We live in an increasingly wireless world where self-powered devices are becoming integral to everyday life. A plethora of next-generation wireless technologies are seeing dramatic growth, involving both consumer and industrial applications. Some of the industrial applications include utility meter reading (AMR/AMI), wireless mesh networks, M2M and system control and data acquisition (SCADA), data loggers, measurement while drilling, oceanographic measurements and emergency/safety equipment, to name a few.

This amazing technological transformation has been accelerated by the development of low-power communications protocols such as ZigBee, Bluetooth, DASH7, INSTEON and Z-Wave. These protocols, in conjunction with low power design, have created the opportunity for certain remote wireless devices to operate for up to 40 years.

Choosing the right power supply
Understanding application-specific power requirements is essential to selecting the ideal power supply. Every application is unique, so all pertinent data needs to be factored in, including:

• Energy consumed in dormant mode (the base current).
• Energy consumption during active mode (including the size, duration and frequency of pulses).
• Storage time (as normal self-discharge during storage diminishes capacity).
• Thermal environments (including storage and in-field operation).
• Equipment cut-off voltage (as battery capacity is exhausted, or in extreme temperatures, voltage can drop to a point too low for the sensor to operate).
• Battery self-discharge rate (which can be higher than the current draw from average sensor use).
• Initial cost, along with long-term maintenance costs, including scheduled battery replacement, where applicable (to determine the total cost of ownership).

Since the design process often involves trade-offs, it is helpful to organize all key performance parameters into a prioritized cost/benefit matrix to ensure that the power supply incorporates all the necessary performance features while also delivering the best overall value.

Don’t be fooled by low initial expense
Billions of consumer primary (non-rechargeable) batteries are manufactured each year to power flashlights, remote controls and toys. There’s also a huge number of consumer-grade rechargeable lithium batteries manufactured each year for use in cell phones, tablet and notebook computers and many other portable and handheld devices. Since most cell phone contracts permit an equipment upgrade every two years, and laptop/tablet devices become rapidly obsolete, OEMs invariably seek the least costly solution by specifying a consumer grade rechargeable battery that lasts about five years and 500 full recharge cycles.

The low initial cost of a consumer battery can be very misleading, as this investment is relatively short lived and carries hidden costs, including the risk of reduced productivity and/or the loss of sensitive data due to battery failure. One must also factor in the total cost of having to replace the consumer-grade batteries every few years, including a significant labor expense, which can be magnified if the device is located in a remote, hard-to-access location.

Comparing common primary (non-rechargeable) batteries
Alkaline cells are readily available and extremely inexpensive, but have well-known drawbacks, including low voltage (1.5 V), a limited temperature range (-0 C to 60 C), a high annual self-discharge rate that reduces life expectancy to two to three years and crimped seals that are prone to leakage and corrosion.

Consumer primary lithium cells are relatively inexpensive, typically delivering either 1.5 V or 3 V, and are capable of delivering the high pulses needed to power a camera flash. These batteries have several limitations, including a narrow temperature range (-20 C to 60 C), a high annual self-discharge rate and a crimped seal that can cause battery leakage.

Lithium thionyl chloride (LiSOCL₂) cells are the preferred choice for wireless applications that require long-term power, especially in extreme environments. Bobbin-type LiSOCL₂ chemistry offers the highest capacity and highest energy density of any lithium chemistry, along with an extremely low annual self-discharge rate (less than 1% per year). This chemistry also delivers the widest temperature range (-80 C to 125 C), and features a glass-to-metal hermetic seal to prevent battery leakage.

All bobbin-type LiSOCL₂ batteries aren’t created equal. For example, an inferior quality LiSOCL₂ battery may deliver as little as 10-year operating life with an annual self-discharge rate of 2 to 3% per year, while a superior-grade LiSOCL₂ battery that features a much lower annual self-discharge rate of just 0.7% per year can operate for up to 40 years. Selecting a better-grade battery could be crucial to lowering your total cost of ownership, especially since the labor costs associated with battery replacement often far exceed the cost of the battery itself.

A Comparison of Primary Batteries

Primary Cell	Li/SOCL2 w/Hybrid Layer Capacitor	Li/SOCL2 Bobbin	Alkaline	Li/FeS2 Lithium Iron Disulfate	Li/MnO2 CR123A
Energy Density (Wh/1)	1,420	1,420	600	650	650
Power	Very High	Low	Low	High	Moderate
Voltage	3.6 to 3.9V	3.6 V	1.5V	1.5V	3.0V
Pulse Amplitude	High	Small	Low	Moderate	Moderate
Passivation	Low	High	N/A	Fair	Moderate
Performance at Elevated Temp.	Excellent	Fair	Low	Moderate	Fair
Performance at Low Temp.	Excellent	Fair	Low	Moderate	Poor
Operating life	Excellent	Excellent	Moderate	Moderate	Fair
Self-Discharge Rate	Very Low	Very Low	Very High	Moderate	High
Operating Temp.	-55 C to 100 C	-80 C to 125 C	-0 C to 60 C	-20 C to 60 C	0 C to 60 C

 

Industrial applications require more robust batteries
Consumer batteries don’t perform well in extreme environments. A prime example is the medical cold chain, where wireless sensors are required continuously monitor the transport of frozen pharmaceuticals, tissue samples and transplant organs at controlled temperatures as low as -80 C. Bobbin-type lithium thionyl chloride (LiSOCL₂) batteries are uniquely suited for the cold chain due to their high specific energy (energy per unit weight), high energy density (energy per unit volume) and their non-aqueous electrolyte, as the absence of water allows certain bobbin-type LiSOCL₂ batteries to operate in extreme temperatures ranging from -80 C to 125 C.

Special considerations for high pulse applications
More advanced wireless devices increasingly require high pulses of energy to power advanced two-way communications and/or remote shut-off capabilities. To conserve energy, these devices must remain in a “standby” mode that requires little or no current, periodically switching to an “active” mode that draws medium to high pulses to support data acquisition and transmission. Standard LiSOCl₂ batteries, due to their low rate design, may experience a temporary drop in voltage when first subjected to this type of pulsed load: a phenomenon known as transient minimum voltage (TMV).

One possible way to minimize TMV is to use super capacitors in conjunction with lithium batteries. A major drawback of the super capacitor is its relatively high self-discharge rate, which can cause the battery to fail prematurely. A super capacitor made up of two 2.5-V capacitors also needs a balancing circuit to extend its service life. Super capacitors also have a limited temperature range, which may prohibit their use in harsh environments.

An alternative solution would be to use a hybrid version of a bobbin-type LiSOCL₂ battery, the PulsesPlus, which combines a standard bobbin-type LiSOCl₂ cell with a patented Hybrid Layer Capacitor (HLC). The battery and HLC work in parallel, with the battery supplying long-term low-current power in the 3.6 to 3.9 V nominal range, while the single-unit HLC delivers high pulses, thus avoiding the balancing and current leakage problems associated with super capacitors. These hybrid batteries also feature a unique end-of-life performance curve, thus enabling devices to be programmed to deliver low battery status alerts.

Certain bobbin-type LiSOCL₂ batteries can also be modified to deliver moderate to high pulses without the use of an HLC, thus delivering high capacity and high energy density without experiencing voltage drop or power delay. These specially modified bobbin-type LiSOCL₂ batteries utilize available capacity very efficiently, thus permitting battery life to be extended up to 15% in extremely hot or cold temperatures.

Challenging environments demand state-of-the art manufacturing
To ensure long-term battery performance in extreme environments, state-of-the-art manufacturing techniques must be used to produce top quality batteries. The process begins by choosing the highest grade of raw materials, then employing total quality management tools such as six sigma methodologies and statistical process controls (SPC) during all phases of manufacturing to ensure greater lot-to-consistency. Not all batteries are manufactured to such high standards, so do your due diligence to verify that the batteries are UL-approved, and offer a high safety margin due to their ability to withstand extreme temperature, humidity, shock, vibration and puncture.

With battery replacement costs estimated at ten times the initial cost of the original battery, it is important to know the source of the raw materials, the manufacturing processes employed, and to verify the accuracy of all claims involving battery life expectancy based on typical annual self-discharge rate.

Certain wireless applications are ideal for rechargeable batteries
A growing number of wireless applications are proving to be well suited for energy harvesting devices that use rechargeable batteries to store the harvested energy.

Consumer rechargeable battery technology has improved dramatically in recent decades, coming a long way from the earliest Nickel Cadmium (NiCad) batteries that were excessively large, had low energy density, and suffered from a “memory effect,” in which a battery that was not fully depleted could not be fully recharged.

Nickel-Metal Hydride (NiMH) batteries represented a major improvement, as they eliminated the “memory effect” common to NiCad batteries. However, NiMH batteries also have drawbacks, including a high annual self-discharge rate that limits their service life.

Consumer-grade lithium ion (Li-ion) batteries have become popular due to their high efficiency and high power output. The most popular type of Li-ion cell, the 18650, was developed by laptop computer manufacturers as an inexpensive solution that could last approximately five years and 500 full recharge cycles. Consumer grade Li-ion cells experience a gradual degradation of the cathode, making these battery less receptive to future recharging, which reduces their total lifetime value. These batteries also have crimped seals.

Slim profile smart phones and tablet computers mainly use thin lithium polymer batteries that are reasonably inexpensive, but have a limited service life, perform poorly at extreme temperatures, and tend to swell in size over time.

Industrial-grade rechargeable Li-ion batteries offer long-term value 
Standard consumer-grade rechargeable lithium batteries aren’t intended for use in extreme environments due to performance limitations such as limited operating life, higher self-discharge and a narrow operating temperature range. To overcome these limitations, TLI Series industrial-grade rechargeable Li-ion batteries were recently developed.

TLI Series rechargeable Li-ion batteries can operate maintenance-free for up to 20 years and 5,000 full recharge cycles, thus delivering a greater total lifetime return on investment. These rugged and reliable industrial grade rechargeable batteries feature very low annual self-discharge rate, the ability to be recharged in extreme temperatures (-40 C to 85 C), the capability to deliver up to 15A pulses from an AA-sized cell, and a glass-to-metal seal to withstand harsh environments.

With the advent of industrial-grade Li-ion batteries, and the proven long-term reliability of bobbin-type LiSOCL₂ cells, design engineers now have a wider range of alternatives with which to compare consumer and industrial grade batteries, resulting in improved product performance and lower total cost of ownership.