Reliable, long-life batteries are capable of powering remote sensors for over 25 years
Selecting a primary battery capable of delivering 25 years of continuous operation is an important consideration, especially in situations where the battery replacement is difficult or impossible or not cost effective, as battery failure is commonly identified as the first or second major factor contributing to overall system failure.
The need for remote sensors capable of delivering reliable, long-term performance is exploding. The growing list applications includes RFID tags, GPS tracking systems, automatic meter reading (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. Newer generations of remote sensors offer enhanced functionality such as periodic two-way communications or remote shut-off capabilities. These performance features drain power, which, in turn, reduces battery life expectancy. Meeting the power hungry demands of increasingly sophisticated remote sensors can only be achieved through proper battery selection.
Delivering long life requires long-term experience
Among the different battery chemistries available for long-term use in remote sensors, lithium thionyl chloride (Li-SOCl2) is overwhelmingly preferred because it combines high energy density, a wide temperature range, low annual self-discharge, and high voltage.
LiSOCL2 batteries have a proven history of success in remote sensor applications, having first been deployed in utility automatic meter reading applications back the late 1970s. As a result, certain battery manufacturers have accumulated more than 30 years of practical experience in the field, thus able to continuously refine and perfect long-term power management solutions.
Selecting high quality grade components to ensure controlled aging of batteries
Many remote sensors have to withstand demanding environmental conditions, including extreme temperatures ranging from -40°C to +85°C, high humidity (up to 90 percent in some regions), and dust. For example, LiSOCl2 batteries are used power E-ZPass electronic toll tags, which are subjected to extreme heat, vibration and rapid temperature cycling, but must work reliably for decades. LiSOCL2 batteries are also utilized to power tens of millions of wireless automated meter reading (AMR) units operating in all types of conditions, from artic cold to desert heat, RFID tags that must withstand the prolonged heat of autoclave sterilization cycles, and data in loggers that must work continuously in the cold chain.
To ensure 25+ year battery life, experienced battery manufacturers utilize superior quality raw materials coupled with proprietary manufacturing techniques. For example, top quality laser welding of enclosures and high-end glass-to-metal seals are required to ensure that LiSOCl2 batteries remain hermetically sealed for a lifetime. High quality materials capable of fighting corrosion must also be specified to avoid the potential for electrolyte leakage or short circuit, which can degrade long-term performance.
Accurately estimating battery life expectancy requires a detailed understanding of application-specific parameters that can impact overall performance. For example, prolonged exposure to extreme temperatures can affect battery performance, as low temperatures tend to slow down electrochemical reactions, whereas exposure to high temperatures may generate voltage delays or increase the internal resistance. The following application-specific parameters may need to be evaluated in order to accurately predict battery life expectancy:
• Overall energy consumption (base current, as well as the size, duration, and frequency of high current pulses, where applicable)
• Storage periods
• Thermal environments
• Equipment cut-off voltage
Leading battery manufacturers have developed life expectancy models based on application-specific utilization profiles. Such proprietary tools enable lifetime predictions and sensitivity analysis of critical parameters impacting expected battery lifetime can be provided.
Many remote sensors devices operate with “dormant” phases, where daily power consumption ranges from nil to a few microamps, followed by “active” modes requiring high current pulses of up to hundreds of miliamps for short range RF communications, or up to a few amps for certain GPRS protocols. Applications that involve dormant periods at elevated temperatures, alternating with periodic high current pulses, can lead to lower transient voltage readings during the initial phase of the discharge.
This phenomenon, known as voltage delay, is strongly linked to core choices regarding the constitution of the battery electrolyte, or the definition of the cathode. It may vary significantly from one battery manufacturer to another one, depending on their ability to master complex electrochemical phenomena.
Low self-discharge is required for long-term operations
Low annual self-discharge is a critical to long-life performance. While many battery manufacturers claim an average self-discharge of less than 1 percent per year at ambient temperatures, this claim may be invalid depending on the size of the cell, its method of construction, or application-specific temperature requirements.
Significant differences can be observed when comparing results from different manufacturers. Figure 1 compares self-discharge between two competing battery manufacturers with all conditions being equal, using data obtained through microcalorimetry.
While both manufacturers exhibited self-discharge rates that can be considered extremely low, once you factor in the difference of a few microamps as described in figure 1, the long-term impact on battery life over 20 years becomes significant as demonstrated in Figure 2 below.
Manufacturing processes can make all the difference
Achieving reliable battery performance requires the use of superior quality materials and advanced proprietary manufacturing processes. Use of inferior raw materials or non-standardized battery manufacturing techniques can lead to batch-to-batch inconsistency, which severely impacts long-term battery performance, even if initial performance characteristics seemed identical. Considering the potential for poor workmanship associated with manual assembly, only highly automated and reproducible manufacturing techniques should be trusted, especially if there is a product warranty tied to battery life expectancy.
For example, Powercast, a company that designs and manufactures wireless sensors for building automation and HVAC monitoring/control, recently introduced a temperature and humidity sensor capable of providing maintenance-free operation for 25+ years on a single Tadiran LiSOCL2 battery. While Powercast generally utilizes energy harvesting technologies, for this particular application they chose Tadiran LiSOCL2 batteries because they have been field-proven to operate for 25+ years, giving Powercast the confidence to offer a 25-year warranty.
While 25-year service life is becoming an international standard for long-life performance, proving it is still a challenge, as primary lithium batteries cannot be easily tested after manufacturing in conditions that accurately simulate actual in-field use. To ensure consistent performance, total quality management tools are necessary to mitigate any risk of anomaly in the field. For instance, in addition to comprehensive battery qualification programs, use of Six Sigma methodology and statistical process control (SPC) are required to ensure the highest level of product quality.
With battery replacement costs estimated at ten times the initial cost of the original battery, design engineers must perform careful due diligence to verify the accuracy of battery manufacturer claims involving battery life expectancy. In situations where a 25-year battery lifetime is becoming essential, the experience of the battery manufacturer becomes pivotal, as battery performance over time cannot be easily simulated. The heritage of the battery and the consistency of its manufacturing must be known in order to differentiate real versus theoretical battery life expectancy.