Faithfully applying common sense practices during the vendor selection process will help ensure decades of trouble-free battery performance for remote wireless devices,

Remote wireless sensors use lithium batteries to achieve 25+ year service life, leading to increased reliability and reduced maintenance costs

From RFID electronic toll tags mounted on car dashboards, to AMR/AMI utility meters that monitor water or energy consumption at homes and offices, to remote sensors that track the whereabouts of products in transit, remote wireless sensors are now integral to modern society.

Hundreds of millions of remote wireless devices are in use worldwide in applications ranging from RFID and GPS tracking systems, to automatic meter reading (AMR), mesh networks, smart power grids, 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 sensing devices.

As momentum builds for a wireless world, the need for reliable, long-term battery power management solutions has intensified, since batteries serve as the lifeblood of wireless devices, providing the essential power required for reliable data collection, storage and communications. The need for long-life batteries is especially critical for remote wireless applications involving extreme environmental conditions where battery replacement can be difficult or impossible.  

While remote wireless applications are predominantly powered by primary lithium batteries, in rare instances energy harvesting devices have been considered as a means of converting sunlight, heat or vibration into electricity. Energy harvesting devices have limited potential for remote wireless applications due to performance limitations such as high cost, large size, and reliability concerns. In addition, energy harvesting devices typically require rechargeable batteries to store energy, which defeats their value proposition, as rechargeable batteries are expensive, offer reduced service life, and involve chemistries that are less environmentally-friendly.

Contradicting Design Requirements

Oak Ridge National Laboratory recently conducted a study to identify the ideal wireless sensor. Not surprisingly, they concluded that the #1 criteria for the ideal sensor was “adequate battery life,” followed by “the need to be self-powered and small.” These conclusions seem entirely logical, as technological advancements invariably lead to miniaturization, forcing design engineers and battery manufacturers to develop increasingly sophisticated power management systems that utilize smaller footprints without sacrificing power and performance.

Achieving these contradictory goals involves trade-offs, so design engineers need to conduct thorough due diligence in order to ensure that the optimal power management solution is being specified. As a result, numerous variables must be factored into a product specification matrix involving such performance parameters as voltage, capacity, size and weight, expected service life, temperature and/or environmental issues, and cost. In certain instances, requirements for high current pulses or high discharge rate must also be factored into the equation.

Once key performance requirements have been identified, then competing chemistries can be effectively compared. The typical choices include alkaline, zinc-carbon-ammonium chloride (Leclanche cell), zinc-carbon-zinc chloride, zinc-air, and lithium.

If the application is easily accessible and requires relative short battery service life within a moderate temperature range, then standard alkaline, poly carbon monoflouride (Li/CFX), or manganese dioxide (Li/MNO₂) chemistries may be acceptable. However, if the application requires extremely long battery life, an extended temperature range, or reduced size and weight, then lithium is the preferred chemistry due its intrinsic negative potential, which exceeds that of all other metals.

As the lightest non-gaseous metal, lithium offers the highest specific energy (energy per unit weight) and energy density (energy per unit volume) of all battery chemistries. Lithium cells, all of which use a non-aqueous electrolyte, have normal OCVs of between 2.7 and 3.9V. The absence of water allows certain lithium batteries to operate in extreme temperatures (-55°C to 125°C), making them ideal for use in remote locations where hardwired AC power is either unavailable or not cost effective.

Choosing Among Lithium Batteries

Contrary to popular misconceptions, the lithium family of chemistries is quite diverse, including poly (carbon monoflouride lithium (CF) X-Li; manganese dioxide lithium (LiMnO₂); lithium thionyl chloride (LiSOCl₂); lithium sulfur dioxide (LiSO₂); lithium sulfuryl chloride (LiSO₂Cl₂); and lithium iodine (LiI₂). These chemistries are all named after the cathode material.

Lithium chemistries differ substantially in terms of performance characteristics such as nominal, minimal and maximum voltage; initial, average and maximum discharge current; ability to handle intermittent operation; amplitude and duration of minimum and peak current requirements; maximum service life; maximum and minimum voltage capabilities; temperature range; safety factor; and storage requirements.

Cylindrical poly carbon monoflouride lithium cells are manufactured with spiral shaped cathodes and crimped elastomer seals, delivering an OCV of 2.8V and moderately high energy density. Although generally safe, under extreme conditions, the elastomer seal can fail, causing reactive constituents to escape.

Manganese dioxide lithium cells deliver roughly equivalent energy density, OCV and safety factors as poly carbon monoflouride lithium cells, but offer only half the expected service life. Manganese dioxide lithium cells are best suited to applications with relatively high continuous-current or high current-pulses due to their low internal impedance.

Lithium iodine cells are considered highly safe because the cell constituents are solid. A major drawback to lithium iodine chemistry is its high internal impedance, which does not support low current draw requirements.

Sulfur dioxide lithium cells, commonly used in military and aerospace applications, feature lower energy density than manganese dioxide or poly carbon monoflouride lithium cells, and only half the energy density and service life of lithium thionyl chloride cells.

Lithium thionyl chloride batteries are extremely well suited for long-term applications with low continuous-current or moderate current-pulse requirements. These batteries feature the highest energy density of all lithium chemistries, as well as very low self-discharge and an extremely wide temperature range.

A Choice Of Lithium Thionyl Chloride Cells

Within the lithium thionyl chloride family of batteries there are two distinct designs: spiral wound (jelly roll) cells; and bobbin-type cells.

Spiral wound lithium thionyl chloride cells feature an energy density of 800 Wh/I, a temperature range of -55°C to 85°C, and a maximum service life of approximately 10 years. Although capable of delivering high current pulses, these batteries cannot offer the capacity and operating life of bobbin-type cells, as their multiple wound layer design increases surface area, resulting in higher current draw and higher self-discharge.

Bobbin-type lithium thionyl chloride cells are ideally suited to long-term remote wireless sensor applications due to their high energy density (1420 Wh/I), high capacity, their ability to withstand extreme temperatures (-55°C to 125°C), and their extremely long service life (20+ years), resulting from very low self-discharge (less than 1% per year). These batteries are also designed with a unique safety feature that protects the cell against extreme temperature, pressure, puncture, shock and vibration. Bobbin-type cells are also extremely well adapted to low current applications due to their high energy density and low self-discharge.

Lithium thionyl chloride batteries have a proven track record in remote wireless applications. For years, these batteries have been used to power EZ-Pass electronic toll tags. Also, in 1984, Hexagram (now Aclara), introduced their first automatic meter reading (AMR) devices for the gas and electric utility market, which was powered by AA-size lithium thionyl chloride batteries. Over 3 million of these devices have been deployed worldwide, and virtually all continue to operate on their original batteries after 25 years. Long-term reliability is especially critical to the utility industry, as extended battery life translates into higher productivity and profitability by eliminating the need for system-wide battery change outs.

Meeting The Needs Of High Current-Pulse Applications

A growing number of remote wireless sensor applications require high current pulses to capture, store and transmit data via cell phone, the internet, or low power RF communication protocols such as ZigBee. To address this growing need, engineers at Tadiran developed PulsesPlus®, a patented hybrid battery that combines a bobbin-type primary cell with a high rate, low impedance hybrid layer capacitor (HLC) to store and discharge high current pulses. 

PulsesPlus batteries exhibit qualities of both a battery and a capacitor. The battery and HLC working in parallel, with the battery supplying long-term low-current power while the HLC stores current-pulses up to 15A, eliminating the voltage drop that normally occurs when a pulsed load is initially drawn. The rate at which energy can be stored by the HLC varies from 280 A/Sec. with smaller HLCs, to 1,120 A/Sec. with larger size HLCs. PulsesPlus batteries also offer the potential for an end-of-life indication when the battery reaches 5-10% of its remaining available capacity. PulsesPlus battery technology is currently employed in millions of wireless sensors. Variations of this hybrid technology are also being utilized to provide short duration high rate power for military and medical applications.

One alternative solution involves combining a discreet capacitor with a primary cell, which is unnecessarily bulky and results in a higher rate of charge leakage, as the discreet capacitor continuously discharges the battery, albeit at a low rate.

Another alternative involves the use of super capacitors working in series with the primary cell. These electrochemical systems store electrical charges in bulk electrolytes rather than on plates. Super capacitors are small and lightweight, but they cause higher impedance, which limits the amount of instantaneous current available to deliver high current pulses. A super capacitor made up of multiple 2.3V units working in series will tend to have balancing and current leakage problems that limit their service life, whereas the HLC is a single unit that works in the 3.6V to 3.9V nominal range to avoid the balancing and current leakage problems associated with super capacitors.

Wireless sensors need to be intelligently designed in order to minimize power consumption and extend battery life. For example, AMR meter transmitter units (MTUs) typically operate in three modes: a sleep or standby mode, where power consumption is nil or a low background current; a measurement or interrogation mode, which requires a few hundred milliamps; and a transmission mode, which may require high current pulses for relatively brief intervals before returning to an energy-saving sleep or standby status to extend battery life. 

Manufacturing Processes Can Affect Battery Life

While the theoretical service life of a bobbin-type lithium thionyl chloride battery is over 20 years, actual service life varies based on the self-discharge rate, which is governed by the chemical composition of the electrolyte, the manufacturing processes used, as well as mechanical and environmental considerations. Battery performance and self-discharge can also be negatively affected by high levels of impurities in the electrolyte, as well as by impedance resulting from the internal resistance created by the electrolyte, the anode, and the cathode. Experienced battery manufacturers can control impedance by blending special additives into the electrolyte.

Since manufacturing and quality systems can have a significant effect on battery service life, design engineers need to perform thorough due diligence to verify that the lithium battery being specified can deliver as promised. With knock-off products flooding the marketplace, added vigilance is require to ensure product quality and authenticity, including 100% product traceability back to the raw materials. As part of the vendor selection process, it is recommended that potential battery suppliers be required to provide a list of customer references along with fully documented and verifiable test results for parameters such as battery pulse, low-temperature pulses, discharge and repeatability.

Faithfully applying these and other common sense practices during the vendor selection process will help ensure decades of trouble-free battery performance for remote wireless devices.

For more information email sales@tadiranbat.com or visit www.tadiranbat.com.