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Proceedings of SPIE, 4393, pp, 230-240 (2001)
Solar Battery Recharge Options for Unattended Ground Sensors
Paul E. Sims, AstroPower, Inc.
ABSTRACT
The operational lifetime of an Unattended Ground Sensor (UGS) depends on the power consumption of the package
and the space allocated for batteries. Solar cells have the potential of dramatically increasing operational lifetimes of UGS
instruments by providing supplemental power, but in this application solar cells are subject to a number of non-traditional
constraints. There are UGS applications where the solar array will need to be covert or have a high shock resistance. It is
also possible that a UGS-solar array will be placed in a shaded area or randomly oriented with respect to the path of the sun.
This paper will first survey conventional approaches towards solar battery charging and then discuss non-conventional
approaches applicable to randomly oriented and covert UGS solar cell arrays.
Keywords: Battery-Recharge, Solar-Cell, Unattended-Ground-Sensors
1. BATTERIES
Batteries are electrochemical energy storage devices. Primary batteries are designed for a one-way chemical
reaction and are not rechargeable. An example is the lithium sulfur dioxide (LiSO2) primary battery, the standard chemistry
used in military applications. Secondary batteries are designed to accommodate a reversible chemical reaction and can
typically be recharged more than 500 times. Table 1 displays some of the metrics of the most common rechargeable battery
chemistries compared to the LiSO2 battery. A cursory examination of Table 1 might lead one to the conclusion that a Li-
based battery is the best choice for UGS applications. However, there are a number of other considerations and the proper
choice of battery technology is not obvious without a detailed examination of the UGS operational profile. For instance, in a
pulse-discharge scenario, a Li battery would perform poorly while a NiCd would perform well due to the large differences in
the internal resistance of these battery types.
Table 1. Battery Technology Comparison1
Battery Chemistry Nominal
Voltage
(V)
Energy
(Wh/L)
Weight
compared
to LiSO2
Capacity
compared
to LiSO2
LiSO2 (Primary) 3.0 415 1.00 1.00
Sealed Lead Acid 2.0 90 2.04 0.30
Sealed NiCd 1.2 80-105 1.80 0.39
NiMH 1.2 175 1.73 0.75
Li-ion 3.6 200 1.32 0.78
Li-polymer 3.2 350 1.45 0.89
Battery Metrics
Voltage. The instantaneous battery voltage depends on the state of charge and the temperature. The nominal voltage of a
cell is merely a representative value. An important parameter is the End of Discharge Voltage (EODV). This is the
voltage where the battery is completely discharged. Electronics powered by batteries must be designed to operate down to
the EODV in order to utilize the rated capacity of a battery.
Capacity. The capacity of a battery is the amount of energy it can store when it is fully charged. This is expressed in Watt
Hours (Wh) or, when divided by the nominal terminal voltage, in Amp Hours (Ah). Under ideal conditions, this is also the
amount of energy that can be extracted from a battery. A simple rule of thumb is that a 1 Wh battery can, in the best case,
supply a 1 W electric load for one hour. Factors that can reduce the amount of energy that can be extracted from a battery
include temperature variations, very high or very low current discharge levels, voltage mismatch and self-discharge.
C rate. The C rate is the amount of current a battery can supply in one hour, i.e. a 2 Ah battery has a C/1 rate of 2 A, C/10
rate of 200 mA, etc.
Proceedings of SPIE, 4393, pp, 230-240 (2001)
Battery performance estimates using Ragone Diagrams
A Ragone diagram2 plots specific power vs. specific energy and can be used to compare many different power
storage technologies. Diagonal lines in a Ragone diagram have the units of time so that required weight for constant
discharge situations can be compared for different energy storage devices. An example of the use of a Ragone Diagram is
shown on Figure 1. Assume that a 10-hour total discharge time is required at 650 mW for a battery pack. By drawing a 10-
hour line to the Li-ion performance envelope, and then over to the Specific Power axis, we see that ~ 18 W/kg can be
expected for this C/10 discharge rate. By dividing the draw power (650 mW) by the Specific Power (18 W/kg) the required
battery pack weight is calculated to be 36 grams. Similarly, a NiMH battery (0.65 W / 6 W/kg) requires 108 grams. By
starting with a power and weight target, this procedure can be reversed to determine the total discharge time available for
different battery technologies. It is obvious from Figure 1 why lead acid and NiCd batteries are preferred in high-discharge
applications and NiMH and Li-ion batteries are preferred in long-life consumer electronics since they require a lower weight
for the same amount of energy.
Figure 1. Ragone Diagram of some battery technologies.
Battery charge procedures.
With solar power charging, the NiCd,
NiMH, and Li-ion chemistries are all feasible
for UGS applications. However, the NiCd and
NiMH batteries have an advantage when solar
trickle charging is considered because they do
not require any charge control electronics,
which may have significant parasitic power
drains in low power conditions. Lithium-ion
batteries always require the use of protective
circuits to prevent destructive over- and under-
voltage conditions and manufacturers will not
sell batteries without these circuits or
approving your circuit design. Figure 2
illustrates the simplest Li-ion battery
protection strategy. If area is not an issue, it is
simple to implement solar power strategies.
However, for many UGS applications, area is
severely limited and the instrument may have
to remain on standby for extended periods
while the solar cell array converts sufficient
power to resume normal operation.
High
Discharge
Current
Long Discharge Time
0.01 Hour
100 Hours
10 Hour
Discharge
0
0.5
1
1.5
2
2.5
0 1 2 3 4 5 6 7 8
V_max
V_min
Normal Operating Range:
Battery is allowed to charge 
and discharge
Above V_max:
Battery is allowed to discharge
Battery is not allowed to charge
Below V_min:
Battery is not allowed to discharge
Battery is allowed to charge
Li-battery charge protection strategy
Figure 2. Li-battery charge protection strategy.
Proceedings of SPIE, 4393, pp, 230-240 (2001)
Maximizing battery capacity when there are volume and weight constraints is aided by using low system voltages
combined with DC-DC converters to boost voltage as needed. For example, three 70-gram Li-ion batteries connected in
series to provide 10.8 V has a capacity of 650 mAh. A 3.6 V battery pack with the same weight would have a capacity of
1950 mAh. If only a small portion of the circuit requires the higher voltage, a significant savings in power efficiency can be
achieved through voltage conversion.
The best way to charge a battery is to follow the manufacturer’s recommended procedure. This usually involves
delivering a constant voltage or a constant current or some combination of both to the battery. The recommended charging
procedure for a Li-ion battery is adjusting the voltage to deliver a constant current until an upper voltage limit is reached and
then holding that voltage until the battery is fully charged. This type of charging procedure requires a charge control circuit
and assumes that the required power is readily available. For solar cells, there can be differences in charging procedures,
especially in cases where the solar power is limited.
2. SOLAR CELLS
Solar cells are large-area diodes where the top surface is patterned to allow light to penetrate into the bulk of the
device. Figure 3 displays a generic solar cell structure. In this figure, the n-type regionat the top of the solar cell is called
the emitter and the p-type region is called the base. Typically, the emitter is made as thin as possible to maximize light
absorption in the base region. Photons with energy greater than the bandgap are absorbed by the semiconductor material and
generate electron-hole pairs. If the minority carrier of the pair (electrons in p-type material) diffuses to the p-n junction
without recombining with a majority carrier, it is swept across by the electric field and becomes an excess majority carrier
that can be used to power an external load. Figure 4 displays the current voltage curve of a solar cell and Table 2 defines
some common terms associated with solar cell characterization. The principle of superposition applies as the dark diode
curve is shifted downward by the photocurrent, IL.
Figure 3. Generic solar cell structure. Figure4. Current voltage curve of a typical solar cell in
the light and in the dark.
There are many different semiconductor materials that can be used to fabricate solar cells. One useful way of
comparing these technology choices is to plot a theoretical conversion efficiency as a function of the bandgap of the
semiconductor material under consideration. Figure 5 displays the maximum terrestrial module efficiency achieved to date
on such a plot3. Crystalline silicon is the only material close to the theoretical limit. At the module level, the other
mainstream technologies, copper indium diselenide-based (CIS or CIGS or CIGSS), cadmium telluride (CdTe),
multicrystalline silicon (mc-Si) and amorphous silicon based (a-Si) are well below theoretical limits. The typical conversion
efficency of a commercial silicon solar cell module is 11-12%. However, like the voltage of a battery, the conversion
efficiency of a solar cell is a highly variable quantity under real conditions and is usually only quoted for Standard Test
Conditions (STC). These are defined as 25°C cell temperature, 100 mW/cm2 insolation and a particular spectrum, AM1.5G,
which is a specific angle through the atmosphere (air mass) under low scattering conditions.
Proceedings of SPIE, 4393, pp, 230-240 (2001)
Table 2. Solar Cell Terminology
Symbol Description
Voc Open-circuit voltage. The voltage a solar cell charges up to while under illumination if no external load
is connected.
Isc Short-circuit current . The current an illuminated solar cell will deliver through an external short
circuit.
Pmax Max power point . The point on an illuminated solar cell’s IV curve where the product of the voltage
(Vmp) and current (Imp) is a maximum.
FF Fill factor. A measure of the “squareness” of the IV curve. Fill factor is defined as FF = Pmax/Voc*Isc.
Rs Series resistance. The internal series resistance of a solar cell which is strongly affected by solar cell
geometric design. The final lumped series resistance can be determined by the slope of the IV curve at
Voc.
Rsh Shunt resistance. The internal shunt resistance of a solar cell which is strongly affected by solar cell
processing flaws. The final lumped shunt resistance can be determined by the slope of the IV curve at
Isc.
R Reflection. The wavelength dependent reflection of a solar cell.
EQE Extenal Quantum Efficiency. The wavelength dependent measure of the number of photons impinging
on a solar cell and the percentage of carriers collected.
IQE Internal Quantum Efficiency. The wavelength dependent measure of the number of photons
penetrating the surface of a solar cell and the percentage of carriers collected. IQE = EQE / (1-R)
h Conversion Efficiency. Pmax / (Total Power of the light incident on a solar cell). For terrestrial solar
cells, the standard test conditions (STC) are at 25°C using 100 mW/cm2 insolation with a spectrum
defined by AM1.5G.
Figure 5. Modeled solar cell conversion efficiency as a function of the energy gap of various semiconductor materials. Also
shown are record module efficiencies for the four most mainstream materials.
Calculation of the average power that can be produced by a solar cell
Figure 6 displays the electrical parameters of a typical 100-Watt crystalline silicon module. The installation details
must be known to estimate the amount of energy that this module will produce. Typically, a flat plate module is oriented
towards the equator and tilted at an angle from level ground equal to the latitude of the installation +/-15° (steeper to shed
snow, shallower to improve performance under cloudy conditions). However, this is simply a guideline and the system
designer should calculate each installation for maximum performance. Historical databases for the US can be found on the
Internet at rredc.nrel.gov/solar/old_data/nsrdb/redbook.
Proceedings of SPIE, 4393, pp, 230-240 (2001)
Figure 6. Electrical parameters of a typical 100-Watt crystalline silicon module.
For the month of June in Delaware the average solar radiation for a module tilted at latitude is 5.6 kWh/m2/day and
the average temperature is 21.9°C. An average daily insolation of 5.6 kWh/m2 is equivalent to a day with exactly 5.6 hours
of one-sun illumination. So, if the module were at 25°C, the average amount of energy produced by a 100 W module is 100
W x 5.6 hours = 0.56 kWh/day. From the data sheet, The Normal Operating Cell Temperature (NOCT) is 45°C for average
illumination and 20°C ambient temperature. Assuming linearity, the module is expected to operate
45-20 = 25°C above the ambient temperature so that the average module temperature is expected to be 21.9+25 = 46.9°C.
For silicon solar cells, the approximate temperature-power correction factor is 0.5% per degree C and the STC temperature is
25°C. So, the temperature correction factor is (46.9-25)*0.5% = 10.95%. When the energy value calculated above is
corrected for temperature, on average a 100-Watt module in June in Delaware will produce 0.50 kWh/day.
The module for which data is shown in Figure 6 consists of 36 single crystal silicon solar cells connected in series.
This is a common configuration designed to directly charge a 12 Volt lead acid battery under typical outdoor conditions and
we note that under typical conditions, the knee of the module curve is above 12 Volts. The commercially available small
solar modules are also designed for specific battery packs. Cell phone battery packs are typically 3.6 V and small modules
can be purchased commercially to charge them. These COTS modules may be adaptable to UGS applications.
Batteries are not resistive loads. A photovoltaic (PV) module directly connected to a battery is always biased to the
instantaneous battery voltage in a narrow range between the full-charge voltage to the EODV. Design of a solar cell module
requires, at a minimum, specification of the full-charge battery pack voltage and the minimum amount of useful current
delivered by the solar cell array.
Solar cell current is linear with light intensity while voltage is logarithmic with light intensity. The Vmp under
different light conditions is the parameter of interest in module design. To maximize the amount of charge, the bias point of
the solar cell should never exceed Vmp, which is near the knee of the IV curve ensuring that the solar cell is always operating
on the flat part of the IV curve and maximizing the available current as the light level varies.
Proceedings of SPIE, 4393, pp, 230-240 (2001)
The Vmp is best determined using a model rather than by using a rule of thumb. A one-dimensional finite difference
model, PC-1D, is commonly employed to predict the performance characteristics of individual solar cells under different
light conditions and temperatures4. Figure 7 displays PC-1D model results for a GaAs solar cell illuminated by diffuse light
at two different intensities. This is an unusual condition that might be encountered in a covert UGS deployment where the
PV arrayis in the shade but can “see” diffuse light from the sky. This model indicates a “best” operating voltage near 0.75 V
per cell and the current density is predicted to range from 2.8 mA/cm2 at low intensity and 13.8 mA/cm2 at the higher
intensity.
-0.015
-0.01
-0.005
0
0.005
0.01
0.015
-1.5 -1 -0.5 0 0.5 1 1.5
VOLTAGE (V) 
C
U
R
R
E
N
T 
(A
) 
1 cm2 GaAs solar cell model
under diffuse sunlight
10 mW/cm2 intergrated insolation
50 mW/cm2 integrated insolation
Figure 7. Modeled performance of a GaAs solar cell under diffuse sunlight at 300K.
For a Li-ion battery, the full-charge cell voltage is 4.1 V and the voltage drop for a germanium blocking diode is
0.3 V. Six GaAs solar cells in series are required to charge this battery. The solar cell module bias will follow the battery
state of charge characteristic from 3 V to 4.1 V (plus the blocking diode). The average individual cell bias point is then
0.55 V to 0.73 V. On a per cell basis, there are four points of interest enumerated in Table 3 that define the operating
envelope for this example.
Table 3. Individual solar cell power density
Light Level Discharged Battery Full battery
Low 0.55 V x 2.8 mA/cm2 = 1.54 mW/cm2 0.73 V x 2.8 mA/cm2 = 2.04 mW/cm2
High 0.55 V x 13.8 mA/cm2 = 7.6 mW/cm2 0.73 V x 13.8 mA/cm2 = 10.1 mW/cm2
Now assume that the desired current under low light is 10 mA. This specification requires that each of the six solar
cell segments have an area of 3.57 cm2 so the total array area is 21.4 cm2, with a form factor of 1.5 x 2.2 inches. The
maximum power produced by this array is then 6 x 3.57 cm2 x 10.1 mW/cm2 = 216 mW while it is connected to the battery.
For some UGS applications, there are two problems with this simple approach. First, the use of six solar cells
connected in series assumes that each one of the cells receives the same illumination. Partial or complete shading of module
segments will cause a significant degradation in module current due to diode mismatch. If the array is mounted on a curved
surface, the cosine losses under uniform illumination will also cause mismatch-induced current degradation. Second, this
module has an awkward size, it is too small from a fabrication perspective since it requires the laborious interconnection of
discrete solar cell elements and it is too big for most of the targeted UGS application envelopes.
Proceedings of SPIE, 4393, pp, 230-240 (2001)
3. ADVANCED CONCEPTS
For UGS applications, solar power collection systems should be small, lightweight, shock resistant, insensitive to
orientation and partial shading, deliver useful power while in full shade, covert, and feasible from a manufacturing
standpoint.
Light collection and concentration using scintillating fiber-optic bundles
One possible approach meeting these criteria is to use infrared scintillating plastic fibers or preforms to collect light
and deliver it to a solar cell array. With this approach, scattered UV and blue photons enter along the length of a plastic
multimode fiber and are converted by an organic scintillant in the core to longer-wavelengths that are transmitted along the
axis of the fiber to a shock-hardened photovoltaic converter. Three possible approaches for fiber incorporation in the
GLIMPS projectile are shown in Figure 8.
(a)
Wrap
(b)
Flag
(c)
Slinky
Figure 8. GLIMPS specific deployment options.
With the scintillating fiber approach, there is a trade-off between increased solar cell conversion efficiency due to
the near-monochromatic light spectra and power losses due to a reduced number of photons with energy high enough to
excite the scintillant compared to a direct conversion approach using the photovoltaic effect. Figure 9 shows the spectral
content of sunlight. A typical scintillant is excited by light with wavelengths shorter than 550 nm. For comparison, a silicon
solar cell converts light with wavelengths shorter than 1100 nm (htyp, AM1.5 = 14%) and a GaAs solar cell converts light with
wavelengths shorter than 880 nm (htyp, AM1.5 = 20%). For a direct semiconductor solar cell the conversion efficiency is
maximized at a wavelength where the absorbed photon energy is just above the bandgap energy. For GaAs, this corresponds
to > 1.41 eV or equivalently < 880 nm. A solar cell under this type of illumination will exhibit high conversion efficiency. A
GaAs laser power converter may have efficiency greater than 60% due to a combination of monochromatic illumination and
high injection levels.
Figure 9. Various solar spectra. The diffuse component is of primary interest for this application5.
Proceedings of SPIE, 4393, pp, 230-240 (2001)
Experimental light collection data for individual commercial red fibers are shown in Figures 10 and 11. Self-
absorption in the fiber limits the amount of optical gain or concentration that can be achieved. The maximum useful length
for a fiber is on the order of 10 feet and most of the optical gain occurs within the first five feet. The larger diameter fibers
are found to deliver significantly more light to the photodiodes. We believe that these results indicate that optical losses due
to the escape of light from the fiber and cladding are dominating compared to optical losses due to self-absorption within the
fiber. Optimization of the core/cladding interface should reduce this “escape loss” and increase the amount of light that can
be delivered to the solar cell array. There are reports in the literature that thicker cladding material and double layer cladding
material significantly improves optical confinement for scintillating fiber-based particle detectors. However, the differences
between absorption of UV light compared to a particle absorption event indicate that a different optimization of layer
thicknesses will be required.
0
0.5
1
1.5
2
2.5
3
3.5
0 1 2 3 4 5 6
ILLUMINATED FIBER LENGTH (ft)
PH
O
TO
C
U
R
R
EN
T 
(u
A)
0.04" Dia.
0.03" Dia.
0.02" Dia.
0.008" Dia.
Figure 10. Photocurrent as a function of illuminated length.
0
0.5
1
1.5
2
2.5
3
3.5
0 0.02 0.04 0.06 0.08
ILLUMINATED FIBER VOLUME (in^3)
PH
O
TO
C
U
R
R
EN
T 
(u
A)
0.04" Dia.
0.03" Dia.
0.02" Dia.
0.008" Dia.
Figure 11. Photocurrent as a function of illuminated volume.
We have constructed various fiber bundles by using ferrules and optical epoxies to constrain the ends of the fibers.
The brightest bundle constructed to date consists of 41 10-ft long fibers constrained by a 0.25 x 0.25 inch square ferrule.
Using calibrated (AM1.5G) gallium arsenide solar cells, we measured the intensity of the light that was collected and
delivered by these fiber bundles. These photocurrent results were then translated into a GaAs “sun equivalent” value. The
1-sun equivalent response of a GaAs solar cell fiber combination is defined as being the same cold light induced photocurrent
that the GaAs solar cell would have under true 1-sun AM1.5G measurement conditions. The best experimental result to date
is ~2.5 times the 1-sun photocurrent (Figure 12). We believe that these results can be improved significantly, at least by a
factor of two, with improvements in light coupling and custom fiber design.
The low-light performance of the large fiber bundle is particularly encouraging. This fiber bundle collects and
delivers significant amounts of light while in the shade and during overcast conditions. A shaded fiber bundle collects and
delivers more light during hazy conditions compared to clear conditions due to an increase in the spectral content of blue
light under these conditions. Partial shading of a fiber bundle is not an issue; the light delivered at the end of the bundle is
spatially uniform under these conditions.
The power generation capability of a fiber bundle– solar cell system can be calculated by multiplying the measured
sun equivalent value delivered by a fiber bundle by a reasonable estimate of the AM1.5G conversion efficiency of a GaAs
solar cell. These estimates are shown in Table 4. These calculations indicate that energy scavengers can reasonably be
expected to produce power levels ranging from 5 to 100 mW/cm2 of device area, depending on the size of the bundle and
placement conditions. However, to achieve this, it is also necessary to fabricate small area solar cell arrays to generate
enough voltage to couple to an electronics package.
Proceedings of SPIE, 4393, pp, 230-240 (2001)
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Figure 12. Measured light intensity (converted to “sun-equivalents”) for two different fiber bundles under various outdoor
illumination conditions.
Table 4.
Predicted Performance for GaAs Solar Cells
Illuminated by Scintillating Fiber-optic Bundles
Insolation
(sun-
equivalents)
18 % Module
Performance
(mW/cm2)
20% Module
 Performance
(mW/cm2)
0.1 1.8 2
0.5 9.0 10
1.0 18.0 20
2.0 36.0 40
5.0 90.0 100
Monolithically Interconnected GaAs Solar Cell Array Development
The fiber-bundle approach implies a need to develop a small solar cell array fabrication process that has high
conversion efficiency while illuminated by near-monochromatic infrared light. Solar cells based on the AlGaAs/GaAs
material system were selected. This is due to two factors, (1) the GaAs system is well developed for high-efficiency solar
cells, and (2) GaAs semi-insulating wafers are available which allows the fabrication of small arrays on a single substrate.
Figure 13 displays a schematic of the array interconnect technology for an ion-implant-based fabrication sequence we
originally proposed.
· A high-efficiency AlGaAs/GaAs solar cell structure is epitaxially grown on a semi-insulating GaAs substrate
· Channels are etched through the emitter into the base layer for the n-type contact
· Device isolation is accomplished using patterned H+ implantation to selectively damage and convert the material to
high-resistivity
· Metal contacts and interconnects are deposited to create an integrated array structure
Large bundle
Small bundle
Proceedings of SPIE, 4393, pp, 230-240 (2001)
Figure 13. High voltage GaAs solar cell array design schematic.
This ion-implant approach is feasible, and Figure 14 shows the IV curve of an array fabricated by this method.
However, we have found that there is a yield problem associated with ion-implanting these devices and have changed our
fabrication method to one based on the use of etch sequences to define array segments. This process uses a photopatternable
polyimide to insulate metallization interconnect tracks and prevent junction shunting. This approach is beginning to
consistently yield functional devices and we are presently optimizing the details of the etch sequence to maximize the solar
cell array conversion efficiency. An IV characteristic of one of these GaAs-based devices is displayed in Figure 15. This
device consists of five rectangular GaAs elements interconnected using photolithographic processes on a semi-insulating
substrate.
Figure 14. IV characteristics of a five element LPE-based monolithically interconnected, ion-implant isolated GaAs solar
cell array.
Integration of solar cell arrays and fiber-optic bundles will be accomplished by the use of a combination of optical
epoxies to improve light coupling and mechanical clamps between a printed circuit board and the fiber ferrule to improve
shock resistance. Inside the instrument, the shock resistance of the assembled light collector and solar cell array should be
similar to that of properly potted electronics. We expect that the mechanical weak-link in a high-shock application will be
shearing of the fiber bundle at the point where it exits the instrument. The best exit point for a fiber bundle in a projectile is
directly down the rear of the instrument centered on the axis. This, however, is prime real estate and we invite system
designers to suggest other feasible approaches.
 Voc = 5.027 V
 Isc = 39.34 mA
 FF = 66.8
Proceedings of SPIE, 4393, pp, 230-240 (2001)
Figure 15. IV Curve for GaAs module using the polyimide approach.
6. CONCLUSIONS
Incorporation of solar power systems and rechargeable batteries in UGS devices can dramatically increase
operational lifetimes. However, each mission will require a separate assessment of the best approach and the trade-offs
involved. For a non-covert, hand emplaced, non-area limited scenario, a COTS approach is recommended and simple
guidelines for selecting batteries and PV modules have been presented. In this situation standard solar power technology can
easily supply power for years of continuous instrument operation. For covert, orientation-independent, and high-shock
scenarios, the fiber-bundle approach has promise, but will result in limited power levels and require significant top-down
interaction between the respective system designers. A key issue is minimizing the draw power while the instrument is in
standby mode to allow the solar cells to charge the battery during operational downtime.
ACKNOWLEDGMENTS
This work has been supported by DARPA and SPAWAR under Contract Number N66001-99-C-8516.
REFERENCES
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3. M.A. Green, et al., “Solar Cell Efficiency Tables (Version 17)”, Progress in Photovoltaics: Research and
Applications, M.A. Green, ed., 2001, 9:49-56.
4. P.A. Basore, IEEE Transactions on Electronic Devices 37(2), 337 (1990).
5. C. Hu and R.M. White, “Solar Cells, From Basic to Advanced Systems”, McGraw-Hill, 1983.
OTHER READING
1. David Lindon, “Handbook of Batteries”, McGraw-Hill, 1995.
2. C. Honsberg and S. Bowden, “Photovoltaics – Devices, Systems & Applications”, (Interactive Computer Disk)
ISBN 0-7334-0596-7.
Voc = 4.2 V
Isc = 1 mA