1 month. This t e s t  r e s u l t  w a s  used as a basel ine f o r  p r i o r  
t o  launch predict ions.  
The spacecraf t  ba t t e ry  vol tage  shows t h e  trend f o r  t h e  end of e c l i p s e  
voltage s ince  launch. The first 6 e c l i p s e s  t y p i c a l l y  required the  ba t t e ry  t o  
experience 60 t o  75% depth of discharges. I n  add i t ion  the  spacecraf t  loads 
required each b a t t e r y  t o  be discharged i n  excess of 4 amps during t h e  e c l i p s e  
operations. I n  t h e  i n t e r e s t  of obtaining maximum ba t t e ry  l i f e  (new goal  of 10 
years  l i f e )  power conservation measures were implemented t o  l i m i t  t h e  maximum 
ba t t e ry  discharge t o  4 amps per bat tery .  The s e l e c t i o n  of the  4 amp r a t e  was 
predicated on l i f e  t e s t  experience, 
The Crane da ta  depicted i n  f i g u r e  6 i l l u s t r a t e s  t h e  r e s u l t s  of r e a l  time 
da ta  from snychronous o r b i t  t e s t i n g  a t  Crane wi th  c e l l s  from the  f l i g h t  l o t ,  
The Crane d a t a  a l s o  follows t h e  voltage predic t ion curve f o r  e c l i p s e  seasons 1 
through 7 but  a t  a s l i g h t l y  higher vol tage  l e v e l  than t h e  predicted curve, 
During t h e  e a r l y  e c l i p s e  seasons t h e  s i g n a l  e lec t rode  degraded t o  a point  
where the  current  t aper  w a s  being control led  by t h e  b a t t e r y  voltage con t ro l  
alone. During e c l i p s e  7 t h e  con t ro l  was switched t o  another s i g n a l  e lec t rode  
c e l l  i n  t h e  t e s t  pack. During e c l i p s e  seasons 8 and 9 no improvement w a s  
evident  i n  con t ro l l ing  t h e  ba t t e ry  overcharge, P r io r  t o  ec l ipse  season 10 the  
charge con t ro l  w a s  modified t o  dupl ica te  t h a t  being used by the  spacecraf t ,  a s  
depicted i n  2b. 
The Crane t e s t  a l s o  provided f o r  capacity discharge during t h e  peak of t h e  
e c l i p s e  seasons. Cells were s e l e c t i v e l y  discharged beyond the  80% depth of 
discharge l e v e l  t o  1 , O  v o l t ,  For e c l i p s e  season 1, c e l l s  9 and 10 were 
discharged, e c l i p s e  season 2, c e l l s  8 and 9 were discharged, For each e c l i p s e  
season a n  a d d i t i o n a l  c e l l  w a s  added t o  each discharge group, u n t i l  a l l  c e l l s  
had been capacity discharged. Analysis of the  da ta  from these  type 
capacity checks indicated  t h a t  i t  was  c rea t ing  a vol tage  divergence between 
t h e  c e l l s  during discharge. Consequentially, p r i o r  t o  ec l ipse  season 11 t h e  
capacity discharges were modified t o  incorporate a l l  cells being discharged 
simultaneously. The da ta  as i l l u s t r a t e d  i n  f i g u r e  6 f o r  e c l i p s e  seasons 11 
through 14 i n d i c a t e  a progressive increase  i n  t h e  test pack voltage a t  t h e  80% 
depth of discharge l e v e l .  
Figure 7 i l l u s t r a t e s  t h e  lowest vol tage  l e v e l  that t h e  spacecraf t  
b a t t e r i e s  reached a t  t h e  end of discharge of each e c l i p s e  period of seasons 1, 
6, and 12. The d a t a  give an  ind ica t ion  of t h e  r a t e  of bat tery  voltage 
degradation over t h e  l i f e  of t h e  spacecraft .  The cons t ra in t s  ( l imi t ing  t h e  
ba t t e ry  discharge rates t o  4 amps) were e f f e c t i v e  i n  minimizing ba t t e ry  
degradation. 
Figure 8 i l l u s t r a t e s  t h e  discharge p r o f i l e  f o r  a day i n  e c l i p s e  season 2 
and 12 where shadow period were of equal t i m e .  The discharge f o r  day 6 of 
e c l i p s e  season 12 w a s  constrained by t h e  maximum discharge r a t e  of 4 amps. 
CONCLUSION 
It can be concluded that the  changes made i n  t h e  ba t t e ry  operat ional  
procedures have enhanced t h e  operations. The l o s s  of ba t t e ry  s i g n a l  e lec t rode  
s e n s i t i v i t y  i n  f l i g h t  was accommodated by changes i n  t h e  bat tery  opera t ional  
procedures, It w a s  an t i c ipa ted  t h a t  t h e  ba t t e ry  would s t a r t  t o  show some 
aging e f f e c t s  a f t e r  severa l  e c l i p s e  seasons and t h a t  t h e  e f f e c t s  would be 
somewhat amplified by t h e  80C temperature d e l t a  t h a t  t h e  b a t t e r i e s  have 
experienced during t h e i r  l i f e t ime .  The o v e r a l l  objec t ive  has been t o  a d j u s t  
ba t t e ry  operat ions without unduly constraining science a c t i v i t i e s .  Based on 
t h e  performance of t h e  f l i g h t  b a t t e r i e s  over t h e  f i r s t  6 years, it i s  believed 
t h a t  these  b a t t e r i e s  w i l l  meet t h e  new 10 year design goal.  
Figure 1. 


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POWER VOLTAGE A v a i l a b l e  Power And Voltage  V s  
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Q. Russell, Space Communications Company: When your batteries are used to 
supply the spacecraft power do you have to command them into service 
or does it happen automatically? 
A. Tiller, GSFC: It happens automatically. The batteries are on 1 ine all 
the time so it just automatically picks it up if they should slue the 
spacecraft to an angle that we lose data acquisition the batteries 
automatically since they are connected to the main bus automatically 
that go into operation. 


Performance of the IUE Spacecraft Batteries After 70 Months

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