A process for determining the waste heat generated in a 50-ampere-hour, nickel cadmium battery as a function of the discharge rate is described and results are discussed. The technique involved is essentially calibration of the battery as a heat transfer rate calorimeter. The tests are run at three different levels of battery activity, one at 40-watts of waste heat generated, one at 60, and one at 100. Battery inefficiency ranges from 14 to 18 percent at discharge rates of 284 to 588 watts, respectively and top-of-cell temperatures of 20 C

Mueller, V. C.

English

NASA Technical Reports Server

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NASA 50 AMP HOUR  NICKEL CADMIUM BATTERY WASTE HEAT  DETERMINATION 
V. Mueller 
McDonnell  Douglas 
The  test  that  we  ran  on  the  qualification  test  sample  was to determine  the  waste  heat gener- 
ated  in  the  battery as  a  function of the  discharge  rate.  The  technique involved is essentially 
calibration  of  the  battery  as  a  heat  transfer  rate  calorimeter. We think  the  test  procedure is rather 
simple,  and  it gives consistent  results. 
(Figure 3-77) 
As I said,  the  objective  of  the  test  was to determine  the  waste  heat  generated  as  a  function 
of  discharge  rate.  What  we do,  essentially,  is  that  we  mount  the  battery  on  a  cold  plate,  which  has 
a  circulating  fluid  through  it  and  which is temperature  controlled,  sufficient  to  maintain a constant 
temperature  at  the  planned levels of battery  activities. 
We ran the  tests  at  three  different levels of battery  activity,  one  at 40 watts  of  waste  heat 
generated,  one  at 60, and  one  at 100. We start  the  test  by  overcharging  at  some  fixed  rate, 46- t o  
100-watt level. We fill the  top  and  bottom. Cell temperatures  are  stable  and  remain  within  I/lO"C 
over  at  least  a  l-hour  period. 
By that  time,  we go directly  into  a  discharge. Our objective  is to maintain  the  same  tempera- 
ture  differentials  and  temperature  at  each  location in the  battery.  In  some cases,  we  did  a  very  good 
job   of  this.  In others, we had  a  little  bit of a  problem  trying  to  get  that  waste  heat  rate  adjusted 
properly. 
After we have got  that  point in general, we continued discharge for at least 2 hours, we 
discharge and  repeat  for  the  next  level. 
(Figure 3-78) 
This is an exploded view of our  50-ampere  hour  battery,  and I show it simply to indicate 
how  it was instrumented. 
We have instrumentation  on  the  thermal fins.  See  the  thermal  fins  interspersed  between  rows 
of cells. There  are 22 cells, 1  1 rows,  and 10 thermal  fins. We have  instrumentation  on  fin 1, which  is 
the  first  one  on  the  end  with  the  connectors,  fin  number 6 and fin number 10. The  fin  that we  used 
to determine  top  and  bottom  of cell  level was  fin  number 6. 
(Figure 3-79) 
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https://ntrs.nasa.gov/search.jsp?R=19800012358 2020-03-21T18:19:49+00:00Z
This  shows  how  the  thermocouples  are  placed  on  the  thermal  fins, 1,  6 and  10.  Fine  number 
6 has  the  greatest  number  of  thermocouples,  and  thermocouple  number 2 at   the  top is what  we 
refer to in later  charts as top   o f  cell temperature.  Thermocouple  number 10 is  the  bottom  of cell 
temperature,  and,  of  course,  thermocouple  number 10 is directly  on  the  heat  transfer  surface. 
(Figure 3-80) 
What I have  done  here  is  plotted  up  some  of  the  data  that  we  got  from  our  40-watt case, the 
first  case  we  ran.  On  the  left  of  this  chart  we  show  the  overcharge  phase,  where  we  are  overcharging 
at 40 watts. 
There is the  bottom  of cell temperature,  top  of cell temperature. We have  a  delta  of  2°C. As 
you  can  see,  the  variation  is  fairly  minor.  The  data I plotted  here is just  at  10-minute  intervals, 
and 1 just use  straight  lines to  connect  them. So you  do  see some  jogging  around. 
At  this  point,  70  minutes  on  this  chart,  we  went  from  charge to discharge,  and  we  initially 
set  the  power level hopefully  to  maintain  the  same  temperature  differential  and  the  same  tempera- 
ture. As you can  see,  we  were  a  little  low  at  first,  and  the  battery  started to cool. When the  operator 
saw it  starting  to  cool,  he  started  cranking  up  on  the  current. 
We do  the  discharge  at  constant  current.  He finally got  the  temperature  to  stabilize,  but  he 
overcompensated,  and  now  it  started  to  heat.  Again,  he  compensated  for  the  fact  that  it  was  heating 
and  tried  to  maintain  that level temperature of about  20.5"C  here.  The average power  over  this  time 
interval  was  284  watts. 
(Figure 3-8 1 ) 
We do  the  same  kind of tests  for  each  of  the  three  activity levels that we tested,  and  this 
chart  plots  them as values of delta T versus  overcharge  and  discharge  rates. You can  see on over- 
charge  it is fairly  linear.  The  three  points  that  we  ran  are  noted  on  the  chart: 40, 60, and 100 watts, 
roughly. 
However,  on  discharge  we  did  see  some  curvature. We think  that  curvature is due to I* R heat- 
ing,  which gives a  little  bit  greater losses at  the  higher level in  terms of percentages,  and  also  the 
battery  temperature,  top  of cell temperature  for  that  last  run, was  roughly  2.5 to  3" C, hotter  than 
the  previous  ones.  The  previous  ones  run  at 20"  C.  The  last  one  at  about 23.6" C. 
I might  point  out  that  those  percentage levels, 14, 15 percent,  etc.  are  just  the  ratio  of  the 
overcharge  rate,  the  waste  heat  rate to the  discharge  rate. 
(Figure  3-82) 
Our  conclusions  from  the  test  are  that  the  battery  inefficiencies range  from 14 to  18 percent 
at  those  discharge levels and  top  of cell temperatures  at  roughly 20°C. As I noted  before,  we feel 
that  the  test is simple to implement,  and  it gives consistent  results,  much  easier  than  a  calorimeter 
would  be,  for  example. 
270 
F 
I DISCUSSION 
THIERFELDER: Were these  done in vacuum or in  air? 
MUELLER: In air. 
THIERFELDER: Were they well insulated? 
MUELLER:  They  were  in  a  pilot  box,  and  they  had 3 to 4 inches of styrofoam all around 
the  sides  and on top.  And  the  base  was  mounted on a  cold  plate. 
HENDEE: Were you  able to run  any  stabilized  tests  throughout,  say,  the  discharge cycle? 
You must have noticed  your  percents  changing as  a  function of the  state  of  charge  or  state  of 
discharge. 
MUELLER: On the  second  test  we  ran  at 60 watts  of  waste  heat. I don’t  know  why  we  were 
so fortunate,  but  we  happened to pick  the  waste-heat  rate  or  the  discharge  rate  exactly. I t  wasn’t 
necessary to  change  it  throughout  the  entire  distance. We did  see  a  constant  discharge  during  that 
60-watt  test. 
HENDEE:  How  deep  depth of discharge? 
MUELLER: I think  on  the  60-watt case  we discharged  for  roughly 2 hours,  and  probably I 
would  say  was 75- 85-percent  DOD. We don’t  see  instantaneous - there’s  no way to  determine 
instantaneous  heat  generation  here. 
PALANDATI: I have two  questions.  What was the  maximum discharge currents  that  you 
performed  the  tests  on? 
MUELLER: I believe it was between 15 and 20 amperes. 
WEBB: On  the  third  test  it  was  close  to 23 amperes. 
PALANDATI : Basically then C/2? 
MUELLER: About 23 amperes, right. 
WEBB: May I please add: We did on the  third  run,  since  we  were  allowing  ourselves 2 hours 
of discharge  during  the  determination - and  at  that  rate  we  did run into  a  condition  at  the  end of 
the  third  run  where  we  were  depleting  capacity  and  efficiency  changed  there. I believe John  took 
data  from an  earlier  plateau  where  heat  was  stable  on  that  one.  But  at the end of the  test,  it  did 
show  an  increase. 
PALANDATI: I have  one  other  question.  On  your  temperatures  now,  did you always  obtain 
the  highest  temperature  up  at  the  top of the  cell,  or  did  you  see  variations? 
27 I 
MUELLER: No. the  highest  temperature was  measured on   top   o f   the  cell,  yes. 
SCOTT: Did  you  find  at  a  constant  current  that  the  efficiency  or  percent  dissipation  was  a 
constant?  Or,  could  you  tell  whether  it  might be changing  with  the  depth  of  discharge? 
MUELLER:  I  don't really think  that  we  determined  that.  What  we  did  was  that  we  estab- 
lished a discharge rate and our objective was to try  and  hold  that  discharge  rate  for  roughly 2 
hours  while  we  maintained  the  delta  temperatures  between  the  top  and  bottom  of cell constant  and 
the  temperatures  constant.  But  instantaneously I don't  really  know  whether  the  dissipation 
changed. 
SCOTT:  If  you  continued to test  for  that  long  at  the  high  end  of  your  discharge  rate,  you 
covered  a  significant  change  in  depth of discharge  from  the  beginning to the  end. 
MUELLER: Yes. 
SCOTT:  And  you  didn't  see  any  difference  from - during  that  time  and  the  percentage 
dissipation? 
MUELLER: No. The  only real  measure of that  one we had was the  temperature  instrumenta- 
tion. We did  not see that  the  temperature was  significantly  changing. 
SCOTT: 1 think  the  theory  would  predict  that  you  should  get an increase  in  dissipation as 
the discharge  proceeds  from  one  depth of discharge to another. 
MUELLER: I don't  know  how  to  comment  to  that. 
WEBB: May I? On the second test, Dr. Scott, the rate that we selected was maintained 
constant  throughout  the  time  that we  were  looking  at  the  delta  T's  in  the  batteries.  This  was  over 
a  good  2-hour  period. So that  the  rate or the DOD on  the  battery was  changing  over  that  period. 
We adjusted  the  wattage  continually  to  keep  it  at  the  level,  and  we  set  the  criteria  initially 
at 0.2 degree  change  in  1/2-hour  period. 
MUELLER: One-hour period. 
WEBB: One-hour  period.  Things  were  going so well that  the  thermodynamics  changed  the 
criteria to 0.1  of  a  degree  change  in  the  hour,  and  we  did  not  detect  a  change  of  that  magnitude 
over the 2-hour period, even though the  depth  of  discharge was continually increasing on that 
second  run.  It  was  very  stable. 
HENDEE: In my mission simulator,  I d o   i t  slightly  differently.  It's  a  computer-controlled 
one. I know  that  the  efficiencies  change,  the  more  deeply  you  go  into  discharge. So if you  would 
look further,  you  could see.  I  agree  with  Scott  of  TRW  that  you  could  see  this  if  you  looked  a 
little closer. 
272 
MUELLER: I would  expect  some  change as  Dr. Scott says, as the discharge continues. Of 
course,  the  voltage is decreasing, the  current is increasing, and  certainly I* R  would  be  increasing 
significantly. So I don’t  know that  they were  a  large  percentage of the  total. I don’t  think that  they 
were. 
273 
50 A.H. EARERY MECHANICALISTRUCTURAL DESIGN 
WASTE  HEAT  DETEPFINATION 
N 
4 
P 
O B J E C T I V E :  
o DETERWINE  WASTE  HEAT  GENERATED AS A  FUNCTION OF 
0 ISCHARGE RATE 
PROCEDURE: 
o MOUNT BATTEKY ON COLDPLATE  VITH  SUFFICIENT  COOLING 
CAPACITY TO M A I N T A I N  CONSTANT  EIIPERATURE  AT 
PLANNED  LEVEL OF BATTERY  ACTIVITY 
OF CELL  TEMPERATURES  REACH  STABILITY  (GOAL  $O.l 'C 
CIIANGE/HOUR) 
o DISCHARGE  THE  BATTERY  AT  A  RATE TO M A I N T A I N   S T A B L E  
TEIIPERATURE  CONDITION  ABOVE 
o RECHARGE AND REPEAT  FOR  NEXT  EST CONDITION 
o OVERCHARGE AT  F IXED RP.TE U N T I L  TOP AND BOTTOM 
Figure 3-77 
WASTE HCAT DETERMINATION 
LOCATIOf I  OF THERFOCOUPLES 
Figure 3-79 
Figure 3-78 
WASTE HEAT DETERMINATION 
TYPICAL  PARAtlETER  VALUES 
Figure 3-80 
I 
WASTE HEAT DETERMINATION 
BATTERY TOP-TO-BOTTOR/\TEFPERATURES vs POWER RATES 
OVERCHARGE R A T E * U A l l S  
100 200 300 400 
BATTERY  DISCHARGE R A T E 4 U A l T S  
Figure 3-8 1 
CONCLUSIONS - WASTE HEAT DETERMINATION 
o BATTERY INEFFICIEIICY RANGES  FROFl 14 TO 18 PERCENT 
AT DISCHARGE RATES OF 2 5 4  TO 588 IIATTS,  RESPECTIVELY 
AND TOP-OF-CELL TEMPERATURES OF 20°C APPROXIMATELY 
o TEST PROCEDURES  ARE SIMPLE TO APPLY AND GIVE 
CONSISTENT RESULTS 
Figure 3-82 
275 


NASA 50 amp hour nickel cadmium battery waste heat determination

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