University of South Carolina
Faculty PublicationsElectrical Engineering, Department of
A Compact Digitally Controlled Fuel Cell/Battery
Hybrid Power Source
University of Miami, email@example.com
Roger A. Dougal
University of South Carolina - Columbia, firstname.lastname@example.org
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Published inIEEE Transactions on Industrial Electronics, Volume 53, 2006, pages 1094-1104.
© 2006 by IEEE
1094 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 53, NO. 4, AUGUST 2006
A Compact Digitally Controlled Fuel Cell/Battery
Hybrid Power Source
Zhenhua Jiang, Member, IEEE, and Roger A. Dougal, Senior Member, IEEE
Abstract—A compact digitally controlled fuel cell/battery hy-
brid power source is presented in this paper. The hybrid power
source composed of fuel cells and batteries provides a much
higher peak power than each component alone while preserving
high energy density, which is important and desirable for many
modern electronic devices, through an appropriately controlled
dc/dc power converter that handles the power flow shared by the
fuel cell and the battery. Rather than being controlled to serve
only as a voltage or current regulator, the power converter is
regulated to balance the power flow to satisfy the load require-
ments while ensuring the various limitations of electrochemical
components such as battery overcharge, fuel cell current limit
(FCCL), etc. Digital technology is applied in the control of power
programmability, less susceptibility to environmental variations,
and low parts count. The user can set the FCCL, battery current
limit, and battery voltage limit in the digital controller. A control
algorithm that is suitable for regulating the multiple variables in
the hybrid system is described by using a state-machine-based
model; the issues about embedded control implementation are
addressed; and the large-signal behavior of the hybrid system is
analyzed on a voltage–current plane. The hybrid power source is
then tested through simulation and validated on real hardware.
This paper also discusses some important issues of the hybrid
power source, such as operation under complex load profiles,
power enhancement, and optimization of the hybrid system. The
design presented here can not only be scaled to larger or smaller
power capacities for a variety of applications but also be used for
many other hybrid power sources.
Index Terms—Battery, digital control, fuel cell, hybrid power
source, microcontroller, power capacity.
ics, hybrid electric vehicles, remote communication facilities,
remote-ground support stations, etc. –. However, many
applications have a common characteristic in their load profiles,
that is, they have a relatively low average power demand but
a relatively high pulse power requirement. The typical pulse
duration in these applications generally ranges from hundreds
of milliseconds to minutes, with power levels depending on
UEL CELLS have shown promising potential for several
areas of applications such as those in portable electron-
Manuscript received December 24, 2004; revised May 4, 2005. Abstract
published on the Internet May 18, 2006. This work was supported in part by
the U.S. Marine Corps under Conrtract N00014-03-1-0952 and in part by the
U.S. Office of Naval Research under Contract N00014-02-1-0623.
Z. Jiang was with the Department of Electrical Engineering, University of
South Carolina, Columbia, SC 29208 USA. He is now with the Department of
Electrical Engineering, University of New Orleans, New Orleans, LA 70148
USA (e-mail: email@example.com).
R. A. Dougal is with the Department of Electrical Engineering, University
of South Carolina, Columbia, SC 29208 USA.
Digital Object Identifier 10.1109/TIE.2006.878324
battery showing that the output power capacity in the active hybrid is higher
than that in the passive hybrid.
the applications. Fuel cell/battery hybrid power sources can
and efficiency than the fuel cell alone while still preserving
high energy density –. The simplest hybrid configura-
tion, for example, passive hybrid, results from connecting both
the fuel cell and the battery directly to the power bus .
However, this passive hybrid allows less flexibility in system
design compared to the active hybrid that will be discussed
below, because the nominal voltages of the fuel cell stack and
the battery in the passive hybrid must be similar in order to not
overcharge the battery, yet similar voltages then determine in a
rather fixed way the amount of power that can be supplied from
the fuel cell to the battery or to the load, as illustrated in Fig. 1.
As an alternative to the passive hybrid, a dc/dc power con-
verter can be placed between the fuel cell and the battery
so that they may have different voltage levels –. As
shown in Fig. 1, the active hybrid can greatly augment the
peak output power while not increasing the system weight
and volume a lot, as will be discussed later. Active hybrid
fuel cell power sources require a much more complex control
scheme that must ensure efficient and robust power transfer
from sources without risks of their rapidly degraded reliability
due to prolonged overcurrent and/or undervoltage conditions.
fuel cell/battery sources must provide an uninterrupted power
flow to the load. Therefore, rather than achieving a single
voltage or current regulation goal at the output, the control
system must regulate the power converter to balance the power
flow of both sources so as to satisfy the load requirements while
ensuring the various limitations of electrochemical components
such as battery overcharge, fuel cell current limit (FCCL), etc.
Previous power controllers for hybrid power sources mostly
employed complicated analog circuits . Although analog
Illustration of voltage–current (V –I) curves of the fuel cell and the
0278-0046/$20.00 © 2006 IEEE
JIANG AND DOUGAL: A COMPACT DIGITALLY CONTROLLED FUEL CELL/BATTERY HYBRID POWER SOURCE 1103
COMPARISON OF THREE POWER SOURCES
during the period of interest, assuming that the final charge
of the battery is at the same level as the initial charge) was
higher than 92%, which makes the hybrid power source really
attractive. The size was optimized according to power and
voltage requirements. The objective was to achieve a power
source with four times fuel cell power capacity and compatible
voltage output. The fuel cell voltage was 17.3 V when in a
maximum power output, so four lithium ion cells (with an
average terminal voltage of 16 V) were connected in series
to achieve a higher efficiency of the power converter since
the battery voltage was compatible with the fuel cell voltage
and the duty cycle was around 85%. When the load drew a
peak power of 140 W, the battery needed to provide 105 W
of power. Two strings of series-connected lithium ion cells
were chosen to achieve such peak power, taking into account
the safe discharging current of the lithium ion cell (4 A for
It is worthwhile to note that the size of each component in
the hybrid power source such as fuel cell, battery, and power
converter could be optimized in a global sense to achieve
maximum system efficiency, which is beyond the scope of
This paper has presented a compact digitally controlled
fuel cell/battery hybrid power source. Such a hybrid power
source provides much higher peak power than each component
alone while preserving high energy density, which is important
and necessary to many modern electronic devices, through
an appropriately controlled dc/dc power converter that han-
dles the power flow shared by the fuel cell and the battery.
Rather than being controlled to serve only as a voltage or
current regulator, the power converter is regulated to balance
the power flow to satisfy the load requirements while en-
suring the various limitations of electrochemical components
such as battery overcharge, FCCL, etc. Digital technology is
applied in the control of the power converter due to many
advantages over analog technology such as programmability,
less susceptibility to environmental variations, and fewer part
counts. The digital power controller circuit primarily consists
of a synchronous buck converter that is controlled by a PIC
microcontroller, with features of small size and lightweight.
The user can set FCCL, BCL, and BVL in the digital con-
troller. A control algorithm that is suitable for regulating the
multiple variables in the hybrid system is described by us-
ing a state-machine-based model; the issues about embedded
control implementation are addressed; and the large-signal
behavior of the hybrid system is analyzed on a voltage–
The hybrid power source is then tested through simulation
and validated on real hardware. Simulation and experiment
results show that the control algorithm is able to correctly
select the regulation mode and appropriately limit the fuel cell
current, the battery charging current, and the battery voltage.
This paper has also discussed some important issues of the
hybrid power source, such as operation under complex load
profiles, power enhancement, and optimization of the hybrid
system. The design presented here can not only be scaled to
larger or smaller power capacities for a variety of applications
but also be used for many other hybrid power sources.
The authors gratefully acknowledge the help of R. Leonard,
Dr. H. Figueroa, and Dr. A. Monti in setting up the processor-
in-the-loop simulation environment.
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Zhenhua Jiang (S’01–M’03) received the B.Sc. and
M.Sc. degrees from Huazhong University of Science
and Technology, Wuhan, China, in 1997 and 2000,
respectively, and the Ph.D. degree from the Univer-
sity of South Carolina, Columbia, in 2003, all in
He was a Postdoctoral Fellow at the University of
South Carolina before joining the University of New
Orleans, New Orleans, LA, as an Assistant Professor
in 2005. His research interests include digital control
of power electronics, fuel-cell power sources and
systems, renewable energy, energy storage, hybrid power sources and systems,
integration of distributed energy resources, and modeling and simulation of
Roger A. Dougal (S’74–M’78–SM’94) received the
Ph.D. degree in electrical engineering from Texas
Tech University, Lubbock, in 1983.
In 1983, he joined the University of South Car-
olina, Columbia. He is the Director of the virtual
testbed project, a multidisciplinary multiuniversity
effort to develop a comprehensive simulation and
virtual prototyping environment for advanced power
sources and systems, integrating power electron-
ics, electromechanics, electrochemistry, and controls
into a common testbed. VTB is unique in allow-
ing the simulation of multidisciplinary systems by importing models from
discipline-specific source languages to a common workspace. In addition to
modeling and simulation, his expertise includes power electronics, physical
electronics, and electrochemical power sources.
ing Award and has been honored as a Carolina Research Professor.