This testing was designed to evaluate various voltage-temperature (V-T) charge curves for use in low-Earth-orbit (LEO) applications of nickel-cadmium battery cells. The trends established relating V-T level to utilizable capacity were unexpected. The trends toward lower capacity at higher V-T levels was predominant in this testing. This effect was a function of the V-T level, the temperature, and the cell history. This effect was attributed to changes occurring in the positive plate. The results imply that for some applications, the use of even lower V-T levels may be warranted. The need to limit overcharge, especially in the early phases of missions, is underlined by this test program

Bondeson, D. W.

Timmerman, P. J.

English

NASA Technical Reports Server

N87-11092
VOLTAGE-TEMPERATURE CHARGE VERIFICATION TESTING
OF 34 AMPERE-HOUR NICKEL-CADMIUM CELLS
Paul J. Timmerman
and
Donald W. Bondeson
Martin Marietta Denver Aerospace
Denver, Colorado
ABSTRACT
This testing was designed to evaluate various voltage-temperature (V-T)
charge curves for use in low-earth-orbit (LEO) applications of nickel-cadmium
battery cells. The trends established relating V-T level to utilizable
capacity were unexpected. The trends toward lower capacity at higher V-T
levels was predominant in this testing. This effect was a function of the
V-T level, the temperature, and the cell history. This effect was attributed
to changes occurring in the positive plate. The results imply that for some
applications, the use of even lower V-T levels may be warranted. The need to
limit overcharge, especially in the early phases of missions, is underlined
by this test program.
INTRODUCTION
V-T compensated charge control is the primary method of battery charging
in low-earth-orbit (LEO) spacecraft. The careful selection of V-T levels is
crucial for mission longevity. The ability to change V-T levels during
operation gives the necessary flexibility to adapt to changing cell
electrical characteristics. To evaluate the three most likely curves
available in this spacecraft, a matrix of nine V-T combinations was
selected. For each of these points, a series of tests was performed. The
test series allowed for the evaluation of utilizable capacity as well as
cyclic parameters. Based upon the results of this testing, implications
regarding cell and system operation can be made.
267
https://ntrs.nasa.gov/search.jsp?R=19870001659 2020-03-23T14:58:07+00:00Z
TEST ARTICLES
The testing was performed concurrently on two (2) separate six (6)
cell-packs of thirty-four ampere-hour (34 AH) sealed nickel-cadmium cells.
The two cell packs have different test histories, although both were
manufactured from the same plate lot. The group referred to as the "new
cells" had no previous testing aside from pre-ATP, ATP, and cell receiving
and matching. The "old cells" had been in test for approximately one and
one-half years prior to this testing. The testing performed on the old cells
includes minimum trickle charge evaluation (i), various characterization
cycles, and a small amount of hot case testing. In all, approximately sixty
100% depth-of-discharge (DOD) cycles were performed. Each of the cell-packs
was mounted in a restraining fixture to provide physical support and
electrical isolation. The cells are series wired. The cell-packs were
instrumented for battery and cell voltages and skin temperature. All tests
were performed in an environmental temperature chamber.
TEST OPERATIONS
Prior to the start of any test series, the cells were shorted with
one-quarter (0.25) ohm resistors. The testing was performed at 0°C, 150C,
and 30°C. At each of these three temperatures, three V-T levels were
evaluated. Each of the V-T levels was evaluated using the following sequence:
Initial Capacity Check
A capacity check is performed following temperature stabilization. This
test is performed once per temperature for each cell pack. The test uses a
C/20 charge rate (1.70 amperes) for forty (40) hours. Discharge is performed
immediately, at a C/2 (-17.0 amperes) rate, until a pack voltage of 6.0 volts
(I.0 volt/cell) is reached. The recorded discharge capacities are then
interpolated to the exact voltage cut-off level. Cell shorts are then
installed, bringing the cells to less than twenty millivolts (20 mV) before
continuing.
Pre-Charge
The cells are then charged at a C/20 rate (1.70 amperes) for forty (40)
hours.
V-T Cyc ling
Following the pre-charge test, the simulated LEO cycling was initiated.
Each of the cycles is one-hundred and eight (108) minutes long, with a
seventy-two (72) minute charge phase, and a thirty-six (36) minute discharge
phase. The charge current is limited to fourteen amperes (14 A) and tapers
268
at the assigned voltage limit. The discharge current is a constant eight and
one-half amperes (-8.5A). This results in a consistent fifteen percent (15%)
DOD. Operation of the simulated LEO cycing is continued until the cyclic
parameters have stabilized, as determined through the use of specialized
software routines.
Upon stabilization of recharge fraction, end-of-charge current, and
end-of-discharge voltage, the test is stopped. This test halt is executed at
the end of the charge phase. The set-up and test start of the next test is
performed immediately. See Figure i for the V-T levels tested.
Post-Cyclic Capacity Check
This discharge test is very similar to the last half of the capacity
check. It uses the same C/2 discharge rate and cell short-down procedure.
At this time the testing proceeds to the next V-T series. Either a
temperature stabilization or a pre-charge is performed, depending upon
whether a change in temperature is required.
TEST RESULTS
To preface the discussion of the effects of V-T levels on cell
performance, it is important to first look at the constant current testing
performed. Both of the cell-packs displayed normal performance, considering
the respective histories. Figure 2 displays the results of the capacity
check tests for both cell packs at a range of temperatures. As would be
expected, the old cells showed marginally lower capacities, with extreme
temperatures perturbing the difference. The general shape of the curves fits
very well with classical capacity curves (2). Figure 3 depicts the maximum
charge voltage for the respective cell-pack as a function of temperature.
The effect of test history on the old cells manifests itself again as higher
charge voltage. This effect is most pronounced at low temperature. The
formation of large crystals of active material, and the resulting reduction
in the oxygen recombination rate is considered the primary cause of this
phenomenon (2, 3, 4).
The cyclic testing can be evaluated by a number of electrical
parameters. The parameters of greatest interest in this testing are recharge
fraction, end-of-charge current (EOCI), end-of-discharge voltage (EODV) and
temperature. The raw data from these parameters would constitute a large
volume. For this reason, only the most representative data was presented.
Figures 4 and 5 depict the data presented in Table i. Figure 4 depicts the
effect of V-T level on the EOCI of the two cell-packs as function of
temperature. Figure 5 depicts the effect of V-T level on the recharge
fraction of the two cell-packs as a function of temperature. This data
represents values for the cycling just prior to the discharge.
269
The strong correlation between Figures 4 and 5 would be anticipated.
Note that the plots for adjacent V-T levels of different cell-packs are very
similar. This is another indicator of the general state of health of the
respective cell-packs. The trends showntoward higher values at higher
temperatures and/or V-T levels is normal. The fact that the new cells have
consistently higher values is witness to the inherently lower charge
voltages, as seen earlier.
The data presented thus far fit well into the expected trend for LEO
applications of sealed nickel-cadmium battery cells. The results of the
post-cyclic capacity checks do not fall into the range of anticipated
values. Figures 6 and 7 present the ampere-hour capacities for the two
cell-packs, as functions of the V-T level and the temperature. The most
notable trend in these figures is that of decreased capacity with increased
V-T level. Note also that this effect is a function of temperature as well.
There are notable exceptions to this trend, as seen in Figure 6, "Effect of
V-T Levels on Capacities of NewCells." While the 30°C data showclear and
consistent trends, the 15°C and 0=Cregions are less clear. The differences
in the values at 15°C are quite minimal, considering the range of capacities
recorded. The unexpected low value for the 0°C measurementfor V-T curve
numbertwo can be attributed to an excessive numberof cycles needed to
achieve stabilization.
Figure 8 represents the general trends for capacities achieved by the new
cells. Thecurves generated are interpretations of the data in Figure 6.
Modification to the trends are based upon aforementioned factors.
Figure 7, "Effect of V-T Levels on Capacities of Old Cells", gave clear
values for the 0°C and 30°C regions, with the 15°C region showing an
excessively low capacity for V-T curve 2. This resulted from improper charge
control and an excessive numberof cycles being performed. Figure 9 is an
expression of the trends established in Figure 7.
There are two major differences between Figures 8 and 9. First is the
reversal in the general trend for the old cells at low temperature. An
increase in discharged capacity correlated with increased V-T level for this
one case. The exact temperature at which the crossover in trend occurs is
beyond the scope of this testing. This testing lacks the necessary precision
to identify that point. Secondis the smaller reduction in utilizable
capacity in the old cells. The presence of this reduction in utilizable
capacity, and the magnitude of this effect, can be related to the data
presented earlier in this paper.
270
DISCUSSION
The results of this testing represent a deviation from the traditional
effects ascribed to V-T levels with respect to capacities. This paper does
not, however, contradict reports which show increases in capacity with
increased V-T level where only one cycle was performed (4). Earlier reports
showed instances of phenomonon similar to that seen here (5). The presence
of such an effect was attributed to a high self-discharge rate, as a result
of elevated plate temperatures. This explanation is not feasible in light of
the test method used in this testing. The self discharge rate would have to
be in the C rate range (-34 A) for this to be the cause, based on maximum
residual capacity and the average set-up time for the post-cyclic discharge
test.
The phenomenon of lower capacities upon discharge can be attributed to
many causes. A simple approach is to look at the individual components and
analyze them separately. First is the separator. Degradation and drying out
of the separator are gradual and primarily irreversible trends (6). Since
the effect observed in this report is reversible, the separator is not a
likely cause. The negative plate is a likely candidate to have the effect
ascribed to it. The trend toward a decrease in surface area, and thus
effective excess negative active material, is well known. Reconditioning of
cells increases the surface area by decreasing the cadmium crystal size.
Thus, the effects of the negative electrode can be considered somewhat
reversible, like the effect reported herein. The fact that the decrease in
utilizable capacity was most dramatic in the new cells, as seen by comparing
Figures 8 and 9, is an indication that the negative plate is not the cause.
It has long been known that overcharging of nickel-hydroxide electrodes
results in the formation of charged active material of higher valence states,
gamma-nickel-oxy-hydroxide (NiOOH) (7). Additionally, it has been noted that
electrodes with high concentrations of the Gamma-NiOOH experience poor
efficiency upon discharge (8, 9). The capacity unavailable at high rate is
referred to as the residual capacity. Further evidence and explanations of
the residual capacity effect were presented in a recent study of charge and
discharge efficiencies (i0). This paper shows a significant inefficiency for
discharging at states-of-charge less than twenty-five percent (25%). In
addition, the level of inefficiency is related to both the charge and
discharge profiles used. These parameters affect the existence of depletion
and defect layers in the positive electrode.
These previous studies do not address all of the intricacies of the data
reported here. One example is the variation in the magnitude of residual
capacity for the cell-pack with different levels of degradation. This
phenomenon is simply an artifact of the higher charge efficiency of the new
cells, and the resulting overcharge level. Consistent with the above
theories, the higher level of overcharge results in a lower charge
utilization. It is, however, unanticipated that the C/2 rate would result in
residual capacity of the magnitudes seen. Another example is the lack of the
manifestation of the residual capacity loss at low temperatures for the old
271
cells. It is commonknowledge that cold temperature oxygen recombination,
especially in cells with an extensive history, can be a problem, due to the
morphological changes in the negative plate. A simple explanation would be
to attribute the rise in capacity with V-T level to the increase in
temperature of the electrodes. Another possible influence maybe the effect
of intra-cell oxygen pressure upon the equilibrium of the cell reactions.
The effect of a reduction of charged excess negative active material may also
play a role in this phenomenon.
CONCLUSIONS
This report presents data which supports the most modern theories on
charge and discharge efficiencies and the related cell electro-chemistry. In
addition it gives indications of the conditions under which the residual
capacity effect will manifest itself. Applying these principles to system
operation, several things become apparent. Primarily, the adverse effect of
overcharging cells is highlighted. At the other extreme, the lower threshold
for V-T charge effectiveness was not firmly established. These test results
do indicate that effective charging at low temperature can be accomplished
with extremely low recharge fractions. It becomes apparent that to some
extent lower V-T curves are better for most cases. In conclusion, an
approach of maintaining a minimum recharge fraction is a good approach toward
monitoring the state-of-charge of a spacecraft electrical power system.
Maximum life and higher operating efficiency may be obtained at recharge
fractions lower than currently being used. In addition, the use of
non-linear V-T curves might give more accurate charge control, especially in
highly variable thermal environments.
272
B IBL IOGRAPHY
.
.
.
.
5.
6.
7.
.
.
i0.
P. Timmerman, "Minimum Trickle Charge Testing of 34 Ampere-Hour
Nickel-Cadmium Cells", 1984 GSFC Battery Workshop Proceedings, 1984, pp.
369-386.
W. R. Scott and D. W. Rusta, "Sealed Cell Nickel-Cadmium Battery
Applications Manual", NASA NAS5-23514, Ref. Publ. 1052, 1979, p. 96.
Nickel-Cadmium Battery Application Engineering Manual", Second Edition,
Ed. by J. C. Grant, General Electric Company Publication Number
GET-3148A, 1975, para. 6.5.5.
W. R. Scott and D. W. Rusta, Ibid, p. 94.
W. R. Scott and D. W. Rusta, Ibid, p. 97.
W. R. Scott and D. W. Rusta, Ibid, p. 213.
S. U. Falk and A. J. Salkind, "Alkaline Storage Batteries", John Wiley
and Sons, Inc., New York, NY, 1969, p. 50.
R. Banard, G. T. Crickmore, J. A. Lee and F. L. Tye, "The Cause of
Residual Capacity in Nickel Oxyhydroxide Electrodes", Journal of Applied
Electro-Chemistry, i0-i, January, 1980, pp. 61-70.
R. Banard and C. F. Randall, "Studies Concerning Charged
Nickel-Hydroxide Electrodes. VIII. The Relative Potentials of the
Beta-Nickel-Oxyhydroxide Reduction Process", The Journal of Applied
Electro-Chemistry, 12-1, January, 1982, p. 121.
A. H. Zimmerman and P. K. Ella, "Charge Efficiency and Charge
Utilization in the Sealed Nickel-Cadmium Cell", Report SD-TR-82-91, The
Aerospace Corporation, 20 December 1982.
273
_eSTemperature_
0°C
15°C
30°C
Table 1. DISCHARGE CAPACITIES
Baseline
Capacity
Test, Ah
(41.18, 40.93)
(41.83, 41.72)
(40.03, 39.21)
V-T Level
One, Ah
1.45 (V/Cell)
(37.73, 38.03)
1.42 (V/Cell)
(38.37, 38.77)
1.38 (V/Cell)
(37.39, 37.46)
V-T Level
Two, Ah
1.47 (V/Cell)
(34.82, 38.40)
1.44 (V/Cell)
(38.90, 36.73)
1.40 (V/Cell)
(35.73, 34.71)
V-T Level
Three, Ah
1.49 (V/Cell)
(35.10, 39.01)
1.46 (V/Cell)
(37.58, 38.73)
1.42 (V/Cell)
(33.06, 36.20)
Note: All Capacities are _iven
as new cells & old eells.
274
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Temperature, °C
Figure 1. V-T LEVELS TESTED
276
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35
30
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-i0
o "218--New Cells
o "221--01d Cells
I I i i I I I l ,
-5 0 5 I0 15 20 25 30
Test Temperature, °C
Figure 2. BASELINE CAPACITIES VERSUS TEMPERATURE
277
1.60
1.55
1.5o
1.45
s 1.40
.,4
_4
1.35
Q ~218--New Cells
"221--01d Cells
1.30 ! I i i , t J t
-i0 -5 0 5 i0 15 20 25 30
Test Temperature, °C
Figure 3. MAXIMUM CELL VOLTAGE FOR C/20 CHARGE RATE
278
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I
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/
/
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/
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..,,,,// _..,..,,._,,.,e V-T Level 2, New Cells
.-_ ...--" ....-B V-T Level 3, Old Cells
_,s_ "_ _ _._._ .-" "" "' "
s" _.._- .... _ V-T Level 2, Old Cells
..":..I_ .... --V-T Level I, New Cells
u,-.... --r..-_-i------=- ..... "8 V-T Level I Old Cells
i t I t I I t i
0 5 i0 15 20 25 30 35
Test Temperature, "C
Figure 4. EFFECT OF V-T LEVEL ON END-OF-CHARGE CURRENT (EOCI)
279
=o
.,4
o
m
o
1.6
1.5
1.4
1.3
1.2
I.i
_I.0
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~221--01d Cells
S _
V-T Level 3, New Cells
/
/
I
/
/
/
/
/ ......_ V-T Level 3, Old Cells
i/ _
/ _._ ..........._V-T Level 2, New Cells
/ ___ _ ............ ._ V-T Level 2 Old Cells/
/ V-T Level i, New Cells
d /_"..._" __-4>------- -_V-T Level i, Old Cells
_" I i i _ i , i p
0 5 i0 15 20 25 30 35 40
Test Temperature, "C
Figure 5. EFFECT OF V-T LEVEL ON RECHARGE FRACTION (RF)
280
.M
O
C_
CD
45.
42.5
40
37.5
35.
32.5
30
-I0
I i I I
0 i0 20 30
Test Temperature, °C
V-T Level i
V-T Level 2
V-T Level 3
Figure 6. EFFECT OF V-T LEVELS ON CAPACITIES OF NEW CELLS
281
45.
.M
.M
0
C_
L_
42.5
4O
37.5
35.
32.5
30
-i0
I I I I
0 i0 20 30
Test Temperature, "C
V-T Level
V-T Level
V-T Level
1
3
2
Figure 7. EFFECT OF V-T LEVELS ON CAPACITIES OF OLD CELLS
282
W
,i-I
w4
u
42.5
40
37.5
35.
32.5
0
V-T Level I
V-T Level 3
V-T Level
30 i i I I
-i0 0 i0 20 30
Test Temperature, oC
Figure 8. EFFECT OF V-T LEVELS ON CAPACITIES OF CELLS NEAR BEGINNING-OF-LIFE
283
42.5
u
40
37.5
35
32.5
30
V-T Level 3
V-T Level i
V-T Level 2
V-T Level 1
V-T Level 3
0 I I I i I I I t
-i0 0 l0 20 30
Test Temperature, *C
V-T Level
Figure 9. EFFECT OF V-T LEVELS ON CAPACITIES OF CELLS NEAR MIDDLE-OF-LIFE
284


Voltage-temperature charge verification testing of 34 ampere-hour nickel-cadmium cells

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