A high rate active Li/SOCl2 cell was designed for use in a 28 volt, 250 amp-hour space battery system. The lithium battery is being considered as a replacement of its heavier silver-zinc counterpart on board the Centaur-G booster rocket which is used to launch payloads from the Space Shuttle cargo bay into deep-space. Basically a feasibility study, this development effort is demonstrating the ability of the lithium cell to deliver up to 90 amps safely at power densities of approximately 25 watts per pound. Test data on 4 prototype units is showing an energy density of 85 watt-hours per pound and 9.0 watt-hours/cu in. The cells tested typically delivered 280 to 300 amp-hours under ambient temperature test conditions using alternating continuous loads of 90, 55, and 20 amperes throughout life. Data from four cells tested are presented to demonstrate the capability of Li/SOCl2 technology for a C/3 discharge rate in active and hermetic cell units

Tura, D. D.

Zolla, A. E.

English

NASA Technical Reports Server

N88- 11028
250AH/90A ACTIVE LITflIUM-THIONYL CHLORIDE CELL FOR CENTAUR-G APPLICATION
A. E. Zolla and D. D. Tura
Altus Corporation
San Jose, California 95112
ABSTRACT
A high rate active Li/SOCI 2 cell has been designed for use in a 28 volt,
250 amp-hour space battery system. The lithium battery is being considered as
a replacement of its heavier silver- zinc counterpart on board the "Centaur-G"
booster rocket which is used to launch payloads from the un-manned space
vehicles.
Basically a feasibility study, this development effort is demonstrating
the ability of the lithium cell to deliver up to 90 amps safely at power
densities of approximately 25 watts per pound. Test data on 4 prototype units
is showing an energy density of 85 watt-hours per pound and 9.0 watt-hr/in 3.
The cells tested demonstrated 280 to 300 amp-hours under ambient temperature
test conditions using alternating continuous loads of 90, 55 and 20 amperes
throughout life.
The cell is hermetically sealed in a welded stainless steel envelope of
103 in 3 volume and weighs 10.9 Ibs. The cell's internal impedance is a low
3.5 milliohms achieved by means of close electrode spacing, a low resistance
internal bussing structure and large 3/8 inch diameter nickel pin terminals.
Data from four cells tested are presented to demonstrate the capability
of Li/SOCI 2 technology for a C/3 discharge rate in active and hermetic cell
units.
A simple thermal model was developed, which predicts the heat transfer
characteristics and cell temperatures as a function of time and current.
DESIGN
Four cells were constructed and tested individually during this
development effort. They were de,signed to provide slightly over 250
ampere-hours to achieve the capacity required and to have reserve capacity to
guard against possible over-discharge (reverse voltage) of a cell in a
multiple series circuit.
A cylindrical case configuration was chosen due to its inherent strength
and ease of manufacture. The case is 4.50" O.D. and 5.06" tall. Internal to
this stainless steel case, a pile construction of multiple anodes and cathodes
with non-conductive but porous separator material was selected. The current
collectors within these electrodes have tabs which protrude from the pile with
a 120 ° dispersion angle between anode and cathode tabs. These anode and
cathode tabs are separately brought together and welded for the electrical
parallel connection to the respective electrode terminals.
61
https://ntrs.nasa.gov/search.jsp?R=19880001641 2020-03-20T08:51:18+00:00Z
The anode disc is 4.0 inches in diameter and 0.007 inches thick. Two
such discs of lithium sandwich the nickel current collector plate. Each
hi-anode assembly has correspondingly 162 cm 2 of active surface area and a
theoretical capacity of 5.94 AH.
The cathode is of similar diameter to the anode and contains 1.24 g of
carbon in each disc. Two such cathodes, separated by a nickel current
collector plate, form the bi-cathode assembly. Each cathode assembly
supports 5.2 AH of cell reaction and therefore limits the cell's capacity
delivery.
The overall cell contains a substantial excess of electrolyte which is
accessible to the cathode and separators by capillary attraction.
Two minor variations of the basic design were built and tested. The
first two cells (S/N 001 and 002) consisted of 57 bi-electrode pairs. The
second two cells (S/N 003 and 004) were constructed with 54 hi-electrode pairs
within the same cell envelope. At the 90A peak current the two designs
operated at 9.7 and 10.3 mA/cm 2 respectively.
The latter cell design variation is preferred from the standpoint of ease
of manufacture and the resultant capacity and performance still meet
technical objectives.
Special emphasis was placed on the internal bussing design of the cell.
It was recognized that at the intended high rates, the internal resistance
must be minimal to avoid excessive internal heating, loss of voltage and
internal pressure build-up. The cell was therefore designed with the largest
nickel conductor cross-sectional area possible within the constraints of space
available and manufacturability.
The measured voltage dropat 90 amperes across this internal bussing
system is 58 millivolts, indicating 0.64 milli-ohms resistance per terminal.
The sum of the two terminal assemblies is therefore 1.3 milliohms for the
nickel conductors. Resistance contributions from electrochemical components
raise the cell's total internal impedance to about 3.5 milliohms; an
acceptable level for 90A delivery.
The bulkiness of the bussing system necessitated large terminal boxes to
be located around the bussing terminations (at 120 ° locations around the cell).
The large size allowed easier and more expedient manufacturing and assembly of
the cells. These terminal housings are completely filled with electrolyte and
are reservoirs for the substantial excess over the capacity requirement.
The 3/8" diameter nickel - teflon feed-thru terminals were designed
specially for this cell. These were thoroughly tested for hermeticity and
reproducibility in manufacture. Special care was taken in the design of
welding joints in the cell case to locate the welding zones away from internal
lithium. This was accomplished by "dishing" the lids on the case so that the
welds are low heat butt-welds located more than I/4" away from the internal
stack. Similarly, the terminal boxes are welded to interface plates which
extend from the main cell I/2" away from lithium. In addition, a full set of
heat sinks were developed to maintain the cell at low temperature during
welding operations.
62
TESTING
The cells were equipped with pressure transducers and were further
prepared for testing by attaching 7 thermocouples, and lead wires. The power
leads were attached to the 3/8" nickel terminals using a clamp-connector
attached to welding cable. The transducer, thermocouples, electrode potential
leads and power leads were remotely connected to test instrumentation. The
instrumentation sampled data every minute and a strip chart recorder
continuously tracked data throughout the test. A computer was used to compile
the data into tabular and graphical formats. Figure numbers 1,2, 3 and 4
present the data graphically for serial numbers 001, 002, 003 and 004,
respectively.
A summary of the test conditions for the 4 cells follows:
S/N 001 - This cell was tested on a 4.0" diameter copper block which was
located on a large slab of steel. The copper block was interfaced to the cell
with a layer of thermal grease. The cell was discharged 4 times with a 60
minute 90-55-20 amp constant current discharge profile (20 minutes at each
level). The cell was then discharged to 2 volts at 20 amps. After a weekend
at open circuit, the cell was depleted of energy on a 0.i ohm resistor.
S/N 002 - This cell was tested on a piece of wood to create a less
thermally advantageous situation than in the previous test. Also loaded 4
times with a 90-55-20 amp discharge profile, this cell was then discharged to
2 volts at 20 amps. Subsequently the cell was depleted of its energy on a 1
ohm resistor.
S/N 003 - Since this cell contains 5% less anodes and cathodes than the
previous 2 cells, it operates at a slightly higher current density. This cell
was, therefore, tested at conditions identical to those for S/N 002 in order
to assess the differences in operation due to this current density change.
The cell was tested on a wooden block with the same current profile to 2 volts
as in S/N 002. On this cell, however, the 20 amp constant current discharge
was continued past the 2 volt cutoff, through 0 volts into reverse voltage
(about -0.I volts) for a further 330 AH.
S/N 004 - Since the ultimate application for the cell is in space (vacuum
environment), it was decided to test this cell in a near adiabatic condition
to determine its behavior under this extreme situation. Both conduction and
convection were precluded as far as possible with the exception of negligible
conduction down the power leads. The cell was tightly wrapped in foam and
placed in a covered styrofoam box tightly packed with styrofoam packing
material to minimize heat loss. The cell was discharged at the same 90-55-20
amp constant current profile as its predecessors. After the third profile,
however, it was determined that the cell temperature and pressure were high
enough, (as expected) to abandon the fourth profile and the remainder of the
test was conducted at 20 _mps constant current. The cell was finally depleted
of its energy on a | ohm resistor.
63
DATA SUMMARY
Table l includes the major data points necessary to summarize the
performance of the cells. Table 2 contains detailed data on average voltages
of the cells.
CELLS S/N 001 AND S/N 002 - Delivered 301 and 294 amp-hours (to 2.5
volts), respectively; well above the 250 AH goal. The energy of both cells,
973 and 956 WH were also very close. Both cells remained at very moderate
temperatures and pressures throughout their respective test cycles. S/N 001
rose to a maximum case temperature of 48.9"C and a maximum pressure of 27 psig.
S/N 002 achieved 58.1°C and a maximum pressure of 37 psig. The lower values
for S/N 001 are clearly due to the fact that heat was conducted from the base
of the cell whereas for S/N 002 it was precluded. These cells operated at 9.7
mA/cm 2 on the 90 amp current discharges and they both delivered high energy
densities (to 2.5 volts) of about 90 watt-hours per pound.
CELLS S/N 003 AND 004 - Delivered predictably lower capacities than their
predecessors due to the 5.3% reduction in carbon c_thode weight. A capacity
of 281.8 AH is mathematically projected and S/N 003 delivered this capacity.
Cell S/N 003 attained a pressure of only 28 psi during discharge - a very
pleasing result. This cell was allowed to enter forced discharge (reverse
voltage) at 20 amps, for 16+ hours after discharge was complete, in order to
access the safety characteristics in this abusive condition. The cell case
maintained its structural integrity and hermeticity throughout the entire
reverse voltage period.
Because S/N 004, in its near adiabatic condition, operated at a much
higher average temperature than S/N 003, it manifested a significantly higher
average voltage.
Cell S/N 004 reached a temperature and pressure maximum of I05.1°C and
172 psig respectively during its last few minutes of constant current
discharge, due to its near adiabatic state. Again, the cell maintained its
structural integrity and hermeticity through-out the entire test.
The design of cells S/N 003 and 004 was validated by the test results and
the resultant 85 watt-hours/ pound after 90A (at 10.3 mA/cm 2) delivery exceeds
the technical objectives.
THERMAL MODEL
In order to develop a grasp of the thermal operating characteristics of
the cells prior to actual testing, a thermal model was developed. The model
is based on a simple energy balance where the cell is assumed to be a
homogenous thermal mass. The calculated heat capacity of a typical cell is
0.223 cal/g/°C. The predicted average temperature profile is determined by
calculating the accumulated energy in the cell as a function of time and
dividing by heat capacity and weight. Three paths for heat transfer were
identified for the cells - conduction to the surface on which the cell rested,
convection to air, and conduction of heat out of the cell through the nickel
terminals. Heat transfer coefficients were determined for these paths, and
used to formulate an overall heat transfer coefficient for the planned test
conditions. The cell model is represented by Figure 5.
64
The integral of the three thermal transfer modes were calculated to be
1.19, 0.84, 0.84 and 0.09 watts/°C above ambient for cells tested as S/N l,
S/N 2, S/N 3 and S/N 4 respectively.
The results of solving the heat balance for each cell indicated that we
could expect moderate cell temperatures for cell tests I-3. As seen in Table
#l, these predictions were quite accurate. The prediction for test number 4,
the near adiabatic test, indicated a high temperature of 93.4°C at the
termination of testing. Based on this .information, a successful test in the
adiabatic environment was predicted. As seen in Table #I, the original model
is marginally less accurate at high temperatures. The original thermal model
was helpful in planning the testing for this development effort. Recent
refinement of the thermal model considered accelerated self discharge at
elevated temperatures and this new model is useful in the design of the full
28V battery. The model credibly predicts the battery temperature and
operating voltage under the various scenarios for solar and earth flux
radiation and operating current.
CONCLUSION
This work has demonstrated a safe and efficient lithium thionyl chloride
cell design for high capacity (250 AH) with high rate performance (C/3). The
cell can deliver a sustained 1 hour at 90A and full 250 AH capacity delivery
within an adiabatic environment.
The demonstrated energy density of 85 WH/LB can readily be raised to 115
WH/LB using a less conservative steel case design which is still competant for
predicted internal pressures.
The developed thermal model predicts a full 9-cell battery operating
safely in a space environment with the factor of two weight reduction over a
silver-zinc system for un-manned space vehicle power.
This work suggests that the present high rate Li-SOCI 2 technology is
ready for full system hardware development and subsequent analysis through
testing.
ACKNOWLEDGEMENT
The authors express their appreciation to Ron Mikkelson of General
Dynamics - Convair Space Systems Division whose interest and support has made
this work possible.
65
$170A
E
v
0
0
E
0
0
Z
<
o
66
_IScl CINId 3 S33.'_83(I
I I I I I I I I
| | " "i %.1
• , u,. 4 "-..
i" %1 "%,
._, I \
/ t4 "1
• I
, • / _;
l _ .I_91 _Jl
i'_ _,,,,l _j
,-,, , ,iJ _ _'_
I > .':4 .") ,-'_ /
/ ..."" _ • .m"L., _,_q ...- _,_
/ "1 " "l E _/
5 '
4,> "- -..,
,i .q,,. ,. i
G " ,"'".., :"',.W
F 4, ,v'---1
h _ ,,,-JI _1: _.1
"1 _"... _._--_
, A
_.. ,.,<
IL._LJ O
SIGO_
(9
_J
E
.,.-#
-,-I
"Ci
0
:E
,--I
:,-I
r'_
E"
@
,-..#
.,-i
Or)
0
0
Z
r,j
:::::
0
L_
t'q
, ,,,,,_
6'7
$170A
00
OJ
E
v
.,-I
_J
'0
0
_7
,--I
r_
O_
c7
OJ
,--I
E
-,-I
4
.'--4
0
0
Z
0
b'7
68
• 4
J
1: c_
SiqO_
'Jl
-,-4
v
.,-t
,--4
_D
0
E
..c:
,-4
c_
.,-I
0
E
O;
>
0
0
0
Z
<
0
,4
LI.
69
I=.'_
• A
E •
W O
O
i
. .,..,
©
@
E
u_
7O
Table 1.
DATA SUMMARY
SERIAl.NO. __0_0_i_002 003 004
CAPACITY TO 2.5 VOLTS
(AHRS) 301 294 282 260
ENERGY TO 2.5 V (WHRS) 973 956 950 899
CELL WEIGHT (GRAMS) 4892 4972 4914 4969
ENERGY DENSITY TO 2.5
(WHR/LB) 90 87 88 82
INTERNAL RESIST. (M OHM) 4.6 3.7 3.3 3.2
NO. OF BI-ELECTRODE PAIRS 57 57 54 54
CASE TEMPERATURE (°C)
LOW 11.6 15.3 16.8 19.0
AVERAGE 33.3 39.5 41.4 83.5
HIGH 48.9 58.1 53.9 105.1
PREDICTED HIGH TEMP. 45.5 54.4 54.5 93.4
PRESSURE DATA (PSIG)
LOW 0.0 0.0 0.0 0.0
AVERAGE 11.1 17.0 14.9 92.8
HIGH 27.0 37.0 28.0 172.0
71
.Q
X
U_
<
0
I>
Z
U_
U
i
i
I
I
)
I
I
U'_
CO
I
i ,4
_4 A
zu_
_Z
=<
U
O
"Ior)
f..
c_
?..
f..
c_
o
u_
u_
u_
c4
O
u_
o
o4
O
u_
u_
O
u_
u_
u_
u_
u_
O
_D
c_
O
o0
u_
u_
o
O
c_
o
u_
O
O
O
u_
c_
u_
u_
O
O
c_
6,4
c_
c_
c_
u_
U_
u_
O
u_
_D
0
r.n
r.u
0 _ _
I>
r.u
72


The 250AH/90A active lithium-thionyl chloride cell for Centaur-G application

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server