Powering Remote Wireless Devices to Run a 40-year Marathon

Summary

The Industrial Internet of Things (IIoT) is exploding, resulting in the dramatic growth of remote wireless devices being powered by primary (non-rechargeable) lithium batteries. Often, these devices need to operate maintenance-free for decades without having to replace the batteries.

To achieve such extended battery life, energy losses must be minimized. This is especially true for lowpower devices that draw average current measurable in micro-amps while operating mainly in an energysaving “standby” state, fully awakening only to sample or transmit data.

Battery-powered wireless devices deployed in extreme environments and hard-to-access locations are generally illsuited for consumer grade alkaline cells that are extremely short-lived, primarily due to a high self-discharge rate of up to 60 percent per year. Lithium chemistries can last much longer.

Lithium features a high intrinsic negative potential that exceeds all other metals, functioning within an operating current voltage (OCV) ranging from 2.7 V to 3.6 V. Lithium chemistries are also non-aqueous, able to endure extreme temperatures with less risk of freezing. Chemistries such as iron disulfate (LiFeS2 ) and lithium manganese dioxide (LiMNO2 ) can deliver medium to high rates of energy discharge. However, there is a trade-off, as these chemistries feature annual selfdischarge rates that are far lower than alkaline but far greater than lithium thionyl chloride (LiSOCl2 ) (Table 1).

Not all batteries can handle a 40-year marathon

Among all commercially available lithium chemistries, bobbin-type LiSOCl2 batteries are preferred for long-term deployments in remote locations and extreme environments due to their ruggedness and ultra-long-life potential. Bobbin-type LiSOCl2 cells deliver the highest capacity and highest energy density of any lithium chemistry, along with an extremely low annual self-discharge rate (less than 1 percent per year), enabling certain low-power devices to operate for up to 40 years on their original battery. Bobbin-type LiSOCl2 batteries also offer the widest possible temperature range (-80°C to 125°C), along with a glass-to-metal hermetic seal that resists battery leakage. Common applications for LiSOCl2 cells include AMR/AMI metering, machineto-machine (M2M), supervisory control and data acquisition (SCADA), tank-level monitoring, asset tracking, and environmental sensors, to name a few (Figure 1).

Harnessing the passivation effect

All batteries experience some amount of annual self-discharge as chemical reactions occur even when the battery is disconnected or not in use. Self-discharge can be significantly reduced by harnessing the passivation effect, which is unique to LiSOCl2 chemistry.

Passivation occurs when a thin film of lithium chloride (LiCl) forms on the surface of the lithium anode to impede the chemical reactions that result in battery self-discharge. Whenever a load is placed on the cell, the passivation layer causes high initial resistance, resulting in a temporary drop in cell voltage until the discharge reaction slowly dissolves the passivation layer—a process that repeats each time a load is applied.

Passivation levels can be affected by several variables, including the current capacity of the cell, the length of storage, storage temperature, discharge temperature, and prior discharge conditions, as partially discharging a cell and then removing the load can increase the amount of passivation relative to when the cell was new. While passivation can serve to reduce a battery’s self-discharge rate, too much of it can cause energy flow to be blocked.

The ability of a bobbin-type LiSOCl2 cell to harness the passivation effect can vary significantly based on the quality of the raw materials and the method by which the battery is manufactured. For example, lower quality bobbin-type LiSOCl2 batteries can lose up to 3 percent of their original capacity annually due to self-discharge, thus exhausting 30 percent of their initial capacity every 10 years, making 40-year battery life impossible. By contrast, the highest quality bobbin-type LiSOCl2 batteries can feature a self-discharge rate as low as 0.7 percent per year, retaining 93% of their original capacity after 10 years, thus enabling up to 40-year operating life.

The analogy to marathon running

Distance: Distance is equivalent to the battery/device operating life. The farther a runner can travel equates to the more years a device can potentially operate (Figure 2).

Incline: An incline is equivalent to the rate of battery self-discharge. The larger the incline, the higher the rate of self-discharge (Figure 3). When athletes run up a steep incline, they expend greater amounts of energy, which shortens the maximum duration of the run. Similarly, higher rates of battery self-discharge expend greater amounts of energy to reduce battery operating life.

Drawbacks of simulated tests

Long-term battery performance cannot be easily duplicated using short-term testing procedures. As a result, specialized test methods must be used to predict expected battery operating life. These testing methods include:

• Long-term laboratory testing: The ideal way to monitor longterm battery self-discharge is by continually testing random samples under various operating and environmental conditions. Over time, the accumulated data points can be used to accurately predict expected battery life based on cell size, temperature, load size, etc.
• ​Accelerated testing: The Arrhenius equation uses a two-fold increase of reaction rate for every 10°C rise in temperature to simulate long-term battery operation. Arrhenius tests are run at 72°C, which is equivalent to about 32 times the theoretical lifetime of a battery stored at 22°C. Unfortunately, the Arrhenius equation tends to yield inaccurate results with short-term tests.
• Calorimeter testing: An extremely accurate tool for predicting battery life is to measure the amount of actual heat energy being lost using a high-quality microcalorimeter. This device can detect small amounts of energy being dissipated as low as 0.1 W. This heat energy can be generated several ways: entropy change, often referred to as reversible heat; cell over-protection, often referred to as irreversible heat; chemical reactions that can affect self-discharge reactions and cell capacity; and side reactions that do not affect cell capacity. Calorimeter testing can be especially useful for measuring battery capacity losses resulting from longterm storage or device operation (including self-discharge), which can be measured using thermodynamic equations and cell voltage considerations. To ensure accuracy, prior to undergoing calorimetric testing, the battery needs to be stabilized for a minimum of one year as the self-discharge rate during the first year tends to be much higher than in subsequent years.
• Lithium titration: Lithium titration can also be used to measure available cell capacity. The battery is cut open, and then titration is used to dissolve the remaining lithium to determine its volume. High rates of self-discharge will accelerate the reduction of lithium found in the cell. Lithium titration techniques can be useful for measuring the effects of extreme temperatures or prolonged high pulses.
• Field results: Perhaps the best form of validation is to test a random sample of cells being deployed in the field. For this reason, Tadiran customers are asked to provide random samples of cells being used in the field to measure the real-life impact of longterm exposure to extreme environmental conditions. For example, AMR/AMI batteries deployed by Aclara (formerly Hexagram) in the mid-1980s were tested after more than 28 years in the field and were found to contain plenty of unused capacity after nearly three decades.

Final thoughts

_{This feature originally appeared in the ebook Automation 2022 Volume 6: IIoT & Industry 4.0.}