Battery Size Matters

Battery Size Matters
Battery
Size
Matters
Five fundamental
considerations for a
battery-powered, wireless
IoT sensor product.
www.silabs.com | Smart. Connected. Energy-Friendly.
The Internet of Things (IoT) is upon
us. Gartner recently reported a typical
family home may contain more than
500 smart devices by 2022 (Gartner
news, Sep 8, 2015). The promise is
that every day home automation
products will be smarter through the
introduction of low-cost IoT technology
including wireless transceivers and
innovative sensors. But these features
take energy, and a majority of these
devices will be powered by batteries.
This presents an interesting challenge. To successfully design a
product for the connected home, designers must generally build
a small form-factor product, with the cheapest bill of materials
(BOM), and reliable operation over several years without battery
replacement.
IoT Home Automation Challenge:
Designers Must Create...
“small form factor
products,”
“with the cheapest
bill of materials,”
“that reliably perform for years
without a battery replacement.”
Five fundamental
considerations for
a battery-powered,
wireless IoT sensor
product.
1
Different Markets, Different
Requirements?
Before you can begin designing it is
important to understand the target
market, as each present different
requirements on cost, reliability and
battery life. Below are two extreme
examples of how market requirements
force designers to make trade-offs.
Subscription-based Service Providers
A subscription-based service provider
(e.g., cable or satellite internet service)
places greater importance on long-term,
reliable operation than achieving the
lowest BOM cost or attaining the smallest
form-factor.
These companies weigh product cost
versus the cost of sending a technician
on-site for trouble shooting, which costs
between $200 and $2,000 per trip,
according to various sources. Of course,
if the issue is merely a drained battery, the
cost trade-off is obviously to put a bigger
battery or a lower-power design in place.
Do-it-Yourself Home Automation
While this segment values reliability,
these consumers often select products
based on cost, size and appearance. This
market is where batteries are typically
small to accommodate an aesthetically
pleasing enclosure. Additionally, the
product cost must be low to achieve a
low shelf price. If the do-it-yourselfer
gets the product home only to have it
fail, the replacement cost of a defective
consumer product is much lower than
the professional service provider market;
therefore, product designers are willing to
make these kinds of trade-offs.
2
Battery Efficiency and
Wireless – Not Always an
Obvious Choice
When a product needs wireless
connectivity, the choice of which protocol
to use will be a major factor affecting
battery life. There are several wireless
choices to consider, some of which may
be preferred or even required by the
target market.
Wi-Fi is a commonly used protocol in
consumer wireless products. Wi-Fi
accommodates large streams of data
at high throughput rates and is more
power-hungry compared to other wireless
protocols. Wi-Fi products are often
plugged into power outlets or frequently
recharged as they are not optimal for a
product running off a battery compared
to products that use very low throughput
rates.
For example, most home automation
sensors such as magnetic door and
window sensors and passive infrared
(PIR) motion detectors, are static for long
periods of time and would not benefit
from large data stream Wi-Fi offers.
These sensors can utilize battery-friendly
wireless standard such as Bluetooth®
Smart (also known as “BLE”) or ZigBee®.
Bluetooth Smart targets direct pointto-point communication, while ZigBee
targets multi-node mesh-networking.
Wireless Eats Energy
Transmitting and receiving streams of wireless
data consumes milliamps of energy per hour,
well beyond what can normally be supported
with two low-cost AAA batteries.
ZigBee might be best suited for multiple
smart devices such as thermostats,
door sensors and even window shades
that can be configured to communicate
autonomously with very little user
interaction. A mesh network also has
built-in redundancy that can eliminate
single points of failure, making it more
reliable than point-to-point technology.
As a point-to-point technology, Bluetooth
Smart or “BLE” might be best suited for
querying a smart device in the home such
as a door lock. For mesh networking,
Standards-based
Standards Body
Application Focus
Type of Battery
Number of Nodes
Required Throughput
(Kbps)
Typical Range (Meters)
Network Topology
ZigBee
Wi-Fi
Bluetooth Smart
Yes (802.15.4)
Yes (802.11)
Yes
ZigBee Alliance
Wi-Fi Alliance
Bluetooth SIG
Monitoring & Control
Web, Email, Video
Sensors
Coin-cell
Rechargeable (Li-ION)
Coin-cell
<10 to 1,000+
<10 to 250
<10
<250
>500
<250
1 to 100
1 to 100
1 to 70
Self-healing Mesh, Star, Pointto-Point
Star, Point-to-Point
Star, Point-to-Point
3
Duty Cycle – How
Often Does the Device
Communicate?
Sleep-state power numbers
within the electrical
specifications of a wireless
transceiver should not be the sole
input when examining a device’s
wireless power budget.
Once a wireless technology is chosen,
the next factor facing the designer is
determining the required transmission
strength, duration and duty-cycle
between active and sleep states.
Most modern wireless transceivers
offer sleep modes when not in use to
save power. The longer a device is in
sleep state the less power it uses, which
extends battery life. However, the sleepstate power numbers within the electrical
specifications of a wireless transceiver
should not be the sole input when
examining a device’s wireless power
budget. The wireless transceiver’s wakeup times and pre-processing algorithms
prior to transmission and return to sleep
also affect power and should be included
when calculating the total wireless power
budget.
The frequency, or duty-cycle, of wireless
transmissions will directly affect the
battery life of the product. Duty-cycle
is determined in-part by the wireless
standard requirements, the software
algorithm and how the device is normally
used. For example, a door sensor with
an open/close event will cause a wireless
data transmission to occur; however,
this sensor may also send and receive
periodic wireless polling events to and
from other mesh network nodes for status
updates. Figure 1 provides an overview of
this process.
4
Sensibly Designing-in
Sensor
In general, each application requires
a certain type of sensor; a carbon
monoxide detector needs a CO sensor,
a smoke detector needs an ionization or
photoelectric sensor, a motion detector
needs a PIR sensor, and so on. Once
the type of sensor is selected, how it
is designed and what MCU is selected
can save power and cost. The reference
designs below provide some insights.
The Silicon Labs RD-0030-0201 ZigBee
Contact Sensor Reference Design
monitors entry points such as doors
and windows. The main sensor is a reed
switch to detect the presence of a nearby
magnet and determine if an entry point
is open or closed. Once a magnetic field
reaches a certain threshold, the sensor
assumes the entry point status has
changed and a wireless notification cycle
is initiated.
When it comes to the number of entry
events, while it has an effect on battery
life, the number itself is a small factor
when measured across multiple entry
events, as seen in Figure 2.
Contact Sensor Battery Life (years)
Battery Life (years)
6
5
4
3
2
1
0
0
10
20
30
40
Number of Door Open & Close Events/Day
Figure 2: Contact Sensor Battery Life
50
However if the reed switch is held static
in a specific state (open or closed) for
long periods of time, it can affect battery
life if the state creates a very low series
impedance circuit across the battery
supply. To avoid this situation, Silicon
Labs developed a reference design with
kilo-ohms of impedance across the
battery supply when the reed switch
is opened or closed so the state of the
reed switch is negligible on the chosen
battery. In other words, a door can remain
open or closed for many days with very
little effect on the battery.
MCU choice can also make a difference in
the sensor node’s power. The RD-00390201 ZigBee Capacitive-Sense Dimmer
Switch Reference Design is used to
control ZigBee-based LED lighting nodes.
This design is battery-powered and uses
ZigBee so it can be placed anywhere on a
flat surface, fully wireless from the switch
to the lights. It also employs capacitive
sensors to detect the user’s physical
touch to correspond to particular lighting
commands.
This reference design uses a small, lowstorage CR2032 battery. To conserve
energy, the design uses a Silicon Labs
low-cost, energy-efficient 8-bit EFM8SB1
MCU for touch sensing capability. This
circuit is designed to operate below
1uA to detect multiple touch points and
gestures for light commands, such as on
or off or level control.
Further, these capacitive events are
monitored in a relatively slow, millisecond
duty cycle rate since people are
accustomed to relatively slow human
interface responses compared to most
MCU capabilities. This saves battery
power without sacrificing usability.
All of these factors should be considered
when designing for the targeted battery
life of a product.
Adjusting MCU Polling
Frequency Saves Power with No
User Impact
MCU capacitive events are monitored in a
relatively slow, milli-second duty cycle rate
since people are accustomed to relatively slow
human interface responses compared to most
MCU capabilities. This saves battery power
without sacrificing usability.
5
Space Constraints and
Stored Power in the
Design
Once the target market and electronic
components that affect battery life are
considered, one must look at different
battery options. In general, smaller
batteries have less storage capability than
larger ones and are described in milliamphours (mAh). In addition, a battery’s
chemical composition plays into its cost
as well as its storage effectiveness.
Alkaline batteries, such as 1.5 volt AA and
AAA, are very low-cost and possess high
storage capabilities; however, they are
large in size, prone to high leakage and
are typically limited to handheld devices
such as television remote controls. Also,
two Alkaline batteries are commonly used
to achieve a minimum three volt supply
required in many consumer products.
Lithium batteries, such as CR2 or CR123A
are smaller in size and weigh less than
Alkaline, but have higher costs; therefore
these batteries are favored in designs
which benefit from small size and low
leakage such as security sensors.
Even smaller coin-cell sized batteries,
such as the CR2032 are efficient when it
comes to storage versus size, and have
better leakage rates in comparison to
Alkaline. However given they possess
less than one-fourth the storage to two
AAA batteries, these coin-cell batteries
are mainly used in designs optimized for
very low power use. Table 2 provides an
overview of these differences between
these batteries.
2x AAA
CR2032
CR123A
CR2
Material
Alkaline
LiMnO2*
Lithium
Lithium
Voltage
3V
3V
3V
3V
Capacity
1000 mAh
225 mAh
1500 mAh
800 mAh
Diameter
10.5 mm (x2)
20 mm
17 mm
15.6 mm
Height
45 mm
3.2 mm
34 mm
27 mm
Weight
24 g
3g
17 g
11 g
Results
When these design considerations are employed, the benefit is a
lower-cost IoT sensor that is capable of achieving an increased
battery life for a given target market.
For example the RD-0030-0201 ZigBee Contact Sensor
Reference Design runs up to five years on a single CR2032. And
the RD-0039-0201 ZigBee Capacitive-Sense Dimmer Switch
Reference Design operates longer than typical battery-powered
consumer remotes.
Some of the main contributors to this long battery life are the
low-sleep current on the EM3587 ZigBee Pro transceiver as
well as its ability to efficiently wake up and transmit data. This
particular design is configured to provide non-entry status
updates approximately once per minute, which also minimizes
its battery use.
Jump Start Your Power Savings with
Reference Designs
By carefully assessing the design considerations outlined in this
article, a product designer can effectively achieve a small-form
factor product able to reliably operate with a low-cost battery.
Silicon Labs offers an array of IoT reference designs that
eliminate many of these design complexities and optimize
battery use for particular applications such as a door/window
sensor (RD-0030-0201) and capacitive-sense dimmer switch
(RD-0039-0201). For more information or to evaluate one of
these reference designs, please visit silabs.com/iot.
www.silabs.com | Smart. Connected. Energy-Friendly.
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