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Small Energy Device Powers
Wireless Sensor Nodes
I
n recent years, sensor networks
intended for monitoring and controlling facilities or equipment, or
observing environments or spaces
have been drawing attention. A sensor network is a system in which many
sensor nodes that incorporate a power
supply, a sensor and communication
functions are placed at a distance to collect information from each node. The
collection of information from a node
is performed through wired or wireless
connection; when information is transmitted wirelessly, no wiring is required,
and the degree of freedom of installation location is much higher than when
information is transmitted through wired
connection. Therefore, a wireless sensor network is the technology necessary
for machine-to-machine (M2M), home
energy management systems (HEMS),
building energy management systems
(BEMS), Internet of Things (IoT), and
will probably be increasingly regarded
as important. In this article, sensor nodes
that perform information communication wirelessly are referred to as wireless
sensor nodes.
Wireless Sensor Nodes:
Configurations, Problems
A wireless sensor node consists of a
battery, a sensor, a wireless communication chip, an antenna, a one-chip microcontroller, among others, and needs to
continue operating independently after
Figure 1: Block diagram of a wireless sensor node
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AEI September 2016
Copyright©2016 Dempa Publications, Inc.
installation until the battery runs out.
Figure 1 shows an example configuration of a wireless sensor node.
A wireless sensor node consists of
four main functional sections: a power
supply section that supplies electrical
energy; a sensor section that detects
physical quantity and converts it to electric signals; a microcontroller section
that arithmetically manipulate signals
from the sensor; and a wireless communication section that transmits manipulated signals as data.
On the power supply section, generally, a power storage device is used to
supply power to a wireless sensor node.
Typical power storage devices include
batteries, which are classified into primary batteries (non-rechargeable batteries,
such as manganese dry batteries, alkaline manganese dry batteries, silver oxide batteries, and lithium batteries), and
secondary batteries (batteries that can be
charged and used repeatedly, such as lead
storage batteries, nickel cadmium batteries, nickel hydride batteries, and lithiumion batteries). A high-capacitance capacitor, such as an electrical double-layer
capacitor, is also a power storage device
that can be charged repeatedly.
If primary batteries are used as power
storage devices, it is possible to build a
system at a relatively low cost, but it is
necessary to periodically replace batteries, predicting the amount of battery
power consumed. Particularly in a sensor
network with a larger scale and a larger
number of wireless sensor nodes configured, maintenance for battery replacement becomes a bigger issue. In order to
solve this issue, in recent years, attempts
have been made to develop power supplies for sensor nodes that do not need
replacing, by combining a power generation device and a rechargeable power
storage device (secondary battery) so that
power is generated and stored into the
power storage device on the spot. This is
referred to as energy harvest. Power generation devices that use natural energy,
for 3mAh, 1C is equivalent to 3mA. As a general
lithium-ion battery has a
maximum continuous discharging current of about
1C, UMAC has 10 times
higher power density, and
is suitable for space-saving applications requiring
a power supply of about
30mA. Also, normally,
when a lithium-ion battery is charged, it is necessary to control charging
current for safety reasons,
and so it takes longer time
to charge a lithium-ion
battery. UMAC does not
require control of current
during charging, and can
be charged at a constant current, and so
it is also suitable for applications requiring quick charging. It is a lithium-ion
secondary battery that is optimal when
high-speed battery charging is required.
In terms of the properties necessary
for power storage devices for wireless
sensor nodes, Murata thinks that power
storage devices require four properties.
The first property is a long cycle lifetime to eliminate the need for the regular maintenance of sensor nodes. As it is
necessary to place many sensor nodes in
various locations to collect information,
regular maintenance for battery replacement requires time and costs. Probably
users do not want to carry out regular
maintenance for battery replacement,
for example. As UMAC has a long cycle
lifetime, it should be able to resolve this
problem. Figure 2 shows discharging capacity retention rates against charge-discharge cycles. If the discharge amount
for one cycle is 50 percent, 90 percent or
more of the capacity is maintained even
after 5,000 charge-discharge cycles. If
the discharge amount is less than this, the
cycle lifetime further increases. If discharge is performed once per day, 5,000
cycles are equivalent to about 13 years;
therefore, UMAC should be able to contribute to development of maintenancefree sensor nodes. The second property
is the capability to charge even by weak
current and to retain energy for a long
time. As the capacity of power generation devices like photovoltaic cells is
small, the property of retaining generated scarce energy for a long period without leaking it is required. The figures on
the left and right in Figure 3 show charge
capacity retention rates and 5μA charging properties, respectively. UMAC
retains 88 percent of the capacity even
after left untouched for 90 days from the
time it is fully charged, and its leak current is calculated to be as small as about
0.17μA. Also, it can be fully charged
even at a very small current of 5μA. As
described, it can be fully charged even
with a small-capacity power generation
device, and can retain generated scarce
2.3V
Charge voltage
2.7V
Discharge cutoff voltage
1.8V
Nominal capacity
3mAh
Maximum discharge current
30mA(10C)
ESR
800mΩ
Operating temperature
range
-20℃ to 70℃
Weight
0.29g
Dimensions
such as sunlight, vibration, and heat, are
being studied, but those with small size
capable of being incorporated in small
devices can secure only a little amount of
power. Therefore, power storage devices
need to be capable of being fully charged
even by small power generating capacity
and of retaining energy for a long period
without leaking it. Also, their performance needs to be maintained even after
they are charged and discharged repeatedly. Secondary batteries and electrical
double-layer capacitors of different types
have different advantages and disadvantages, and currently, only a small number of those batteries and capacitors can
satisfy all conditions suitable for energy
harvest.
Application to Wireless Sensor
Node
Murata Manufacturing Co. Ltd.’s
small energy device UMAC is a new
lithium-ion secondary battery that uses
lithium titanate as cathode material, unlike conventional lithium-ion batteries.
Its innovated material system allows
high-rate charging and discharging, long
cycle lifetime, and levels of safety at
which a thermal runaway does not occur; these cannot be achieved by general
lithium-ion batteries.
Table 1 shows the product specifications of UMAC. UMAC has a capacity
of 3mAh, a nominal voltage of 2.3V,
and a maximum continuous discharging current of 30mA (10C). 1C means a
current value at which the battery can be
completely discharged in one hour, and
Continued on page 65
Discharge capacity retention rate
UMAC040130A003TA01
Nominal voltage
110%
100%
90%
80%
50%
Test temperature: 25ºC
60%
50%
1000
2000
3000
4000
5000
Cycle
Figure 2: Discharge capacity retention
rates after charging and discharging
120
3
100
2.5
80
2
60
40
100%
70%
0
Voltage [V]
Product number
Charge (capacity) retention rate (%)
Table 1: Product specifications for the cylinder type (UMAC)
25ºC
20
1.5
1
25ºC
0.5
0
0
20
40
60
Elapsed days (days)
80
100
0
0
0.5
1
1.5
2
2.5
3
3.5
4
Charge capacity [mAh]
Figure 3: Capacity retention property and 5μA charging property
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Copyright©2016 Dempa Publications, Inc.
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Small Energy Device Powers...
Continued from page 59
energy without wasting it. The third
property is a high-rate discharging
property that allows a power storage
device to directly drive a system. If a
power storage device can be charged
but cannot meet the peak load of the
system, peak assist devices such as
capacitors are required, making the
system complicated. UMAC can discharge at 30mA (10C), and so can be
used as a power supply for near-field
communication such as Bluetooth Low
Energy (BLE) and ZigBee to directly
drive a load. Also, in intermittent operation such as wireless communication, UMAC can secure 30mA for a
long time even at -20°C because of its
low equivalent series resistance (ESR);
therefore, it is effective in discharging
at low temperatures unlike lithium-ion
batteries, and can be used outdoor in
cold climates. Figure 4 shows voltage
waveforms for 30mA pulse discharge
at -20°C. For pulses where 30mA is applied for 10msec in a cycle of 30sec.,
UMAC is capable of pulse discharge for
100 hours or more. The fourth property
is the capability to be used immediately
after charging begins. In a capacitor, the
amount of charge stored is proportional
to the voltage between both ends of the
capacitor, and therefore, there is a time
lag between the time charging starts
and the time the system becomes operational. In contrast, UMAC allows the
system to become operational immediately, because it is a lithium-ion battery,
that is, it can reach its nominal voltage,
Figure 4: Voltage transient property and pulse waveforms during 30mA pulse discharge at -20°C
2.3V, when charged to Table 2: Comparison of power storage devices as power
only several percent. supplies for wireless sensor nodes
(More on this property
Electrical
Lithium
double-layer UMAC
is available at Murata’s
ion battery
capacitor
website.)
Described
above are the properties 1) Maintenance-free
⃝
⃝
×
(Cycle life)
that the company thinks
⃝
⃝
2) Low-loss (leak current)
×
are necessary for pow3) Direct load driving
⃝
⃝
×
er storage devices for
(High-rate discharging property)
wireless sensor nodes; 4) Quick start (charging property)
⃝
⃝
×
UMAC should be suitdevice that has the properties necessary
able as power storage
for wireless sensor nodes; the company
devices for wireless sensor nodes.
is hoping to contribute to the coming
Finally, Table 2 summarizes a comage of IoT by continuing to improve
parison in terms of the properties exthe features of this product, expand
plained above.
the product line, and offer proposals to
Future Expansion
customers.
Developments in sensor networks
About This Article:
will take place with the help of wider
use of wireless sensor nodes involving
The authors are Yusuke Tanaka and
energy harvest, which allows increase
Yoshiyuki Tabata from the Planning
in the degree of freedom of installation
and Promotion Section, High Perforor development of maintenance-free
mance Power Device Dept., Chemical
sensor nodes.
Device Group of Murata ManufacturMurata’s UMAC is a power storage
ing Co., Ltd.
AEI September 2016
Copyright©2016 Dempa Publications, Inc.
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