Numerous types of batteries are available for powering wireless sensors, including alkaline, carbon zinc, zinc-air, rechargeable lithium, and primary (non-rechargeable) lithium chemistries. Each of these chemistries offers performance advantages and disadvantages, so choosing the optimal power management system invariably involves tradeoffs. For example, inexpensive consumer alkaline cells may be ideal for an application that requires a few months of service life in moderate temperatures and in an accessible location where replacing the battery is relatively simple. Conversely, inexpensive alkaline batteries are not recommended for applications that demand an extended service life in extreme temperatures where this water-based chemistry is prone to failure.
If the application requires long-term, maintenance-free performance in a remote location where there is no access to the AC power grid or where battery replacement is difficult or impossible, then lithium batteries are the preferred choice because lithium's intrinsic negative potential exceeds that of all other metals. Lithium is the lightest nongaseous metal, offering the highest specific energy (energy per unit weight) and energy density (energy per unit volume) of all available battery chemistries.
Lithium cells, all of which use a nonaqueous electrolyte, have normal open-circuit voltages (OCVs) of between 2.7 and 3.6 V. The absence of water also allows certain lithium batteries to operate in extreme temperatures (–55°C to 125°C), with certain models adaptable to cold-chain temperatures down to –80°C. Recently, Tadiran placed LiSOCl2 cells in a cryogenic chamber and subjected them to progressively lower temperatures down to –100°C; the batteries remained operational.
All lithium batteries are not created equal. Within the lithium family, numerous primary chemistries are available, including poly carbon monofluoride (Li/CFX), manganese dioxide (Li/MNO2), and lithium thionyl chloride (Li/SOCl2). As shown in Figure 1, each chemistry offers advantages and disadvantages. Poly carbon monofluoride and manganese dioxide are best suited for applications that require a shorter operating life without exposure to extreme temperatures. When long battery life, extended temperature range, and reduced battery size and weight are required, the ideal choice is lithium thionyl chloride. These batteries are constructed in two different ways: spirally wound batteries, which offer a maximum service life of approximately 10 years; and bobbin-type cells, which are the only lithium batteries that can deliver 25+ years of service life.
Figure 1. Lithium battery characteristics
Li/SOCl2 with HCL
|Li/SOCl2 bobbin-type||Li/SOCl2 spirally wound||Li/SO2||Li/MnO2|
|Energy density (Wh/l)||1420||1420||800||410||650|
|Voltage||3.6–3.9||3.6 V||3–3.6 V||2–3 V||2–3 V|
|Performance at low temperatures||Excellent||Fair||Excellent||Excellent||Poor|
|Self-discharge rate||Very low||Very low||Moderate||Moderate||Moderate|
|Operating temperature||–55°C to 85°C||–55°C to 125°C||–55°C to 85°C||–55°C to 60°C||0°C to 60°C|
While 25 years of service life is becoming an international standard for battery performance in wireless sensor applications, this milestone is often difficult to prove, as bobbin-type LiSOCl2 batteries cannot be easily tested in conditions that accurately simulate actual in-field use. The cost of battery replacement can be as high as ten times the initial cost of the original battery, so design engineers must be careful to verify the accuracy of manufacturer claims involving battery life expectancy.
Understanding Application-Specific Parameters
Remote sensors are becoming increasingly feature-rich, offering enhanced functionality such as periodic two-way RF communications and remote shut-off capabilities, even in extreme temperatures. These advanced features tend to drain battery power, which in turn reduces battery life expectancy.
To accommodate the higher energy demands of feature-rich devices without compromising battery service life, the wireless sensors designed for 25 years of service must operate in a dormant or 'sleep' mode, where average daily power consumption ranges from zero to a few microamps to conserve energy. The device is also programmed to periodically switch into active mode, where high current pulse loads are required for brief intervals while the sensor progresses through start-up, interrogation, data transmission, and return to dormant mode. The energy required during active mode varies from hundreds of milliamps for short-range RF communications to a few amps for certain GPRS protocols.
Each application is unique, so to specify the right battery, design engineers need to fully understand application-specific power requirements, including:
- Energy consumption during dormant or sleep mode (the base current)
- Energy consumption during active mode (including the size, duration, and frequency of high-current pulses, where applicable)
- Length of time that the battery will be stored because self-discharge during storage diminishes capacity
- Thermal environments for both storage and operation
- Equipment cut-off voltage. As battery capacity is exhausted, or in extreme temperatures, the voltage can drop to a point too low for the sensor to operate
- Battery self-discharge rate, which sometimes can be higher than the current draw from average sensor use
These and other parameters can be combined to create an energy-use profile, a mathematical model that accurately predicts battery life expectancy. Battery manufacturers can also perform sensitivity analysis of critical parameters, such as prolonged exposure to extreme temperatures, which helps to optimize system performance and extended battery life.
Among the different battery chemistries available for use in wireless sensors, lithium thionyl chloride (LiSOCl2) is overwhelmingly preferred for long-term use in extreme environments because of its performance characteristics, including its high energy density, high capacity, extremely low self-discharge, and extended temperature range.
LiSOCl2 batteries (Figure 2) have a successful history in remote wireless applications dating back to the late 1970s.
Figure 2. Lithium thionyl chloride (LiSOCl2) chemistry is the preferred choice for applications requiring extremely long battery life, extended temperature range, and reduced size and weight
Figure 3. Aclara AMR meter transmitter units (MTUs) installed in 1984 are still operating 27 years later on their original LiSOCl2 batteries
For example, in 1984, Aclara, formerly Hexagram, began using LiSOCl2 batteries to power meter transmitter units (MTUs) for automated meter reading systems used by water and gas utilities (Figure 3). Many of these early devices are now reaching the end of their operational lives but the older units were still operational after 27 years, using their original Tadiran batteries. The proven track record of LiSOCl2 batteries allows today's automated meter reading (AMR) and automated metering infrastructure (AMI) equipment manufacturers to offer long-term performance contracts for large-scale infrastructure upgrades that enhance customer service and billing and permit continuous monitoring of customer demand and usage.
While Aclara's example demonstrates that 25+ years of battery life is achievable, one cannot assume that all LiSOCl2 batteries are created equal. Good batteries are products of battery manufacturers who use high-quality materials and advanced manufacturing techniques. Long-life performance requires batteries with a low potential for electrolyte leakage or short circuits combined with low annual self-discharge. While many battery manufacturers claim low annual self-discharge rates at ambient temperatures, this claim may be invalid depending on the size of the battery, its method of construction, or the application-specific temperature requirements. A difference of just a few microamps in self-discharge rate can significantly impact battery life expectancy.
Powering Advanced Two-Way Communications
A growing number of remote wireless sensors now offer advanced two-way communications, including utility meter reading (AMR/AMI), wireless mesh networks, system control and DA (SCADA), data loggers, measurement while drilling, oceanographic measurements, emergency/safety equipment, and other remote sensors.
If the application involves dormant periods at elevated temperatures, alternating with periodic high current pulses, then lower transient voltage readings can result during initial battery discharge. This phenomenon, known as minimum transient voltage (MTV), is common to bobbin-type LiSOCl2 batteries because of their low-rate design and is strongly linked to the choice of battery electrolyte or cathode.
One possible solution is to use supercapacitors in conjunction with lithium batteries, but the supercapacitors' relatively high self-discharge rate can cause the battery to fail early. A supercapacitor made up of two 2.5 V capacitors needs a balancing circuit to ensure service life. The limited temperature range of a supercapacitor also prohibits its use in certain applications.
Two alternative solutions have been developed by Tadiran: PulsePlus batteries for high current pulses and TRR Series batteries for moderate current pulses. Both technologies solve the TMV problem inherent to bobbin-type LiSOCl2 cells while retaining the key advantages of long life and low self-discharge.
Tadiran PulsesPlus batteries (Figure 4) combine a standard bobbin-type LiSOCl2 battery with a patented Hybrid Layer Capacitor (HLC). The battery and HLC work in parallel, with the battery supplying long-term low-current power while the HLC supplies current pulses up to 15 A, thus eliminating the voltage drop that normally occurs when a pulsed load is initially drawn. PulsesPlus batteries can also enable low battery status alerts; a 3.6 V system indicates when approximately 95% of battery capacity has been exhausted and a 3.9 V system indicates when 90% of available capacity has been used up.
Figure 4. PulsesPlus batteries combine a standard (LiSOCl2) cell with a patented HLC to deliver the high current pulses required to power advanced two-way communications
The single-unit HLC works in the 3.6–3.9 V nominal range to avoid the balancing and current leakage problems associated with supercapacitors, and has delivered high current pulses and a high safety margin when used in the field.
Figure 5. Tadiran Rapid Response TRR Series batteries are ideal for applications that require moderate current pulses. TRR Series batteries are extremely efficient, able to extend battery life by up to 15%, especially in extremely hot or cold temperatures
Rapid Response TRR Series batteries (Figure 5) are intended for applications that require moderate current pulses. The TRR Series does not require the use of an HLC (although it can use a smaller HLC) to deliver high capacity and high energy density without voltage or power delays. When a standard LiSOCl2 battery is first subjected to load (especially at cold temperatures or when the battery is nearing the end of its operating life) voltage can drop temporarily before returning to its nominal value. TRR Series batteries virtually eliminate this voltage drop as well as voltage drop under pulse (or transient minimum voltage level). The final result is zero delay during the voltage response. TRR Series batteries use available capacity efficiently, extending the operating life of the battery by up to 15% under certain conditions, especially in extremely hot or cold temperatures.
Use of inferior raw materials or nonstandardized battery manufacturing techniques can lead to batch-to-batch inconsistency, which severely impacts long-term battery performance, even if initial performance characteristics seemed identical. Performing the proper due diligence during the battery specification process will help ensure that the battery performs as promised. Since primary lithium batteries cannot be easily tested in conditions that accurately simulate in-field use, it is also helpful to gauge application-specific requirements against similar real-life examples that involve 25+ years of battery performance.
Figure 6. Powercast WSN-1101 wireless sensors quickly convert any structure into an energy-saving smart building. Use of a PulsesPlus battery enables Powercast to offer a 25 year warranty
A Temperature and Humidity Sensor
Powercast Corp. develops wireless power technologies and low-power RF systems, often using energy harvesting. The company chose PulsesPlus hybrid lithium batteries to power their WSN-1101 wireless sensor (Figure 6) that measures temperature and humidity for building automation, HVAC and lighting control, energy management, industrial monitoring, and medical applications. Powercast chose these batteries because of their proven ability to deliver reliable, maintenance-free performance for 25 years in similar short-range RF applications, giving the company the confidence to offer a 25-year warranty.
With its pre-installed, nonreplaceable PulsesPlus battery, the WSN-1101 can transmit data once per minute for >25 years while remaining inconspicuous. Designed for indoor applications with an operating temperature range of –20°C to 50°C, WSN-1101 wireless sensors communicate with Powercast's WSG-101 wireless gateway that supports up to 100 sensor nodes and 800 sensor ports. The WSG-101 gateway interfaces with wired building automation systems (BAS) networks via industry-standard protocols, including BACnet, Modbus, Metasys N2, LonWorks, XML, and SNMP. Data transmitted from these sensors can be integrated with HVAC systems from Johnson Controls, Trane, and other leading manufacturers.
Figure 7. KOHLER touchless commercial faucets combine energy-saving Insight technology with hybrid LiSOCl2 batteries to deliver 30 years of maintenance-free performance
A Touchless Faucet that Lasts 30 Years
Another notable application involves the KOHLER Hybrid energy system, a battery-powered, no-maintenance, water-saving faucet designed to last 30 years (Figure 7). Upon initial installation, KOHLER Insight Technology analyzes and logs feedback from its environment and recalibrates the factory default settings to accommodate its new home. If the bathroom space has low lighting, or highly reflective lighting, the sensor automatically adjusts to eliminate false actuations. This proven sensor technology does not require maintenance if it is used infrequently or if the lighting is less than adequate—challenges that plague other similar systems.
The Kohler Hybrid energy system uses an LiSOCl2 battery equipped with an HLC that collects small electrical charges, which are discharged every time the faucet is activated, allowing the battery to maintain its power storage. The use of energy-saving Insight Technology enables the hybrid energy system to operate continuously for 30 years without battery replacement.
These examples show how reliable 25+ years of battery performance could benefit a variety of commercial applications that require a long operational life in a variety of environments, such as RFID tags, GPS tracking systems, AMR/AMI, mesh networks, system control and data acquisition (SCADA), data loggers, measurement while drilling, oceanographic and environmental measurement, emergency/safety equipment, military and aerospace systems, and other remote sensors.
ABOUT THE AUTHOR
Sol Jacobs is the VP and General Manager of Tadiran Batteries, Lake Success, NY. Tadiran can be reached at 800-537-1368.