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    Power Your Wireless Sensors For 40 Years

    Short-range wireless sensors are experiencing rapid growth in wide range of applications: RFID to GPS tracking, traditional automatic meter reading (AMR) plus advanced metering infrastructure (AMI), mesh networks, system control and data acquisition (SCADA), data loggers, measurement while drilling, oceanography, environmental systems, emergency/safety systems, military/aerospace systems, and more. Many of these applications rely on long-life lithium batteries with a potential lifespan of up to 40 years, especially in remote locations where battery replacement is difficult or impossible.

    However, actual battery life is often difficult to prove because it’s not particularly easy to test primary lithium batteries for lifespan in conditions that accurately simulate in-field use. Therefore, design engineers must be extremely diligent in demanding verifiable information from battery manufacturers to avoid unscheduled battery replacements, which can incur 10 times the initial cost of the original battery.

    LiSOCl2 Enables 40-Year Service

    Bobbin-type lithium-thionyl-chloride (LiSOCl2) chemistry is overwhelmingly preferred forremote wireless sensors because it offers the highest specific energy (energy per unit weight) and energy density (energy per unit volume) of all current battery chemistries(Fig. 1). Lithium delivers high energy density due to its large electric potential, which exceeds other metals. It produces the higher 2.7- to 3.9-V dc voltages typical of lithium batteries.

    1. Bobbin-type lithium-thionyl-chloride (LiSOCL2) chemistry is preferred for remote wireless sensors because of its high specific energy, high energy density, wide temperature range, and very low annual self-discharge rate.

    Lithium cells use a non-aqueous electrolyte, enabling certain LiSOCl2 batteries to operate in extreme temperatures typically ranging from –55°C to 125°C. Certain cells are adaptable to the cold chain, down to –80°C. For example, Tadiran tested LiSOCl2 cells in a cryogenic chamber and subjected them to progressively lower temperatures down to –100°C. These LiSOClcells continued to operate as necessary.

    LiSOCl2 chemistry is also renowned for long life. Formerly known as Hexagram, Aclara began using bobbin-type LiSOClbatteries in 1984 to power meter transmitter units (MTUs) for automated meter reading systems used by water and gas utilities (Fig. 2). These older devices are now being replaced by more technically advanced equipment, but the original batteries are still operating after 28 years in the field. This real-life example gives AMR/AMI equipment manufacturers the confidence to offer long-term performance contracts that increase the total return on investment (ROI) of an AMR/AMI network.

    2. LiSOCL2 batteries have been utilized for decades in AMR applications, with some systems still operating on their original batteries after 28 years.

    Not All Lithium Batteries Are Created Equal

    While many battery manufacturers claim low annual self-discharge rates at ambient temperatures, such claims may be invalid depending on the construction method or specific design requirements. For example, testing on Tadiran batteries shows that these cells have an average self-discharge of approximately 0.7% per year, while other batteries using the same chemistry have 2.5% to 3% annual self-discharge.

    The use of inferior raw materials or non-standard manufacturing techniques can lead to uneven battery performance. This includes batch-to-batch inconsistencies that raise the risk of anomalies in the field, even if initial performance characteristics seem identical. As a result, advanced manufacturing processes based on Six Sigma and statistical process control (SPC) methodologies are required to ensure consistent product quality.

    When it comes to selecting the ideal battery, each application is unique in terms of a set of application-specific parameters:

    • Overall energy consumption during sleep mode
    • Energy consumption during active mode entailing the size, duration, and frequency of high current pulses, where applicable
    • Battery self-discharge rate, which is sometimes higher than the actual sensor average-use rate
    • Equipment cutoff voltage
    • Length of storage periods
    • Thermal environments

    Experienced battery manufacturers know how to create a customer-specific energy-use profile along with sensitivity analyses. The end result is a mathematical model that accurately predicts battery-life expectancy.

    Two-Way Requirements

    Wireless sensors are increasingly providing “on demand” two-way RF communications, with the device operating in two modes. One is a dormant or sleep state where daily power consumption ranges from nil to a few microamps. The other is an active interrogation and transmission mode requiring high current pulses up to hundreds of milliamps for short-range RF communications to a few amps for certain GPRS protocols.

    If a wireless sensor remains dormant for extended periods at elevated temperatures and is occasionally interrupted by a high current pulse, lower transient voltage could result during initial battery discharge, especially in low temperatures. This phenomenon, known as transient minimum voltage (TMV), is strongly related to the quality of battery electrolyte or cathode.

    One alternative is to combine supercapacitors with lithium cells, a solution that tends to fail prematurely due to relatively high self-discharge. A supercapacitor comprising dual 2.5-V capacitors also needs a balancing circuit to ensure acceptable service life. And, supercapacitors have a limited temperature range, disqualifying them for use in some applications.

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