Design framework of a configurable electrical power system for lunar rover

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Design framework of a Configurable Electrical
Power System for Lunar Rover
Baiju Payyappilly
Power Systems Group
ISRO Satellite Centre
Bangalore-560017, INDIA
Email: bbp@isac.gov.in
Sankaran M
Communication and Power Area
ISRO Satellite Centre
Bangalore-560017, INDIA
Email: msankar@isac.gov.in
Abstract —The temperature swing over lunar terrain is harsh
and it can fall to as low as 70K during lunar nights in the polar
region[1], which has significant interest as a preferred landing
site. Most of the lunar rover designs incorporate, radioisotope
heater units (RHU) to maintain the temperature during the
nights. A 50W electrical power system (EPS) design for a
Lunar rover, which does not contain a radioisotope heater
units, is presented. The design approach adopted, provides
wide operational flexibility and enhances the survivability
of the Rover, in the extreme environmental conditions and
unpredictable terrains present over the moon. The design
conserves the battery capacity for recovery from unfriendly
terrains or for an extended power requirement by a scientific
experimental payload. The design concept is verified in lab
conditions, by making a functional verification model.
Keywords —Electrical Power System; Battery Powered
Vehicles; Planetary Rovers; Solar cell arrays
I. INTRODUCTION
Many agencies globally are involved in the development
of lunar rover, for conducting various scientific experiments
on the lunar terrain[3][7]. The temperature swing over lunar
terrain is harsh, as it varies from around 70K during night at the
polar region[1] to 384K at the equator during the day time[2].
The data from Clementine Long-Wave Infrared (LWIR)[2] for
the lunar surface temperature with Sun at noon conditions and
at zero latitude is shown in Fig. 2(a)[2]. The estimated lunar
surface temperature at 90◦S latitude, with zero inclination is
shown in Fig. 2(b)[1].The harsh lunar environment, in terms
of extreme temperature variations are considered to be a
challenge in the survivability of the rover. A number of lunar
missions to land a rover, have ended up prematurely, due
to failures induced by the severe low temperature during the
lunar nights[7]. Most of the lunar rover designs incorporate,
radioisotope passive heating units (RHU) to maintain the tem-
perature during the nights. In a rover design, that does not
have the RHU, the extreme cold swing of the temperature
is particularly damaging to the batteries of the Rover, as the
freezing of electrolytes can cause permanent failure[16].
It is important for the overall survivability of the lunar
mission, which does not contain any RHU, that the design of
power system is more accommodative to the possible failure
scenarios of batteries, due to the lunar operating conditions[8].
Fig. 1. Number of days of sunlight per month during the year for different
lunar latitudes[6]
Power system design approach for the AMALIA Moon rover
has been reported in [4]. Also, energy management strategy
for a lunar rover is discussed in [5].
Reconfigurable systems can attain states with new capabil-
ities. This can enhance the survivability, by maintaining some
level of overall functionality by the way of reconfiguring. The
design should address majority of possible scenarios, which
can lead to a gradual or sudden power failure[9].
The scope of lunar rover’s power requirement differers
widely from the perspective of the overall mission objectives.
This paper presents an electrical power system design for a
small lunar rover of 25Kg class, and having a power budget
of 50Watts. The design utilizes well proven analog space grade
active and passive devices, while keeping the complexity and
weight to a minimum level.
The landing site preferred by many of the missions are
in the southern lunar hemisphere, a zone around 70◦to 90◦
south latitude, which receives a good solar illumination[2].
This zone has many planes those are suitable for conducting
a landing experiment. This lunar day profile for the possible
landing location for different latitudes for an year long period
is shown in Fig. 1[6].978-1-5090-4426-9/17/$31.00 c
2017 IEEE

(a)
Temperature (K)
Time(One Earth year)
60
70
80
90
100
110
120
130
140
150
160
(b)
Fig. 2. Lunar surface temperature (a)Clementine data for Sun at noon and at zero latitude[2] (b) Estimated surface temperature at 90◦S latitude, with zero
inclination[1]
II. OPERATI ONAL F EATUR ES OF POW ER SYS TE M
The power bus design caters to the following operational
features, which are important for improving the survivability
of the rover.
•Minimal drain on the battery during the launch and
transit phase of the mission. During this time rover is
kept in power off condition and assembled to the drop-
down platform of the lander. This is to ensure that,
the battery is holding sufficient state of charge (SoC),
during the initial phase to support the solar panel
deployment and roll down from the lander drop down
platform.
•Sleep and wake-up logics to address the initiation of
automatic power shut-down, at the on set of lunar
night and automatic power ON, whenever lunar day
begins. This makes sure that the entire lunar rover
hardware gets switched off at the onset of lunar night.
This improves the survivability of all the electronic
systems, as they are not qualified for an active state
during the extreme cold weather. Making them to pass
through the lunar night in a passive state, improves the
probability of survival. The wake-up logic ensures that
the activation of all rover circuitry happens only after
the rover temperature builds up to a conjunctive level
of 0◦Cor more.
•A feature to preserve the battery capacity before night
falls, as the extreme cold survivability of battery en-
hances with higher state of charge. This is achieved by
adding a command-able battery isolation, that makes
the power bus to be sourced only from the solar panels.
This is a planned activity, after which no movement
of the rover will be carried out till the wakeup on
next lunar day. This allows for continuation of on-
board experiments, till the rover falls to sleep after
the temperature fall below -4◦Cor after the sun set,
whichever happens first.
•Automatic disable of battery charging whenever the
temperature is ≤0◦C. This prevents possible damage
to the Li-ion battery, as charging at very low temper-
ature can lead to a short mode failure of the cell[16].
The entire solar generation during this time is used for
heating the battery as well as other rover electronic
systems.
III. POWE R SOUR CE S
There are many proposals of the power source options for
the planetary surface vehicles[10][11]. The power sources on-
board the proposed rover is fixed (deployable but not indepen-
dently sun tracking) solar panel, made of space grade triple
junction solar cells, and a 3Ah capacity battery made of [15]
low temperature Li-ion rechargeable cells [15]. The generation
capacity of the solar panel is around 50W, at normal incidence
of solar radiation.
The battery is crucial during the post landing operation
phase, where the rover rolls down from the lander craft after
touch down. Also the operations to deploy the solar panels are
done before the roll down. The batteries can be discharged
till the casing temperature drops to -20◦C, while charging
is done only above 0◦C. This is as per the manufacturer’s
recommendations[15].
The solar panel is deployed by issuing commands from the
ground control center, after the touch down of the lander craft,
and after waiting for the settling of the lunar dust. The solar
panel is oriented to receive maximum solar energy by orienting
the rover accordingly, while moving or otherwise. Solar cells
are placed on both sides of the panel, so that generation can
happen from either side, depending on sun position.
IV. POWER BU S CONFI GU RATIO N
The power bus is planned to be of 16.5V to 21V range,
as the maximum power conception is only around 40W. The
power bus design allows reconfigurability of the bus. The
simplified configuration is shown in Fig.3. The Bus isolation
Logic (BIL) is designed to configure the power bus in follow-
ing modes.
1) Battery tied bus (BIL selects path B): In this case,
battery voltage decides the bus voltage. Battery

gets charged with a current, decided by the in-
stantaneous generation(Igeneration ) and the load
requirement(Iload).
Icharge = (Igener ation −Iload).
2) Sun-lit regulated bus with battery support (BIL selects
path C) : Bus voltage is regulated to 21V during sun
presence, while dipping to battery voltage when sun
is absent. Note that the small size lunar rover can ex-
perience frequent sun absences due to local shadows.
Also battery supports the peak current demand by the
wheel drive motors for climbing up or for running
over obstructions.
3) Sun-lit regulated bus with battery charging(BIL se-
lects path D): This is similar to the previous mode,
with the exception that, battery gets charged with the
excess current (Igeneration −Iload )).
4) Sun-lit regulated bus with disconnected battery (BIL
selects path A): This is solar generation alone config-
uration, where battery is disconnected from the bus.
Advantage of this mode is that, configuring to this
state, just before the lunar night, can help in retaining
the battery capacity, which will enhance the chance
of survival of the battery during the lunar night. Also,
in case the battery does not survive the lunar night,
the mission can still be on track.
The overall power bus configuration is shown in the Fig.4,
and the battery isolation logic is shown in Fig.5. This scheme
provides the flexibility to configure the bus as in different
modes as discussed above. The mode selection for the power
bus can be done by ground command, and some initialization
of the modes are done at turn ON and turn OFF of the rover
automatically, as discussed in the following subsections. The
switches (P-channel MOSFETS) are made ON or OFF and the
four possible combinations results in selection of path A, B,C
or D for the BIL as discussed above.
A. Rover Power ON initialization
Q2is made ON, and Q1kept OFF. Battery can only do a
discharge of the demanded current. In effect, bus is configured
as sun-lit regulated bus with battery support. The switches Q1
and Q2are configured in this fashion, whenever the Rover
is powered ON (S1is closed). This prevents charging current
flow into the battery, immediately after the power ON of the
rover, during that time, temperature may be in a near zero
condition. At the same time battery provides the peak current
support for the wheel drive motors, if necessary, as the switch
Q2is in ON condition.
Initializing the bus to this mode, ensures that, the rover
turns ON, even if the battery has underwent a short mode fail-
ure during the just ended lunar night. If the battery is in a good
condition, no charging of battery takes place automatically, as
the rover battery may be still not in a thermally comfortable
zone to get charged. At the same time, battery discharge is
possible, which allows the rover, to drive to a more comfortable
location to get more sunlight or to do a payload operation.
B. Rover Power OFF initialization
The switches Q1, and Q2are made OFF on power shut
down. So that, the drain on the battery is minimal(≈50nA),
and ensures that the power-bus is not clamped down by a
failed/ faulty battery, during the wake up. This is crucial as,
batteries are prone to failures due to crystallization and seal
leak break during storage in extreme cold conditions[16].
C. Bus re-configuration
There is a command provision to make the switches Q1,
and Q2, ON or OFF separately from the ground, and this
allows the reconfigurability of the power bus. This is shown
in Table I.
TABLE I. DI FFE REN T CON FIG UR AB LE S TATES O F TH E BUS
CO NFI GUR ATI ON
States Q1Q2Functionality Remark
1 ON ON Battery Can get charged
& Discharged
Battery Tied Bus Config-
uration
2 OFF ON Battery can only Dis-
charge
Sun-lit Regulated Bus
configuration,
This configuration is ini-
tialized after Rover is
made ON
3 ON OFF Battery can only get
charged
LTP/UTP control prevents
over charge
from the excess genera-
tion available
4 OFF OFF Battery is fully isolated Preserves the Battery Ca-
pacity,
from the Power Bus When rover goes OFF,
this state is initiated
D. LTP/UTP based Sunlit bus regulation
The control of shunt switches for the bus regulation during
sunlit time utilizes the differential comparator based LTP/UTP
control[13]. The LTP/UTP control range for the bus voltage
rgulation is set between 20.5V and 21V. The value of the
bus-capacitor is chosen to limit the frequency of shunt switch
operation to a value less than 2KHz[12].
E. Pre-wake-up heating & Regulator
The pre-regulator, which is active only when the rover
temperature is less than 0◦C, is a shunt linear regulator, which
regulates the bus to 10V. The dissipated heat is used for
rising the rover temperature to a level above 0◦C(average
temperature inside the rover equipment bay). This also acts as
an initial supply to all the circuitry that need to work before
the wake-up function, like temperature logic. The control range
of the pre-regulator is shifted to 22V, after the above 0◦C
condition is achieved. This voltage level is more than the
LTP/UTP control range of 20.5V to 21V. Hence it acts like
a redundant controller, which can regulate the bus in case of
a failure of the LTP/UTP control or open mode failure of the
shunt switches.
F. Wake-up logic
The block schematic of the proposed rover power system
configuration is shown in Fig.3. The wake-up of the rover
happens, with closure of the switch S1. The activation of wake-
up is based on satisfying the following conditions.
•Rover temperature ≥0◦C. All of the generated solar
array output, before wake-up, is used for heating of the
equipment bay of the Rover. This, together with direct

Pre-Regulator,
Thermal & sleep/
wake-up logic,
Bus configuration-
Initialization
LTP/UTP
CONTROL
LTP/UTP
CONTROL
Solar Array
current sense
resistor
Bus Capacitance
Battery
Battery Current
sense resistor
S1
Solar array string-1
Solar array string-2
Rover
subsystem
Loads
POWER BUS
BIL
A
BC
D
tele-command
to re-configure
Rover ON/OFF
relay
Fig. 3. Proposed Simplified Configuration diagram of the Power Bus formation for the Lunar Rover
sun load helps in building up the rover temperature
after an extreme cold lunar night.
•The solar panel generation can support a load of 1A at
18V or the battery can provide a current of 1A at 18V.
Before the wake-up, health of the battery(the battery
is connected in discharge mode to the bus during this
time) or the generation from solar panel is verified
by connecting the bus to a heater load of ≈1A, for
around 20ms. If bus voltage is maintained above 18V,
with the 1A load, wake-up is carried out. This check
ensures that, after the closure of S1, bus can support
the minimum bus load of 1A.
G. Wake-up Enable or Disable
During the launch/ transit phase, any accidental wake up
of the rover can go unnoticed, and can result in draining of
the entire battery capacity before the rover is on lunar surface.
This can lead to a mission failure, as battery capacity in the
initial phase of the rover landing is very important. To prevent
this, a wake-up disable or enable feature is provided. This can
be initiated only by a wired command, either from the ground
test equipment or from the Lander craft. This is shown in Fig.4.
H. Sleep Logic
the sleep logic is an automatic initialization of a power
shut-down by opening S1in Fig.4. The activation of this is
based on the Bus voltage becoming lower than 16.5V. There is
a provision to initiate the sleep function by tele-command (TC),
provided the lunar night has set in. This command is labeled as
Forced sleep command in Fig.4. This command remains non
functional during the sunlight period. Fig.6 shows the flow
diagram for the sleep and wake up logic operations of the
rover.
V. HA RD WARE IMPLEMENTATION
The functional verification model (FVM) of the rover is
realized and tested. The realization utilized generic space
grade components like LM111 comparator, OP07 operational
amplifier etc. and other passive components. Also, instead of
bipolar transistors, MOSFETs are used in all circuits because
of their better low temperature behavior[14]. The circuit im-
plementation of Battery isolation logic (BIL) is shown in Fig.5.
The configuration of P-channel power MOSFETs Q1and Q2,
realizes the functionality of configuring the power bus. The
two tele-command provisions, “charge mode ON/OFF”, and
“discharge mode ON/OFF”, can configure the battery to get
charged or discharged independently. Charging is enabled with
Q1, while discharging with Q2.
The default value of “discharge mode enable” is zero, and
the default position of S2is ON. Hence, turn ON of the the
switch Q2, happens with, turn ON of the power bus. With rover
OFF, the battery discharge mode gets disabled automatically.
With rover ON, discharge mode gets enabled, as the 5V,
reference voltage is generated after the switch S1.
Discharge mode gets enabled, whenever Rover ON com-
mand is issued from ground checkout (GC) or Lander
Craft (LC), for the command duration of 150ms, prior to the
closure of S1. This helps in verifying the battery health, as
mentioned in section IV(F). A similar check happens during
the automatic wake-up of the Rover, with battery getting
connected in discharge mode for ≈20ms, from the thermal
logic. The battery health check or the solar generation check
happens, as the 18Ωheater is connected to the Normally
Closed (NC) contact of the Relay switch S1. A voltage of
18V, developed across this heater, initiates the rover wake-
up by activating the relay switch S1. Now the 18Ωheater gets
disconnected and Power Bus gets activated.
VI. CONCLUSION
A configurable 50W power bus design for a lunar rover
is presented . The design improves the survivability of the
rover considering the harsh environmental conditions of lunar
surface. The proposed configuration guaranties the operation
of the rover, in case of a short mode failure of the battery,

Battery
isolation
Logic
Battery
Voltage
sense
Shunt
Regulator
Thermal
Logic
Wake-up
and
Sleep
circuit
LTP/UTP
control Mono
Shot
Post ON/OFF
initialization
LTP/UTP
control
BACS
SACS
HEATER-1
OFFON
GC/LC
ROVER ON
GC/LC
ROVER OFF
BUS CAPACITOR
Disable
Enable
BUS
Discharge
Mode ON/OFF(TC)
Charge
Mode ON/OFF(TC)
Forced
Sleep(TC)
Ena/Dis STR-2(TC)
HEATER-2
S1
solar array string-1solar array string-2
GC: Ground Checkout
LC: Lander Craft
SACS: Solar Array Current Sensor
BACS: Battery Current Sensor
Fig. 4. Proposed Block diagram of the Lunar Rover Power System
referance
voltage
Batttery
Discharge
Mode ON/OFF(TC)
ON OFF
Charge
Mode ON/OFF(TC)
OFF ON
S1POWER BUS
FROM
GC/LC
ON
FROM
THERMAL
LOGIC
Q1
Q2
from Solar Strings
S2
LOW, S2 ON &
HIGH, S2 OFF
HEATER
To S1
coil
supply
Fig. 5. Schematic diagram of Battery Isolation Logic

sun
presence?
Rover
Temperture
>0 oC?
Bus Voltage
> 18V ?
Wake-up
logic
Enabled?
Turn ON
Rover
No
Yes
Yes
Yes
Heater ON
No
No
No
Rover OFF
command
from Lander/
GC
Bat. Volt
<16.5V?
Rover
Temperature
<-4 oC?
Sun
Presence
Activate
Rover
Heating
Turn OFF
Rover Received
Forced Sleep
Command?
Yes Yes
Yes
Yes
No No
No
No
AFTER LUNAR NIGHT
Rover ON
command
from Lander/
GC
Yes
LUNAR NIGHT
Enable
Wake-up
Logic
Disable
Wake-up
Logic
AFTER LUNAR DAY
LUNAR DAY
Fig. 6. Logic flow diagram of sleep & wake-up function
while providing the most suitable battery tied power bus for
normal operation of the rover. The design complexity is kept
minimum and operationally address many of the possible
failure scenarios imposed by lunar environment and unknown
terrain. The built in reconfigurability feature of the power bus,
helps in improving the effective operational life of the rover.
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