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Temperature effects on satellite power systems performance
M. Bekhti, M. N. Sweeting,
Abstract - Alsat-1 is the first small satellite for the Centre of
Space Techniques (CTS) – Algeria. It was designed, built,
assembled and tested at Surrey Satellite Technology Limited
(SSTL) at the University of Surrey, with the participation of
11 Algerian engineers covering all aspects of micro satellite
engineering within a technology transfer programme between
SSTL and the CTS. Alsat-1 is an enhanced micro satellite
weighing 100kg (launch mass). The satellite measures 60 by
60 by 62.5 cm, and is powered by four body mounted GaAs
solar panels, with a total power rating of 60 watts. The solar
panels are the primary source of power to the satellite. Twenty
two 4Ah Nickel Cadmium cells are used to power the satellite
during eclipse.[9]
The power system on Alsat-1 has met or exceeded prelaunch
predictions excepting for the NiCd battery pack which started
showing signs of defects in August 2005. [8][10]
This paper shows how, from August 2005 until now, teams
from SSTL and CTS have been working together to monitor
battery health and performance in particular with regard to its
charging cycles. The key points affecting the battery thermal
condition including power consumption during eclipse, heat
generated by the battery itself and the depth of discharge of
the battery are discussed. Some recommendations and ideas
regarding thermal design and battery protection are presented.
Keywords - micro satellite, system performance, degradation,
depth of discharge, temperature, thermal conditions.
I. Introduction
Small satellites have already been launched with considerable
success by many institutions in the developing countries. Their
attraction lays in the promise of low cost and short
development timescales made possible by the use of proven
standard equipment and off the shelf components and
techniques. Coupled with realistic and focussed goals, such
satellites make it possible for a country with even a small
research budget to participate in their development, launch and
operation.
Small satellites thus present an ideal opportunity for training
students and engineers in different disciplines, including
engineering, software development for on board and ground
computers.
The Algerian engineers were trained at Surrey through the
AlSat-1 project with specializations in the following
disciplines: System Engineering, ADCS, Power, RF, OBDH,
Mechanical and Launch Interface, Earth Observation Payload,
GPS, Operation, TTC and Propulsion. [8]
Manuscript received July 20, 2010. This work is part of a research project on
power systems on board small satellites.
M. Bekhti, Centre des Techniques Spatiales, BP 13, Arzew 31200, Algérie.
Tel: +213 41 47 38 26 - Fax: +213 41 47 36 65, Email: m_bekhti@yahoo.fr
M. N. Sweeting, Surrey Satellite Technology Limited,Tycho House, 20
Stephenson Road , Surrey Research Park , Guildford GU2 7YE , United
Kingdom, Tel: 44 (0)1483 803803 - Fax: 44 (0)1483 803804, Email:
m.sweeting@sstl.co.uk.
II. Alsat-1 platform architecture:
The spacecraft is cubical in shape with four body-mounted
panels, with the remaining sides including the spacecraft
launch adaptor, sensors, payload apertures and antennas
(figure 1). The structure is based on aluminium and aluminium
honeycomb panels and was been designed to be compatible
with a range of launchers. The stack of trays carries an optical
platform and between the stack and the panels, the battery
pack, the wheels and the propulsion system are carried. A
propulsion system is required in order for the spacecraft to
carry out initial launcher injection corrections, spacecraft
separation into the final orbital slot, altitude maintenance and
finally an end-of-life manoeuvre to remove the spacecraft
from the operational system. [9]
Fig 1 Alsat-1 platform architecture.
III. Alsat-1 power system description
The primary power to the satellite is supplied via 4 solar
panels (figure 2). The power from each of the four solar panels
is fed into a dedicated Battery Charge Regulator (BCR), i.e.
one BCR per solar panel. The output of the BCRs is connected
to a 22 cell, 4Ah NiCd battery, the Power Distribution Module
(PDM) input and the Power Conditioning Module (PCM)
Advances in Communications, Computers, Systems, Circuits and Devices
ISBN: 978-960-474-250-9
57
input. The solar arrays and BCR outputs are isolated from
each other using one blocking diode per BCR. By isolating the
solar arrays from the bus using a BCR per panel suits the low
earth orbit environment and the nature of the Alsat-1 design
for several reasons:
1. the maximum power point (MPP of an individual panel
can be tracked over the changing thermal conditions whilst in
sunlight.
2. the battery is charged for the majority of the sunlight
period efficiency of the power system.
3. the direct connection between the battery and the bus
ensures maximum efficiency.
The function of the PCM is to convert the raw battery voltage
into a regulated 5V line and a –5V line. The dual redundant
PCMs (1 and 2) are virtually identical.
PCM 1 has a battery under-voltage lockout system
incorporated into its design, providing further protection to the
power system. When the PCM input (battery) voltage goes
below 20V, the PCM switches off completely.
This removes the 5V bus from the spacecraft, switching OFF
the power system CAN microcontrollers and with it all of the
power switches in the power system. The PCM will re-start
once the battery voltage has recovered to approximately
22.7V.
Only PCM 1 has this safety feature, so by selecting PCM 2
this feature will be disabled. PCM 2 will operate down to a
battery voltage of about 11.5V. At voltages lower than this
value, the PCM can no longer regulate the output voltage to
5V, and the voltage goes to zero.[4][5][6]
Fig 2 Alsat-1 power system block diagram.
A. Solar Panels
The solar cells used on Alsat-1 were single junction GaAs/Ge
cells, mounted on aluminium face sheet and aluminium
honeycomb substrate. The cells provide on average 19.52 %
conversion efficiency at ambient temperature.
The solar panels were configured as follows: The panel
substrates were made of 20mm aluminium core honeycomb
with 0.5 mm aluminium faceskins front and rear. The front of
the panels has an insulating layer of 75 µm kapton. The cell
lay down design of the four panels was identical consisting of
6 strings of 48 cells in series (a total of 288 solar cells per
panel).
The cells were arranged on the panel in 12 columns of 24
cells. This convenient arrangement allowed the entire terminal
wiring (redundant positive and negative) for each string to be
done at one end of the panel. 20mm x 40mm GaAs/Ge solar
cells were glassed and welded into solar cell assemblies
(SCAs).
The cells were then individually measured (at 0.86V) to
arrange the cells into their respective current classes. The cell
performance was selected so that the +Y and –Y panels had a
better performance than the +X and –X panels.
The panels were busbarred and wired completely after. The
panels were then rear wired (thermistor, temperature sensor,
and 15 pins D type connector added). Large Area Pulsed Solar
Simulator (LAPSS) measurements of the panel powers pre-
environment tests (EVT) were performed at QinetiQ in
Farnborough on 29/5/02, (figure 3). Panel powers were again
measured on 19/7/02 post – EVT, see table 1.[2][4][5][6][7]
Fig 3 Alsat-1 flight model panel.
Test conditions: Air Mass Zero (1366.1W/m2) at 25°C. [10]
+X Panel Pmax Ptest
Pre test 59.06W 56.5W
Post test 58.63W 56.5W
+Y Panel
Pmax
Ptest
Pre test 60.70W 58.04W
Post test 60.00W 57.60W
-X Panel Pmax Ptest
Pre test 59.68W 56.97W
Post test 58.85W 56.25W
-Y Panel
Pmax
Ptest
Pre test 59.92W 57.30W
Post test 58.85W 56.51W
Table1: Pre and Post EVT power measurements.
B. Battery Pack
Alsat-1 uses a single battery pack (figure 4) that consists of 22
Sanyo N4000 DRL cells in series and has a nominal voltage of
26 to 34 volts (depending on the charge). The cells used are
fast charging cells and have undergone a set of mechanical
and electrical matching tests to produce the flight battery pack.
Advances in Communications, Computers, Systems, Circuits and Devices
ISBN: 978-960-474-250-9
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There is an under voltage protection for the battery provided
by the PCM/PDM module. This protection will stop the
battery voltage dropping below 26 volts and therefore stop any
of the cells from being damaged by over discharge. The
battery pack has 37 Telemetry lines that provide all
temperature and voltage data to the PCM/PDM and BCR
Modules. The PCM/PDM Module provides some of the house
Fig 4 Alsat-1 flight model battery pack.
keeping data processing for the Battery, including the
individual cell voltage telemetry and the battery pack
temperature. The BCR Module provides battery current and
battery voltage telemetry and interfaces with the 4 thermistors
mounted in the battery for temperature compensated control of
the BCR. Fast charging of the battery requires careful tracking
of the maximum power point (MPP) of the solar panel. The
MPP for Alsat-1 panels is roughly 40 volts, a value which is
carefully monitored by a microcontroller associated with the
BCR, varying the voltage of the panel to allow the optimal
production of power.[1][3][10]
C. Battery Level Tests
Once the battery has been assembled, it undergoes battery
level tests that include conditioning cycles, battery capacity
measurements and thermistor / temperature sensor tests. The
battery was subjected to three full charge/discharge cycles at
22.5°C. The charge current was 400mA for approximately
15hrs and the discharge tests consisted of discharge rates of
C/5 (800mA). The capacity of the battery is calculated from
the third and last discharge. The discharge current of 800mA
takes just over 5 hours to fully discharge the battery, giving a
capacity of 4.25Ah. It should be noted that it is possible for
the battery voltage to increase to a voltage of approximately
33.4V if charged at 0°C, and can reach over 31.6V when
charged at room temperature.[5]
D. Battery Charge Regulator
The BCR used on Alsat-1 is a low power buck topology DC-
DC converter. In total, we have 4 BCRs, one dedicated to each
solar array stepping down the 40 volts from the solar arrays to
the 28 volts unregulated bus. The BCRs operating frequencies
are not all identical but separated from each other by 15 KHz
to prevent beat frequencies interfering with sensitive radio
frequency systems such as the receiver. The chosen nominal
frequency of a BCR is 200 KHz. An estimate of the solar array
maximum power point (MPP) is made by sensing the array
substrate temperature using thermistors. The array voltage is
set to the end of life MPP in hardware. The end of charge
(EoC) voltage of the NiCd battery is set in hardware and uses
a thermistor for temperature compensation. Once the battery
voltage reaches the EoC set point, it is held at this value as the
charge current naturally tapers off to a trickle charge level,
thus the battery is never overcharged. Each BCR also has the
ability to be controlled by software, enabling more accurate
tracking of the MPP of the solar array. The software has a
watchdog timer that protects the power system from a
software crash. If the software times out, the BCR will
automatically revert to hardware control. The software control
is managed within the power system which has its own
microcontroller. The BCR efficiency has been calculated from
tests performed at room temperature and varies from 70% to
80% depending on the input power.[10]
E. Power Conditioning Module
The Power Conditioning Module (PCM) is a buck DC-DC
converter and provides a regulated 5 volts supply and a semi
regulated 15 volts from the raw battery voltage. The output
current is 3A.
The PCM provides deep discharge protection to the battery by
shutting OFF when the battery voltage falls below
approximately 26 volts and permitting the system to come
back up again once the battery has recovered to a voltage of
28 volts.[10]
F. Power Distribution Module
The power distribution module (PDM) consists mainly of
MOSFET based power switches. The same switch design is
used for regulated 5 volts and 15 volts and raw battery power
distribution. By using P channel MOSFETS with a very low
RDS (On) of about 20 mΩ (IRF 4905S), the switch provides a
highly efficient interface to subsystems and can easily be
configured to trip at the required current level. The PDM
consists of around 100 P channel MOSFET, which act like
programmable circuit breakers. They can be switched On and
Off, allowing the control of power to the satellite subsystems
and payloads. This switching is controlled by a
microcontroller which switches power to a subsystem when
requested via the CAN. Radiation is, however, still enough of
a factor to determine the choice of MOSFET in the PDM. A P
channel device has been chosen over an N-Channel because it
lasts far longer in conditions of higher radiation.[10]
IV. Telemetry data processing and battery data analysis
Since its launch on the 28th November 2002, Alsat-1 has so far
completed 28 000 orbits and thus 28 000 charge/discharge
cycles for the battery.
The software implemented in the ground station has basic
statistical tools. It can also allow variable telemetry data
Advances in Communications, Computers, Systems, Circuits and Devices
ISBN: 978-960-474-250-9
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comparison and calculation, for example, the power generated
by the solar cells using voltage and current data.
The analysis of the telemetry data files related to the solar
arrays temperature shows for most of the times that it
fluctuates between -35°C when in eclipse and +35°C in
sunlight.
Fig 5-a Cell 19 healthy 30/08/2005.
A more detailed analysis of the telemetry data over the period
2003-2008 gave the results on table 2: [4][5][6]
Solar array
configuration
Min
temperature
Max
temperature
period
-X -30°C to -
10°C
+10°C to
+20°C
2003-2008
+X -35°C to -
5°C
+5°C to
+30°C
2003-2008
-Y -35°C to -
30°C
+10°C to
+35°C
2003-2008
+Y -30°C to
0°C
+5°C to
+20°C
2003-2008
Table 2: Alsat-1 solar arrays telemetry data
The battery environment in orbit (temperature, voltage and
current) was carefully checked all the time using this tool.
Since the battery pack cells are charged during sunshine and
discharged in the shadow, therefore their respective voltage
increases and decreases periodically. As an example, figure 5-
a shows the voltage for cell 19 in its healthy state. The
telemetry data was taken from the 30th August 2005 file and
clearly shows that the cell was charging and discharging
properly. The voltage was increasing from 1.2 volts (nominal)
to 1.45 volts (max).
Fig 5-b Cell 19 acting as a resistor30/10/2006.
Figure 5-b shows, a year later and for cell 19, a sign of defect.
In fact, cell 19 started acting as a resistor from 30th October
2006. The discharge cell voltage was hitting a lower value
well below the nominal value.
A close monitoring of cell 19 data showed later that it was
deteriorating slowly and finally it went to the short circuit
state on the 22nd March 2008 with a voltage of 0 volt, see
figure 5-c.
Fig 5-c Cell 19 short circuit 22/03/2008.
A. Depth of discharge of the battery
Following the start of breakdown of the Alsat-1 battery pack, a
close watch of the temperature effect on the battery
characteristics was daily measured. The depth of discharge of
the pack was also kept under control to make sure it does not
contribute to the damage of the battery. Table 3 gives an
insight of the range of depth of discharge being experienced
on Alsat-1:[4][5][6]
year Battery
temperature
Depth of discharge
2003 16 to 20°C 15 to 25%
2004 21 to 28°C 16 to 25%
2005 25 to 39°C 16 to 22%
Table 3 monitoring of the depth of discharge.
It can be seen from table 3 that the depth of discharge is kept
within the required range for the lower values, i.e., 15%. The
higher value can for some situation be considered good as well
as long as it does not exceed 25%. The most noticeable data
figure is the temperature which has exceeded for the years
2004 and 2005 the optimum value. The battery pack has an
operational temperature limit of 0°C to 40°C but it should be
kept within the range 15 to 20°C for good performance.
B. Temperature and voltage monitoring and control
To protect the battery pack, several plots of voltage/current
against temperature were used to monitor the thermal working
environment according to the variable working modes of the
spacecraft. Figure 6 shows the battery temperature over the
spacecraft lifetime. This shows a steady rise until the
introduction of the software charge control in early 2005. The
Advances in Communications, Computers, Systems, Circuits and Devices
ISBN: 978-960-474-250-9
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most significant drop in early 2006 is due to the spacecraft
being yawed by 180° when not imaging.
Fig 6 Battery temperature.
C. Cell Voltages
Figure 7 shows the individual cell voltages as of the 24th of
November 2006. Cell 19 has effectively failed and is acting
as a resistor. This can be seen from its behaviour with the
changes in current during charging. Effective battery
management and a reduction in battery temperature have
greatly improved the performance of cells 3 and 12 which
have shown signs of defects lately also.
Fig 7 Cells voltages.
D. Battery Voltage
Figure 8 shows the battery voltage over the lifetime of the
Spacecraft. The battery voltage has been steadily decreasing
over the in orbit period, showing signs of recovery around
mid 2005 when the charge control software was
implemented and even better performance since early 2006
when the spacecraft was yawed by 180°, the battery
temperature decreased and cell capacity improved. Battery
voltage is averaging about 23.5V at the present time.
Fig 8 Battery voltage.
Conclusion
The present state of the battery on the spacecraft and the
main factors which led to the damage of a few battery cells
in the pack for Alsat-1 has been discussed.
At present the battery voltage is above the reset voltage but
it is expected that this will fall with age and increased
spacecraft utilisation.
For the time being Alsat-1 is still operating with a close
watch of the battery parameters as to avoid further damage
for other cells.
It is clear on the graphs shown in this paper that temperature
is one factor contributing to the battery damage and here we
could point the fact that for future missions, the battery
position within the spacecraft must be studied carefully.
This is confirmed as being a temperature related problem
because when the satellite was yawed 180°, ie, the battery
pack changed place in the satellite reference frame, the
temperature went down to normal and the cells performance
had gone up noticeably regarding charge/discharge schemes.
References:
[1] C.S. Clark, A.D. Hill, M. Day, Commercial nickel
cadmium batteries for space use: a proven alternative for LEO
satellite power storage, Proceedings of the Fifth European
Space Power Conference, Tarragona, Spain, 21–25
September, (1998), pp. 715–720.
[2] M. D’Errico, M. Pastena, Solar Array Design and
Performance Evaluation for the Smart Microsatellite, 49th
Congress of the International Astronautical Federation, Sept.
28 – Oct. 2, (1998), Melbourne, Australia.
[3] M. Pastena, M. Grassi, Design and Performance Analysis
of the Electronic Power Subsystem of a multi Mission
Microsatellite, Acta Astronautica, vol. 44, n°1, (1999),
pp. 31-40.
[4] Clark, C.S., Alsat-1 Power System TVT Test Procedure.
Surrey Satellite Technology Limited, November (2002),
pp. 1-17.
Advances in Communications, Computers, Systems, Circuits and Devices
ISBN: 978-960-474-250-9
61
[5] Clark, C.S., Alsat-1 Battery Conditioning, Operation and
Handling Procedure. Surrey Satellite Technology Limited,
November (2002), pp. 1-7.
[6] Clark, C.S., Alsat-1 AIT Power System Test Report.
Surrey Satellite Technology Limited, November (2002),
pp. 1-11.
[7] R, Kimber, Alsat-1 solar array interface control document.
Surrey Satellite Technology Limited, December (2002),
pp. 1-8.
[8] M. Bekhti, J.R. Cooksley, Alsat-1 in orbit performance
results, in: Third Disaster Monitoring Constellation
Consortium Meeting, Abuja, 3–4 April, (2003).
[9] M. Bekhti, M. Benmohamed, M.N. Sweeting, The role of
small spacecraft in the developing countries: the Algerian
experience, in: Small Satellite and Services Symposium, La
Rochelle, France, 20–24 September, (2004).
[10] M. Bekhti,M.N. Sweeting, power system design and in
orbit performance of Algeria’s first micro satellite Alsat-1.
Electric power systems research 78, (2008), pp 1175-1180.
Biography:
Mohammed Bekhti received his first degree in electronics from
the University of Science and Technology of Oran, Algeria, a
master degree in power electronics from the University of
Nottingham, UK and a Mastère in space telecommunication
from a High School of Engineering (SUPAERO) in France.
His fields of interests are microstrip technology applied to
filter design and power electronics for micro satellites power
systems design.
Advances in Communications, Computers, Systems, Circuits and Devices
ISBN: 978-960-474-250-9
62