Special Feature

Clean Energy Technology
Takahisa Shodai, Keiichi Saito, Hiroshi Wakaki,
Masato Mino†, Jun-ichi Ohwaki, and Yasuyuki Kanai
Abstract
Enabling anyone to communicate freely at any time and from any place requires highly reliable communication systems. Consequently, there is a demand for a clean and powerful backup power supply.
Here, we introduce i) a high-performance backup power supply system that employs nickel metal
hydride batteries for use in the communication infrastructure and ii) Solar Card Power, a power supply
for ubiquitous mobile devices.

1. High-performance backup power supplysystem
1.1 What is a backup power supply system?
A backup power supply system supplies continuous
48 V DC and l00 V AC to telecommunication equipment from storage batteries when the public power
supply is interrupted by a serious disaster.
1.2 Why is a new power supply system needed?
For over half a century, lead-acid batteries havebeen used for backup storage batteries, but they areheavy and take up a lot of space. Furthermore, thepower consumed by communication equipment has
tended to increase with increases in the speed andfunctionality of data communication services, so thedemand for a small, lightweight power supply system
to replace lead-acid batteries has also increased. As
one step toward meeting that demand, NTT Energyand Environment Systems Laboratories has developed a power supply system based on large-capacity
nickel metal hydride (NiMH) batteries, which have ahigher energy density than lead-acid batteries [1].
1.3 Overview of the new power supply system
The configuration of the new high-performance† NTT Energy and Environment Systems Laboratories
Atsugi-shi, 243-0198 Japan
E-mail: mino@aecl.ntt.co.jp

46

backup power supply system and its basic operation
are illustrated in Fig. 1. An example of a high-performance backup power supply system is shown in Fig.
2. This power supply system stores 10 kWh of energy, so it can provide about three hours of backup
power at the power output of 3 kW during a commercial power outage. It is therefore suitable as the power
supply system for wireless base stations, ADSL
remote subscriber stations, and other such access systems.
1.4 Technological key points
One feature of our system is that it is divided into abattery module and a power supply module. NiMHbatteries can tolerate hot summer environments andhave long stand-by times under full charge. However,
we had to develop new large-capacity NiMH batteries because there were none suitable for use in a backup system. We started with the cycling NiMH batteries used in electric automobiles as a base andimproved the charging and discharging characteristics, the self-discharging characteristics, and the lifetime characteristics for a wide range of use environments at temperatures between –10°C and 65°C by
optimizing the electrode composition, the electrolyte
concentration, and the electrolyte ratios (KOH,NaOH, and LiOH).
For the power supply module, we developed a special charging device, discharging device, and control
device as well as a new recharging control method toNTT Technical Review

Special Feature

Conversion ofalternating current
to direct current

Normally, power is supplied to thecommunication equipment from thecommercial power supply via a rectifier.

Commercial
power supply
Communicationequipment

RectifierIf the battery level
becomes low, it isautomatically charged tostore electrical power.

Charger
Controller

The operational status ofeach component of thesystem is monitored andcontrolled.

Discharger

NiMH battery

Signal line

When the power is interrupted,
power is supplied to the equipmentfrom the batteries without any power
flicker via the discharger.

Normally, the system stands
by in the charged state. Thebatteries discharge if the powerstops.

Fig. 1. Components and basic operation of backup power supply system.

Inverter(for 100 V AC)

RectifierController
Charger
Discharger

NiMH battery modules

Dimensions: W600 × D685 × H1600 mm
Weight: 300 kg

Fig. 2. High-performance backup power supply system.

extract the full potential of NiMH batteries. Specifically, the controller regularly computes the remaining
capacity of the battery and begins supplementarycharging automatically when the capacity falls to aprescribed value. Furthermore, the charging is done
by a constant current charging method, which is themost desirable method for NiMH batteries. A fullcharge is detected by measuring the increase in heat
per unit time (dT/dt), utilizing the characteristic ofNiMH batteries that heat is generated when the batVol. 2 No. 4 Apr. 2004

tery is fully charged. The discharger incorporates ahighly efficient dropper and step-up converter formatching the voltage change range of the battery andthe operating voltage range of the load. It can supply
power to the load without any power flicker whencommercial power is interrupted.
Considering outdoor use, we conducted field trials
in a harsh summer environment. Those tests confirmed that this new system can maintain the same
safety and reliability levels as lead-acid battery systems or even exceed them. Furthermore, this system
received a model certification based on the Fire Services Act, which is the first time this certification has
been granted to a storage battery facility using NiMH
batteries in Japan.
1.5 Results
The battery module of the developed power supply
system is two-thirds the volume of a conventional
system using lead-acid batteries and half the weight,
so the floor load weight and installation footprint canboth be reduced. The battery lifetime is also at least
twice that of lead-acid batteries and the batteries aremaintenance-free. Therefore, the total cost, including
replacement and maintenance costs, is lower than thatfor lead-acid batteries. Furthermore, the completely
lead-free battery module makes this a very environmentally friendly system, so companies that use it
should benefit from a positive boost to their corporate
images.
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Special Feature

2. Solar cell input type mobile power supply:
Solar Card Power
2.1 Power supply for mobile devices that isfriendly to the global environment
In recent years, the popularity of mobile phones,
PDAs (personal digital assistants), digital cameras,
and other mobile devices has risen sharply. The overall energy consumption of these devices has also
greatly increased. We have developed a power supply
for such devices that is more convenient than existingones and only a slight burden on the environment. We
call it Solar Card Power. It features:• Clean power generation from sunlight• Applicability to various types of mobile devices• Compact design and high power output
It is thus useful as a mobile power supply for travelers and as a backup power supply in times of emergency.
2.2 System configuration
As shown in Fig. 3, the system comprises a generating unit and a main unit. The main unit has a storage part that accumulates the electrical energy generated by the generating unit and a converter for supplying that energy at various voltages and currents.
The prototype is shown in Fig. 4. The generating unit
and the main unit are separate and detachable for convenience and ease in carrying. The storage battery
built into the main unit has about twice the capacityof a built-in mobile phone battery and provides about
5 Wh of energy in an easily carried form. The battery

Generating unit

Storage parthas a lifetime of over 500 recharging cycles, so it canprovide clean energy equivalent to more than 1000
AA alkaline batteries.
2.3 Technological key points
Solar Card Power features two technologies developed by NTT: (i) an electric-power generating module with a single solar cell and (ii) automatic settingof the output power.
(1) Generating module
The solar cell conveniently generates electricity
when illuminated. However, the output voltage of acell is only about 0.5 V. Conventionally, from 10 to 12
cells are connected in series to increase the output toabout 5 V, as shown in Fig. 5. Thus, this setup suffersfrom the problem that if even one of the cells is shaded from the sunlight or breaks, the electricity-generating system cannot function correctly. The ultralow-Generating unit

Main unit

Fig. 4. External appearance of the prototype device.

Converter
Automatic
output setting• Constant output
voltage(3.8 V, 5.0 V, etc.)• Constant output
current(100 mA, 1.0 A, etc.)

Single-solar-cellelectric power
generating unit

Ultralow-voltage-input
booster technology

Generating unit

Load(electrical terminals)

Battery

Software controlledconverter technology

Main unit

Fig. 3. Configuration of Solar Card Power.

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NTT Technical Review

Special Feature

voltage-input booster technology that we have developed can raise a voltage of about 0.l V to about 5 V.
That eliminates the need to connect multiple solar
cells in series and allows the use of a single solar cell,thus reducing the manufacturing cost. This technology can eliminate the problem of partial shading andincrease the amount of electricity generated, while
maintaining a stable output.
(2) Main unit
Conventional converters change the output voltage
or current by a switch or other switching mechanism,
which could allow excessive voltage to be applied to
the load equipment if a mistake were made in theswitch setting. Our newly developed software controlled converter technology can output the voltage
and current at the values set for various types ofcables according to the type of cable connected (cellphone cable, 5-V cable, etc.) as shown in Fig. 6. Thisis more convenient for the user. The output specifica-tions for the prototype are 3.8 V to 8.5 V with a maximum output power of 10 W, so one of these devices
can be used for many types of portable devices fromPDAs to digital cameras and video cameras, but notfor notebook PCs.
3. Future plans
We will proceed with our efforts to increase thecapacity of the high-performance backup power supply system and with plans to support power systems
ranging in size up to the main power supply systems
of communication buildings and data centers. Our
goal is to construct a clean energy system that canreduce the cost of electricity as well as the emissionof CO2 by coordinated use of the commercial power
supply and fuel cell facilities (electric power generators).
We plan to continue our work on increasing the(a) Conventional solar cell configuration(b) New configuration

Single cell(+)(+)
Boostercircuit

5 V, etc.
(unstable)

5V(stable)0.1 V~(–)(–)

Fig. 5. Configurations of conventional solar panel and our new solar cell.

High-current
supply is possible

Setting
conditions

Softwarecontrolledconverter

Built-in battery(4 V)

90% efficiency

Dedicatedconnection cable

Controlled output(3.8~8.5 V)(~10 W)

Main unit

Fig. 6. Configuration of the main unit with built-in software controlled converter.

Vol. 2 No. 4 Apr. 200449

Special Feature

capacity of Solar Card Power as a solar cell electric
power generation system and to expand the application to the field of telemetry, including remote monitoring equipment. Like solar cells, fuel cells produce
low-voltage output and must be connected in series ina power system. We also plan to develop the ultralowvoltage booster technology for use in micro-fuel-cell

Reference[1]

K. Saito, T. Shodai, A. Yamashita, and H. Wakaki, “High Performance Backup Power Supply System,” Proceedings of Intelec’03,
Yokohama, Japan, p. 261, Oct. 2003.

Takahisa Shodai

Masato MinoSenior Research Engineer, Supervisor, NTT
Energy and Environment Systems Laboratories.
He received the B.S. and M.S. degrees in material development and engineering from Kyushu
University, Fukuoka in 1986 and 1988, respectively, and the Ph.D. degree in engineering fromMie University, Mie in 1998. In 1988, he joinedNTT. He had been engaged in research on Li-ion
battery anode and solid electrolyte materials. Heis currently engaged in research on backup powersupply system. He is a member of the Electrochemical Society of America and Electrochemical Society of Japan, and the Solid State Ionics
Society of Japan.

Senior Research Engineer, Supervisor, NTT
Energy and Environment Systems Laboratories.
He received the B.S and M.S. degrees in metalmaterial engineering from Tohoku University,Sendai, Miyagi in 1982 and 1984, respectively.
In 1984, he joined NTT Electrical Communications Laboratories, Tokyo, Japan, where he has
been engaged in research on thin-film magnetic
devices and micro-converters. He is currently
engaged in research on power electronics technology. He is a member of the Institute of Electrical Engineers of Japan, the Magnetic Societyof Japan, the Japan Institute of Metals, and IEEE
Magnetic Society.

Keiichi Saito

Jun-ichi Ohwaki

Senior Research Engineer, NTT Energy andEnvironment Systems Laboratories.
He graduated from the Waseda University,
graduate school of science and engineering in
1991 and joined NTT the same year. He wasengaged in research on Li-ion battery anodematerials from 1991 to 1995 and new businessdevelopment from 1995 to 2000. He is currently
engaged in research on powering systems.

Senior Research Engineer, NTT Energy andEnvironment Systems Laboratories.
He received the B.E., M.E., and D.E. degrees
in electrical engineering from Nihon University,
Tokyo in 1976, 1978, and 1989, respectively.
Since joining Nippon Telegraph Public Telephone Corporation (now NTT) in 1978, he has
been active in research on thin-film electroluminescent devices and IR-to-visible upconversion
phosphor materials. He is currently engaged inresearch on powering systems. He won the Phosphor Prize in 1996. He is a member of the Institute of Electronics, Information and Communication Engineers, the Phosphor Research Society, the Electrochemical Society of Japan, andAssociation for the Study of Failure.

Hiroshi Wakaki
NTT Energy and Environment Systems Laboratories.
He received the B.E. degree in electronics
engineering from Nagoya University, Nagoya in
1997. He joined to NTT Facilities in the sameyear. Since moving to NTT in 2002, he has been
researching power supply systems using nickel
metal hydride batteries.

50batteries and other small generation systems, thus
promoting the use of clean energy.

Yasuyuki Kanai
Research Engineer, NTT Energy and Environment Systems Laboratories.
He joined NTT in 1994. He has been engaged
in research on power electronics. He is currently
engaged in research on renewable energy systems in the field of wearables.

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