History offers important lessons for designers of today's most advanced wireless sensors.
Twenty-five years ago, Aclara™ (formerly Hexagram Inc.) pioneered the development of networked wireless sensors by installing hundreds of thousands of battery-powered automatic meter-reading (AMR) devices for the utility market. Miraculously, virtually all of these decades-old wireless devices are still going strong on their original Tadiran TL-2100 AA-size lithium thionyl chloride battery, with laboratory tests confirming that they have retained nearly 25 percent of their original capacity.
Aclara's example also demonstrates why continual improvement is essential for maintaining a competitive edge, as Aclara has since introduced a newer generation of STAR® Network wireless fixed-network AMR systems using an upgraded Tadiran TL-4903 "XOL" (eXtended Operating Life) AA-size lithium thionyl battery, which features 33 percent higher capacity (2.4 AH vs. 1.8 AH) and lower self-discharge than the TL-2100 battery.
Longevity Enhances Profitability
After carefully reviewing competing batteries, Aclara selected the TL-4903 battery for its superior performance characteristics, including higher energy density, higher capacity, a temperature range of 55 to 85C and proven 20-plus year service life.
STAR network meter transmitter units (MTUs) are capable of providing multiple daily readings using narrow-band radio frequency to communicate with data collection units (DCUs) strategically positioned on buildings or utility poles located approximately one to two miles apart. These units combine intelligent energy-saving design with advanced lithium thionyl chloride battery chemistry to deliver proven 20-year service life. Energy conservation is also a critical requirement, so STAR MTUs do not employ a receiver to listen for a "wake-up" transmission. Instead, a timer initiates data transmission at programmed intervals. Use of one-way transmission also results in a less complex, lower cost system.
The life cycle savings attainable with a 20-plus year AMR unit are substantial compared to a unit with an expected service life of seven to 10 years, as cost savings begin to accrue in year seven and continue throughout the 20-year product life cycle. For example, a utility with 5,000 service connections could realize up to $200,000 in total cost savings over a 10-year period by eliminating all battery changeouts during that 10-year period, assuming that the cost of each battery changeout is approximately $40 per site. If a second round of battery changeouts was eliminated over a 20-year period, the potential $400,000 in savings could be achieved for every 5,000 service connections. These savings are on top of the labor savings achieved by eliminating visual meter reading.
Lithium Batteries are not Created Equal
Through comprehensive in-house and independent tests, Aclara confirmed that batteries with identical chemistries offered significantly different performance. To date, Aclara has not qualified any other brand because none could match the quality of construction and electrolyte purity of the Tadiran lithium battery. Since a sealed, self-contained power management system was required, energy harvesting devices were considered unsuitable for this application.
The performance of a lithium thionyl chloride cell can vary 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. Self-discharge can also be negatively affected by high levels of impurities in the electrolyte. Extreme temperatures can also reduce battery performance by affecting voltage and self-discharge rate.
Battery performance is further hindered by impedance, resulting from the internal resistance created by the electrolyte, the anode and the cathode. Experienced battery manufacturers control impedance through the use of special additives. Use of fully automated manufacturing techniques also serve to ensure unparalleled lot-to-lot consistency in large volume production. Product quality can verified by ISO-9001:2000 certified quality systems, including fully documented and verifiable test results for parameters such as battery pulse, low-temperature pulses, discharge and repeatability.
Aclara's remarkable feat of 25-plus year service life offers important lessons for design engineers, demonstrating the synergistic value of combining innovative product design with long-life lithium batteries, thus attaining an optimal power management solution that reduces the total cost of ownership and maximizes long-term ROI.