A high power (>35 kW at 215V), low capacity (5.2 Ah), and compact (45L) NiCd battery was developed for the X-38 Crew Return Vehicle (CRV), which is an experimental version of the lifeboat for the International Space Station (ISS). A simple design and innovative approach using a commercial-off-the-shelf (COTS) NiCd cell design enabled the design, qualification, and production of 4 flight units of this highly reliable and safe spacecraft battery to be achieved rapidly (2 years) and cheaply ($13M)

Darcy, Eric

Davies,Frank

Hummer, Leigh

Strangways, Brad

NASA Technical Reports Server

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NASA Johnson Space Center
270V Battery Using COTS NiCd Cells For Manned Spacecraft
Eric Darcy, NASA-Johnson Space Center, Houston, TX, USA
Frank Davies, Lockheed Martin, Houston, TX, USA
Leigh Hummer, AZ Technology, Huntsville, AL, USA
Brad Strangways, Symmetry Resources, Arab, AL, USA
Abstract
A high power (>35 kW at 215V), low capacity (5.2
Ah), and compact (45L) NiCd battery was developed
for the X-38 Crew Return Vehicle (CRV), which is an
experimental version of the lifeboat for the
International Space Station (ISS). A simple design and
innovative approach using a commercial-off-the-shelf
(COTS) NiCd cell design enabled the design,
qualification, and production of 4 flight units of this
highly reliable and safe spacecraft battery to be
achieved rapidly (2 years) and cheaply ($13M).
Introduction
This battery powers the vehicle's electromechanical
actuators, which move its flight control surfaces. The
battery also powers the vehicle's winches, which steer
its very large parafoil. The actuators require power
peaks of > 35 kW (170A) for 100 ms each, delivered at
a voltage exceeding 203V during the re-entry of the
vehicle. The winches require sawtooth power peaks
rising from 6 to 55A over 15 seconds maximum during
steering maneuvers. Since the average baseline current
of this battery is 6A for less than 45 minutes, less than
5.2 Ah are required from the battery. The location of
the battery in the nose of the vehicle imposed very tight
volume constraints (Figure 1). Each battery module
could not exceed a width of 25.25" (64.1 cm), a height
of 6.75" (17.1 cm), and a depth of 16.25" (41.3 cm).
However, center of gravity requirements for the vehicle
did not constrain battery mass.
The X-38 will be launched on a manned spacecraft
(Space Shuttle) and is intended to demonstrate a point
design for the operational lifeboat. Reliability and
safety requirements impose a two-failure tolerance on
both battery system performance and in preventing
catastrophic hazards. Additionally, exposure to high
voltage contacts external to the battery is prohibited.
The reliability is achieved by providing 3 independent
battery modules and 270V buses, each capable of the
meeting all vehicle requirements. Cost and schedule
constraints were challenging and prohibited any cell
design development.
The high voltage requirement coupled with the low
pressure environment during re-entry makes insulation
breakdown due to corona discharge effects a very real
concern, which required special attention to the
electrical design and materials selection.
Figure 1.CRV with 270V battery in non pressurized
nose compartment.
Design Solution
The job of designing, assembling, qualifying and
delivering the flight battery modules and ground
support chargers for the X-38 was contracted by
NASA-Johnson Space Center to AZ Technology, in
Huntsville, AL, teaming with Symmetry Resources, in
Arab, AL. Lockheed Martin helped define the design
requirements of the battery electronics for NASA.
Cell selection — Short and long term cost drivers
dictated the use of a standardized commercially
available cell size that is available from multiple
sources. Operationally, the CRV would be returned to
earth for refurbishment after spending its required 3-
years of service docked to the ISS. Therefore, over the
30-year life of the ISS, ten battery refurbishments are
envisioned, requiring that the cell design selection must
be made with prevention of obsolescence in mind.
Several commercially available lead-acid, NiMH, and
NiCd candidate cells were evaluated on the basis of
capacity and pulse power test performance, and NiCd
subC cells were found to be the building block cell with
the smallest volume battery [ 1 ]. After further evaluation
of NiCd subC cells from Sanyo Energy Corp, model
number CP-2400SCR was down selected. This cell
weighs 58 g and has a diameter of 22 mm and a height
of 42.5 mm. It delivers 2.2 Ah at a C-rate to LOV at
room temperature and kicks out 80A, 10s pulses and up
to 20A continuously. Destructive physical analysis of
cells revealed that the design utilizes sintered positive
and negative electrodes with a nylon separator. The cell
https://ntrs.nasa.gov/search.jsp?R=20100029804 2019-08-30T11:29:32+00:00Z
vent consists of a disc backed with a spring. The
insulator material of the crimp seal is nylon.
Battery Configuration — To perform within the 203 to
300V performance window and not exceed 300V
during 85A charging pulses, strings of 210 cells in
series are needed. These charging pulses can occur
when aerodynamic forces accelerate the flight surfaces
and result in regenerative pulses. To meet the capacity
and power requirements, 4 parallel strings are needed.
This configuration is shown in Figure 2 and yields at
8.8 Ah battery at a C-rate, room temperature discharge
to 210V.
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TOP VIEW
Figure 2. Top view of battery module showing the 4
strings each with 210 series cells along with shunts,
and contactors in the front section.
Deadfacing — Since the output of this battery exceeds
42V, the high voltage connectors are required to be
"deadfaced" to prevent accidental human exposure.
This is achieved by placing contactors in series between
the string power outputs and the external power
connectors of the battery. In order to charge each string
in the battery one at time, each string is in series with a
contactor (Model # Kilovac AP90X-09 from CII). A
block diagram of the battery module is shown in Figure
3. The contactors require 28V input power to be closed
and open whenever this power is removed.
Furthermore, a series of jumper wire interlocks for the
28V power loop are embedded in each high voltage
connector to preclude closing any contactors without all
battery module high power connectors mated with their
mates or safety caps.
Safety — The abuse tolerance of the selected cells was
determined by short circuit, overcharge, reversal, and
heat to vent tests. For each test, the polyvinyl chloride
(PVC) shrink wrap provided by Sanyo was removed.
The cell suffered very little external damage during all
of these tests. No cell explosions, ruptures, or flaming
vents occurred. In addition, the vent and burst pressure
of the cell design was determined to be 190-310 psid
and 800-850 psid, respectively. This was determined by
drilling a hole in the side of the cell can and making a
connection to a nitrogen gas pressure system. The burst
pressure was determined by a similar test but with the
cell vent plugged with an epoxy adhesive. The results
indicate that the cell meets NASA-JSC structural
requirements for sealed containers in that the cell can
design is leak-before-burst and its vent pressure < 2.5 x
its burst pressure.
FBM intcmal block diagram
8
Figure 3. Block Diagram of the Flight Battery Module
(FBM) showing 4 parallel cell strings isolated with
contactors
Each contactor is controlled by a Circuit Breaker Power
Control (CBPC) module which measures the string
current and temperature and trips open when 50A and
80°C are exceeded, respectively. Note that the
temperature threshold is only active during charging.
Since this only provides one level of control for short
circuit failure modes, the resulting hazard must not be a
catastrophic hazard.
Figure 4. A 30 cell-in-series bundle consisting of' 5
sticks of 6 series cells. This photo was taken after the
short circuit.
Therefore, this battery design depends on proper high
temperature (200 °C) material selection for the
construction of the cell strings to limit the hazard of
short circuit to cells venting or leaking KOH
electrolyte. With this approach, shorted cells get hot but
do not cause a propagation of more short circuits due to
melting or burning materials external to the cells in the
battery. No fire, flame, cell rupture, or explosion is
permissible. This result was demonstrated through a
short circuit test of a 6-cell stick at 5 mohms and a 30
cells-in-series bundle at 105 mohms (Figure 4). Peak
currents of 240A and maximum temperatures of 140 °C
were achieved with the cells venting mostly water
vapor and without incurring any external damage.
Figure 5. Four 210-cell strings
Strings of 210 cells were assembled by stacking and
gluing 7 of 30-cell bundles into tight packages as
shown in Figure 5. A top and side aluminum plate were
adhered to the strings to add strength and help with heat
dissipation when in the battery module housing.
Figure 6. The 270V NiCd qualification battery module.
Corona Mitigation — Special precautions taken to
mitigate corona discharge arcing which leads to
insulation breakdown include
(1) Eliminating sharp edges on conducting surfaces on
all bus bars and shunts
(2) Proper wiring (size, type, insulation type and
thickness, termination)
(3) Proper conductor shielding, separation, routing,
and placement
(4) Selecting lowest outgassing materials possible
(5) Minimizing gaseous leaks or vents
(6) Proper ventilation of battery housing to prevent
trapping any gases
Charging — The battery modules are recharged one
string at a time at C-rate using a reverse pulse charge
waveform terminating on a voltage inflection or a
temperature slope. Switching from string to string is
achieved automatically via a programmable logic
controller which monitors the charge termination
control status. Each string receives a "Fast Charge",
followed by a second "Fast Charge/Topping Charge" to
ensure each string is fully charged. Both charge regimes
are sequenced to minimize string overheating. This
charging process is completed by connected the battery
to a Ground Support Equipment (GSE) charger. The
GSE charger also has the necessary resistive load banks
to discharge the battery module and perform internal
resistance measurements.
Cell Acceptance
Over 5700 cells were screened to enable the assembly
of the 5 battery modules, perform cell lot qualification
tests, and have spare cells for engineering assemblies of
30-cell bundles. The plan for acceptance, shown in
Figure 7, includes 100% screens for capacity, internal
resistance, self-discharge, overcharge tolerance,
leakage, mass, dimensions, and visual appearance.
Overall, the performance of the lot was very
homogenous [2]. Average C-rate discharge capacity
was found to be 2.247 Ali
	 a standard deviation of
0.040 Ali 1.8%). The average internal resistance
determined with a 100 ms, 24A pulse was found to be
5.211 mohms with a standard deviation of 0.169
mohms (or 3.2%).
Cell Preparation —remove sleeves, clean
necessary, serialize all cells
as
Resiuual nigh open Circuit Voltage
Discharge All cells
Cold Charge, Discharge
All Cells
I.. Capacity
High rate charge, 3-day rest,
discharge with Re Pulse J_ capacity,
hgh Re
Phenolphthalein leak check
All Cells sealrocnt Lcaks
Physical Measurements, Viswl
Inspection — Mass and dimensions pm ofspm.,
All Cells visual Defects
Passing Cells Rejected Cells
Figure 7. Cell acceptance flow chart.
Only 9 cells were rejected due to reasons other than
visual discrepancies. Specifically, 2 cells each failed the
initial capacity test and the leakage test, 1 cell each
failed the self-discharge test and internal resistance test,
and 3 cells failed the height dimensional test
presumably due to bulging caused by poor
recombination kinetics during overcharge.
The leakage test is achieved by cold charging the cell at
C/10 for 16 hours at 0 °C which causes pressure build-
up by reducing the kinetics of oxygen recombination.
charge, causes a noticeable voltage depression and
capacity degradation which causes the minimum
voltage to sag to 208V under the same current profile.
The pressure stresses the seals of the cell and leakage is
detected with a phenolphthalein indicator for KOH. We
arrived at this method by performing a series of charge
tests with cells connected to pressure transducers. The
connection was achieved at the center of the bottom of
the cell can and great care was taken to minimize the
increased cell void volume by the plumbing. Figure 8
shows that equilibrium pressures of 47 to 61 psia were
achieved on three separate N-1900SCR cells from
Sanyo. This cell design was the precursor to the CP-
2400SCR design.
These results are a good agreement with a snapshot
measurement of cell pressure after 16 hours. In that
independent experiment on the same cell design, cell
pressures averaged 67 psia and were taken immediately
after the 16-hour charge by puncturing the cell to
measure it internal pressure at that state-of-charge.
Figure 23: Sanyo N-1900SCR NICcl Cell Characterization
0 d.g.0 Chang. Voltage. and Internal G.. Pr.......
,m ,m
a
Figure 8. Voltage and cell pressure during a 0
charge at 0 °C for 16 hours with the Sanyo N-1900 SCR
cell.
Since the cell vent begins to operate at 190 psia, the 67
psia means that the cells are in essence proof tested at
only 1/3 rd of the vent pressure. However, this test
creates a pressure differential across the cell seals that
is four times greater than the 15 psid that the cells will
see in space vacuum storage.
Interestingly, lowering the temperature to —15 °C
during the C/10 charge test, produced lower equilibrium
pressures than at 0 °C. Furthermore, with Grate
charging at room temperature, after 100 minutes
equilibrium pressures of 95 to 116 psia were achieved
with cell temperatures reaching around 45 °C, but is
considered detrimental to the cell performance, and
thus, not appropriate for acceptance of flight cells.
Battery Performance
Acceptance tests of the 210 cell strings to %4  of the
battery mission profile indicate that the design driving
pulse is the last 170A EMA pulse. At a string level,
these 42A, 100 ms pulse which occur every 1 second
over a 240 second period, cause the voltage to sag to
215V when testing within hours after recharging. As
shown in Figure 9, a 14-day rest period after the last
270V String Voltage Under Mission Discharge Profile
Cycle 1 (3 Hr Rest) vs. Cycle 2 (14 day Rest)
Room Temperature Discharge
_—._.__.._.. --- ._.—J.__..__........_.-.....
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Figure 9. Comparison 210-cell string discharge voltage
performance under the mission profile with a 3-hour
and 14-day rest after the last recharge.
A thermal analysis of the battery module for the
anticipated worst case hot case (re-entry) indicates the
cell temperature should not exceed 70 °C, with the
boundary condition imposed and using cell heat
capacity and heat generation determined by calorimetry
at Sandia National Labs [3]. The modules large mass
(187 lbs) provides it with adequate thermal capacitance.
This will be verified during environment testing under
this worst case hot condition.
Conclusions
The battery design approach utilizing small commercial
cells as the building block cell can be successful for
demanding aerospace applications like the CRV for the
ISS. This approach saves considerable development
time because it utilizes a commercially established cell
design that requires no further development. The
approach also increases reliability because the failure of
one cell would only cripple '/4 of the battery module
rather than the same failure in a single string battery
approach. Lots of the selected commercial cell from
Sanyo performed with very little cell-to-cell variations.
References
[1] High Power Commercial Cell Evaluation for the
Crew Return Vehicle, SRI report, Job #78C, 1997.
[2] Cell Acceptance Testing, SRI report Job #11X,
2001.
[3] Thermal Property Measurements of SubC Ni/Cd
Cells for the X-38 Crew Return Vehicle, E.P. Roth
and E. Darcy, 2002 Power Sources Conference,
Cherry Hill, NJ, 2002.


270V Battery Using COTS NiCd Cells For Manned Spacecraft

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