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ACTA IMEKO
February 2015, Volume 4, Number 1, 35 – 43
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ACTA IMEKO | www.imeko.org February 2015 | Volume 4 | Number 1 | 35

Power system of the Guanay II AUV

Ivan Masmitjà, Julián González, Gerard Masmitjà, Spartacus Gomáriz, Joaquín del‐Río‐Fernández

SARTI Reserch Group, Electronics Dept., Universitat Politècnica de Catalunya (UPC), Rambla Exposició 24, 08800, Vilanova i la Geltrú.
Barcelona. Spain +(34) 938 967 200. www.cdsarti.org

Section: RESEARCH PAPER

Keywords: State of charge; batteries; AUV; Wireless connection; Ni‐Cd

Citation: Ivan Masmitjà, Julián González, Gerard Masmitjà, Spartacus Gomáriz, Joaquín del‐Río‐Fernández, Power system of the Guanay II AUV, Acta IMEKO,
vol. 4, no. 1, article 7, February 2015, identifier: IMEKO‐ACTA‐04 (2015)‐01‐07

Editor: Paolo Carbone, University of Perugia

Received December 12
th
, 2013; In final form November 8

th
, 2014; Published February 2015

Copyright: © 2014 IMEKO. This is an open‐access article distributed under the terms of the Creative Commons Attribution 3.0 License, which permits
unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited

Funding: This work was supported by the Spanish ministry of economy and competitiveness under the research project: “Sistemas inalambricos para la
extension de observatories submarinos” (CTM2010‐15459)

Corresponding author: Ivan Masmitjà, e‐mail: ivan.masmitja@upc.edu

1. INTRODUCTION

The study of the sea and oceans has become increasingly
important among fishers and biologists especially because
marine species change their behavior depending on
environmental variables such as temperature, salinity and
ph, among others [1]. For these oceanographic observations
different tools are used. Aerospace technologies can focus
on the study of the oceans at a global level but are unable to
carry out detailed observations in a specific sector or depth.
Oceanographic ships, on the other hand, can solve this
problem but the actual planning and mission deployment
required to obtain a satisfactory special-time resolution data
are very expensive. As a solution to these needs, these tools
are migrating towards the creation of autonomous marine
vehicles.

The autonomous underwater vehicle (AUV) Guanay II
[2] [4] is a vehicle developed by the SARTI group of the
Technical University of Catalonia (with the co-financing of
IMEDEA-CSIC) with the objective to provide a platform

for measuring oceanographic variables, such as temperature
and salinity of the water column, with a high
simultaneously spatial and temporal resolution. Figure 1
shows the vehicle. The knowledge of the charge status of
the batteries in an autonomous vehicle is an important
factor to ensure security of the vehicle and mission. This
work presents a measurement system and energy

Figure 1. Autonomous Underwater Vehicle Guanay II AUV.

ABSTRACT
Guanay II is an autonomous underwater vehicle (AUV) designed to perform measurements in a water column. In this paper the
aspects of the vehicle’s power system are presented with particular focus on the power elements and the state of charge of the
batteries. The system performs both measurement and monitoring tasks and also controls the state of charge (SoC) of the batteries. It
allows simultaneous charging of all batteries from outside the vehicle and has a wireless connection/disconnection mode. Guanay II
uses a NiCd battery and for this reason the current integration as a SoC methodology has been selected. Moreover, it has been
validated that it is possible to obtain instant consumption from the SoC circuit. Finally, laboratory and vehicle navigation tests have
been performed to validate the correct operation of the systems and the reliability of the measured data.

ACTA IMEKO | www.imeko.org February 2015 | Volume 4 | Number 1 | 36

management for the Guanay II vehicle. Initially different
solutions for electrically measuring the batteries charge
state are analyzed, and the methodology of current
integration is chosen [5] due to the nature of the batteries
used in the Guanay II. Subsequently, the construction of a
prototype and the experimental tests performed both in
laboratory and field missions in order to validate the
correct operation of the device is presented. Finally, this
device is complemented with a design of a battery charger
system, accessible from outside the vehicle, which allows
simultaneous charging of all battery packs, and also a
wireless connection-disconnection of the battery.

2. POWER SYSTEM OF GUANAY II AUV

The Guanay II’s power system can be divided into three
parts: propulsion system; communication and control
system; and battery system. Below, each of these elements
are briefly described.

The propulsion systems consist of a series of propellers
for longitudinal and directional movement and a motor-
piston set to modify the buoyancy of the vehicle to
perform dives. The principal characteristics of the main
engine, for longitudinal movement, are: 24 V of direct
current (dc) supply; 300W of output power; and can be
controlled via RS232 communication. Two engines are
located on the rear side of the vehicle to control its
direction. The engines have 24 V of power supply,
maximum power output of 110 W each and are controlled
by an RS232 communication port. Finally, the piston-
engine set comprises a motor of 24 Vdc and a piston that
can move up to 1.5 litres. This piston can either take in or
eject seawater. With this action the buoyancy of the vehicle
can change and therefore the dives can be controlled. These
systems consume most of the Guanay II AUV power.

The second part of Guanay II vehicle is the control and
communication system. The main control system of the
vehicle is an embedded computer located inside. This
computer is responsible for controlling and managing the
various elements of the vehicle (sensors, actuators, motors,
communications, etc.) Also, by using a radio link and a
WiFi network the vehicle can be remotely controlled from
a base station.

Finally, an energy storage system has been mentioned.
This vehicle has autonomy of around 4 hours thanks to the
nickel-cadmium (Ni-Cd) battery pack of 24 V and 21 Ah.
This pack consists of a subset of 12 Vdc and 7 Ah batteries
arranged in two series and three in parallel configuration
(2s3p). With these batteries and the average consumption of
electronic components of the vehicle, the autonomy under
normal conditions can be established.

Figure 2 shows the development of the vehicle in 3D
where the three parts of the power system can be seen (1,
Communication and control system. 2, Propulsion system.
3, Battery system).

Finally, in Table 1 the values of the theoretical
consumption of various devices of the vehicle can be seen.
These values are specified in the technical specifications of
the devices.

As seen in table 1, the engines consume most (90%) of
the vehicle’s energy. The other electronic systems consume
the remaining 10%. Given these data and the total capacity
of the batteries, the autonomy of the vehicle can be known.
If the engines run at full power, a maximum of less than 1.5
h has been obtained. However, various factors have to be
taken into account. First, the side engines correct the
direction of the vehicle, but are never on for long time.
With regard to the main engine, a compromise between
speed and autonomy of the vehicle is necessary. Finally, the
engine piston set is only activated to make the dives. In
experimental results, it has been observed that the vehicle
has a range of about 4 hours under normal conditions.
3. MEASUREMENT SYSTEM FOR BATTERY STATE OF

CHARGE

The state of charge (SoC) is mathematically defined as in
the following equation (1).

nom

nom AhdttIAhtSoC /))(()(
0
 (1)

Table 1. Theoretical consumptions of the Guanay II.

Maximum values

V A W

GPS 5 240 m 2
PC104 5 0.9 9.9

Compass 5 <20 m 0.1
Radio‐modem 12 1.5 18
Engine Driver 5 31 m 0.15
Main Thruster 24 16 300
Lateral Thruster 24 4.25 110
Piston‐Engine 24 3 50

TOTAL 25.94 490.15

Figure 2. This figure shows the development of the vehicle in 3D with the
three parts of the power system (1, Communication and control system. 2,
Propulsion system. 3, Battery system).

ACTA IMEKO | www.imeko.org February 2015 | Volume 4 | Number 1 | 37

where I(t) is the current extracted from the battery (which
is assumed positive while discharging the battery), and
Ahnom is the nominal battery capacity. Equation (1) assumes
that the current integration starts at SoC(t) = 100% at t =
0.

Several methods of estimating the SoC of a battery have
been used. Some are specific to particular cell chemistries.
This work is subject to the use of Ni-Cd batteries used in
the Guanay II.

3.1. Methods for SoC estimation

For such batteries there are different methods to
measure the charge, such as the measurement of voltage,
impedance and current integration.

a. Voltage based SoC estimation uses the voltage of

the battery cell as the basis for calculating the
remaining capacity [6]. Results can vary widely
depending on discharge rate and temperature and
compensation for these factors must be provided
to achieve reasonable accuracy. Figure 3 shows the
relationship between the voltage and the capacity
at different discharge rates.

b. Internal impedance measurements can also be
used to determine the SoC. However these are not
widely used because of difficulties in measuring the
impedance while the cells are active as well as
difficulties in interpreting the data and very
complex calculations are required [7] and [8].

c. Current based SoC estimation, known as
coulomb counting [5] [11]. In coulometric
measurements however, the amount of capacity
taken out or put into a battery is measured in
terms of ampere-hour or in %. The coulomb
counting approach basically implements equation
(1) to evaluate the SoC. It uses a more general
definition, defined as can be seen in (2).


t

m
nom

dttI
Ah

SoCtSoC
0

)(
1

)0()( (2)

where SoC(0) is the starting value of SoC and Im(t)
is the measured current.

Charge accumulation techniques where SoC is
determined by monitoring battery charge and
discharge current are impractical in the long term
due to accumulation of errors. For this reason this
technique makes it impractical to be used by itself.
A monitoring technique combining the open-
circuit voltage under no-load condition, and
coulometric measurements under constant load has
been implemented in [13] on a microcomputer-
based circuit. On the other hand, correction
factors are required for different discharge rates
and ambient temperatures [12].

3.2. Gas Gauge IC for power‐assist applications

Current based SoC estimation, known as coulomb
counting [5], which is currently selected, calculates the state
of charge by measuring the instantaneous current of the
battery and integrating in time both in the process of
charging and discharging. The reason for using this method
is because it obtains a good correlation of the measurement
data and also because it can be implemented quickly
because of the already existing specific integrated circuit
(IC). This allows us to rapidly develop a prototype that can
be used to estimate the remaining charge left in the battery
in Guanay II and to perform some field tests. In this
implementation the bq2013 IC (a Gas Gauge IC for Power-
Assist Applications) from Texas Instruments Company,
specifically designed for Ni-Cd batteries [9] has been used.

Figure 4 shows a block diagram of the SoC estimator
where the batteries, motors (load) and charger can be
observed. The bq2013 constantly monitors the current
flowing through the batteries and also measures the voltage
and the internal temperature to compensate for various
factors. Finally, the communication to the SoC estimator is
through a serial communication, allowing the user to read
and write some parameters of the configuration and read
the value of the charge of the batteries with the micro-
controller.

Figure 4. Block diagram of the SoC Estimator.

Figure 3. Relationship between the voltage and the capacity at different
discharge rate. Source: Saft batteries [10].

ACTA IMEKO | www.imeko.org February 2015 | Volume 4 | Number 1 | 38

3.3. Prototype

The prototype developed is shown in figure 5, where
the communication ports with the PC-104 and LEDs for
visual indication can be seen. This IC includes a simple
single-pin serial data interface with command-based
protocol (HDQ plus return). For this reason a RS-232 to
HDQ interface has been developed to communicate with
both gas gauge and PC-104 systems [9].

3.4. The SoC measurement method

As explained in section 3.1 there are some drawbacks in
using a simple coulomb counting SoC. These drawbacks
include the noise, temperature effects, discharge rates and
how to estimate the initial SoC value. The algorithm used
should include these effects to get a reliable reading of the
SoC. The bq2013 implements compensations to minimize
these drawbacks. The operational overview diagram in
figure 6 illustrates the operation algorithm of the IC.

As can be seen in figure 6, the IC compensates charge
current for charge rate and temperature. Discharge current
is load compensated and allows automatically adjustment

for self-discharge. The main counter, nominal available
capacity (NAC), represents the available battery capacity at
any given time. Battery charging increments the NAC
register, while battery discharging and self-discharge
decrease the NAC register and increment the DCR
(discharge count register).

The DCR and LMD (last measured discharge) registers
are used to update and adjust the initial SoC value.

Texas Instruments provides equation (3) to obtain the
value of the internal register NAC (in mVh) of the bq2013
IC, which monitors the voltage drop across a resistor
connected in series between the battery and ground.

ScaletorSenseresis

mAhacityBatteryCapmVhNAC





)(

)()(
(3)

where Senseresistor = 0.01 Ω and Scale = 640 (predefined).
We can see the relation between the NAC register and the
SoC in mAh.

As seen in (3) the NAC value depends on the capacity of
the battery and is determined by charging and discharging.
Therefore, the consumption can be calculated from
expressions (4) and (3):

)(

)(
)(

AhacityBatteryCap
AnConsumptio




 . (4)

Finally, by correcting scale factors, we can obtain the

instant consumption using equation (5):

01.01000640

3600

)(

)(
)(








st

mVhNAC
AnConsumptio , (5)

where 3600 is the time factor correction; 640 is scale factor
determined by the IC; 1000 is the ampere scale factor
correction; and 0.01 is the sense resistor in Ω.

Figure 6. Scheme of charging system and battery connection.

Figure 5 . State of charge measuring prototype. A) Top view B) Bottom view.

ACTA IMEKO | www.imeko.org February 2015 | Volume 4 | Number 1 | 39

4. CHARGING SYSTEM AND BATTERY CONNECTION

In this section the implementation of charging system
and battery connection are described. These two systems
can improve the operability of the vehicle and increase
safety and reliability. Figure 7 shows a schematic model of
this system.

It shows the three battery packs connected in parallel.
The switch (SW) between batteries and the AUV
electronics is radio-controlled and allows the user to turn
on and off the vehicle. Finally, an external charger can be
connected using a two pin connector to charge the
batteries.

4.1. Charging system

The energy of the Guanay II comes from the 2s3p
battery packs that can provide 24 Vdc and 21 Ah.
However, a problem arises with this configuration when
the batteries have to be charged. Many manufacturers do
not recommend charging two battery packs in parallel
because this can cause damage especially at high charge
rates. Until now, types of chargers that can charge a single
pack only were used, but the vehicle must be opened every
time. In order to maximize the mission time a system was
designed for simultaneous parallel charging of all battery
packs through a single external connector. The location of
this single point for charging means the it is not necessary
to disassemble the vehicle’s mechanical and electronic
systems and so the charging operation is faster. Figure 8
shows the connector used for charging the batteries. This
connector is located in one of the covers of the sealed
cylinder. It is manufactured by Subconn Company.
Specifically it is a circular 4-pin capable of supporting up to
600 V and 10 A at pressures up to 1400 bars.

After discussions with the manufacturer, it has been
decided to use a low rate constant voltage charge applied to
all the 2s3p batteries to prevent damage. A constant charge
around 0.2 C is used. By keeping the charge current low
enough so that the battery does not generate any heat, this
method performs charging without using any control. The
calculation formula for semi-constant-voltage charge system
is as follows (6):

 CH C N kV V   , (6)

where: VCH is the output voltage of the DC power supply
for charge; Vc is the single-cell battery voltage (1.45 V/cell:
average battery voltage during charge at 20º C, 0.1 C); N is
the number of cells used; (k) is the stabilizing constant and
must be selected in accordance with the purpose of the
device in which the battery pack is used.

The value of the above-mentioned stabilizing constant
(k) must be selected carefully and after discussions with the
manufacturer, using a value of 1.25 has been decided upon.
Using equation (3) a charging voltage of 29 V has been
obtained. Moreover, the current has been limited to 5 A to
prevent any overcurrent.

Charge rate is often denoted as C or C-rate and signifies
a charge or discharge rate equal to the capacity of a battery
in one hour. Equation (7) shows the relation between the
capacity of battery in Ah and the current in A.

( ) (1/ ) ( )C apacityofBattery Ah C h C urrent A´ = (7)

4.2. Battery connection

In order to have control and easy access to the battery
connection/disconnection to electronics and propulsion of
the vehicle, a wireless device that acts as a switch has been
incorporated. Figure 7 shows a schematic model of this
system.

The user can turn off/on the switch of the vehicle using
the remote controller. This operation enables the
connection or disconnection of the batteries while the
AUV is in the water, obtaining greater security in the
vehicle. A HIR6-433 RF AM 433 MHz receiver/decoder
from Rfsolutions has been used for this purpose. These
types of modules provide a very low power receiver,
combined with a flash programmable controller, supplied
pre-programmed to operate with the Keeloq Transmitter
encoder. The range from such a system can be up to 50
meters Line of Sight (LOS).

Finally, a new antenna with epoxy resin has been
designed to allocate the HIR6-433 and provides the
necessary protection in the water and in the depth. Besides
hosting this module, the antenna is also used to install a
new WiFi communication module and GPS antenna.

5. EXPERIMENTAL RESULTS

Initially both systems, the connection/disconnection of
the battery and its charging and SoC estimation have been
tested in the laboratory, where the proper functioning of all
the systems designed was verified.

Figure 7. Scheme of charging system and battery connection.

Figure 8. Subconn connector for charging in one of the covers of the sealed
cylinder of Guanay II AUV.

ACTA IMEKO | www.imeko.org February 2015 | Volume 4 | Number 1 | 40

Figure 9. shows the two laboratory tests. In 1a charge testing of all packs of batteries in parallel can be seen. In 2b discharge testing and a comparison
between sensed current from the multimeter and sensed current from the bq2013 can be observed.
1.A) Block scheme of the battery charge test. Three battery packs, three digital multimeters, one voltage source and one laptop to monitor the variables. All
of them are connected through a GPIB port.
1.B) Shows the test bench where the battery charging has been tested.
1.C) The current of the three packs (red, green and purple) of batteries during a low rate charging in parallel.
2.A) Block scheme of the battery discharge test. Five resistors of 18 ohms in parallel as a load have been used. The three measurements (from the ammeter,
from the bq2013 and theoretical) have been compared using a laptop.
2.B) Test bench that has been used to check and adjust the SoC sensor of the battery. For this laboratory experiment a bank of five loads in parallel has been
used. Each load of 18 ohms introduces a 433 mA of consumption current.
2.C) Vehicle current measurement error using the SoC circuit (X‐axis is the total current of the vehicle).

(1.A) (2.A)

(1.B) (2.B)

(2.C) (1.C)

ACTA IMEKO | www.imeko.org February 2015 | Volume 4 | Number 1 | 41

One of the most critical parts is the parallel charging of
battery packs, in particular the individual currents of each
pack. Laboratory tests have been conducted to validate the
correct performance of the charging. In this test a power
supply and three digital multimeters to monitor individual
currents of each pack during charging has been used (see
figure 9 point 1.A, Block scheme). Figure 9 in point 1.B
shows the test bench where the battery charge has been
tested. For this purpose, three ammeters and one power
supply connected to a computer by GPIB have been used.
This computer is responsible for controlling and saving
instrument data.
Figure 9 in point 1.C shows the current of the three packs
of batteries during charging in parallel mode. In this picture
it can be seen that after 10 hours of charge the current
drops to 0.15 A and the battery is practically charged. Also,
the difference of the current charge in one of the packs can
be observed. This is one reason why the high rate current
charge is not recommended in order to prevent damages. In
figure 9 point 1.C the rate of current used to charge the
batteries can also be seen. According to the equation (7) a
maximum current rate of 0.17 C is obtained during the first
hour of charge, from which this rate decreases to less than
0.05 C.

On the other hand, as seen in equation (5), the current
consumption of the vehicle can be calculated using the SoC

circuit. A test and a calibration have been conducted to
validate the system, using a digital multimeter and five 18
ohms resistors as a load (see figure 9 point 2.A, Block
scheme). Figure 9 in point 2.B shows the test bench that has
been used to check and adjust the SoC sensor of the
battery. For this laboratory experiment a bank of five loads
in parallel has been used. Each load of 18 ohms introduces
433 mA of consumption current.

Figure 9 point 2.C shows the vehicle’s current
measurement error using the SoC circuit bq2013. The error
varies according to the total current of the vehicle with less
than ±50 mA. Therefore, using the NAC register a
sufficiently accurate method to estimate the power
consumption from Guanay II has been obtained.

Afterwards, these systems have been validated in various
vehicle navigation tests. For example, one of these tests was
performed at the Canal Olímpic de Catalunya in
Castelldefels (The Olympic channel located in Castelldefels)
as shown in figure 10. Figure 11 shows different trajectories
performed by the vehicle during the field test.

Figure 16 shows the instantaneous battery consumption
due to the action of the propulsion engine and two
direction motors of the vehicle during a path on the water.
The first three graphs show the action of propellers in %

Figure 11. Different trajectories performed by the vehicle during the field
test.

Figure 12. Percentage of power to the engine requested vs. current drawn.
(Propulsion propeller).

Figure 10. Guanay II AUV in the field test.

Figure 13. Percentage of power to the engine requested vs. current drawn.
(Direction propeller).

ACTA IMEKO | www.imeko.org February 2015 | Volume 4 | Number 1 | 42

(up to one). In blue the action of the main propeller is
shown and in red and green the action of side propellers is
shown. The fourth graph shows the instantaneous battery
consumption (black line) and the mean of this (dashed line).

One can observe the relationship between consumption
and the action of the engines. Also, the average
consumption from the remaining electronic elements
(PC104, radio modem, GPS, etc.) can be seen. This value
may be contrasted with the consumption values of table 1.
The difference may come due to the time of use of the radio
modem or PC104. In any case, this can be a field to
investigate in future work.

From the experimental results shown in figure 14, the
relationship between the percentage of power to the engine
requested and the current drawn has been made. Figure 13
shows this relationship for the left direction propeller and
figure 12 for the propulsion propeller.

The results show that, at low power, the engine will not
draw power and therefore will not work. This is because a
minimum amount of power is needed to run the motors.
For the direction propellers more than 50% of power has
been required and for the propulsion propeller more than
20%. However, more field test points are needed to obtain a
more accurate relationship.

These tests also confirmed that disconnecting the
batteries when the vehicle is still in the water provides
security at its landing ground.

6. CONCLUSIONS

A system for measurement and for energy management
for an underwater vehicle have been designed. The system
monitors the state of charge of the batteries and the
instantaneous consumption of the thrusters, information
that the control mission needs to optimize and facilitate
navigation. A single external access connector in the vehicle
allows all batteries to be charged and a radio frequency
control switch is used to connect and disconnect the
vehicle’s power supply even in water. The navigation tests
performed confirm that the system operates correctly and
the data measured is reliable.

ACKNOWLEDGEMENT

This work was supported by the Spanish Ministry of
Economy and Competitiveness under the research project:
“Sistemas Inalambricos para la Extension de Observatorios
Submarinos” (CTM2010-15459).

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Figure 14. Consumption of the vehicle versus power of the propellers.

ACTA IMEKO | www.imeko.org February 2015 | Volume 4 | Number 1 | 43

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