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Achieving Long

Jun 27, 2023Jun 27, 2023

Ultra-long-life lithium batteries power remote wireless devices throughout the IIoT, with certain cells operating up to 40 years. This feature originally appeared in the IIoT & Industry 4.0 edition of Automation 2023.

Extended life batteries are essential to remote wireless devices utilized throughout the IIoT, providing a major cost benefit by reducing or eliminating the need for battery replacements. Use of an ultra-long-life battery can translate into significant cost savings for remote wireless applications by eliminating the labor expenses related to battery replacement, which invariably exceeds the cost of the battery itself. This money-saving benefit is especially important for remote wireless devices being deployed in remote locations and hostile environments, where battery access can be highly cost prohibitive and sometimes impossible.

There are two types of low-power devices. The vast majority of these devices operate mostly in a "stand-by" state and draw average current measurable in micro-amps with pulses in the multi-amp range to power two-way wireless communications. These applications generally rely upon industrial-grade primary (non-rechargeable) lithium batteries, especially when battery access is limited or in harsh environments. If the battery is easily accessible for replacement and operates within a moderate temperature range, then consumer grade batteries could be considered as a more economical solution.There are also certain niche applications that draw average energy measurable in milli-amps with pulses in the multi-amp range, consuming enough average energy to shorten the operating life of a primary battery. These higher drain applications could require the use of an energy harvesting device in conjunction with a rechargeable Lithiumion (Li-ion) battery to store the harvested energy. Industrial grade Li-ion batteries are now available that can operate for up to 20 years.Numerous types of primary (non-rechargeable) chemistries are available, each offering unique performance benefits and trade-offs. These chemistries include alkaline, iron disulfate (LiFeS2), lithium manganese dioxide (LiMnO2), lithium thionyl chloride (LiSOCl2), and lithium metal-oxide (Table 1).Table 1: Bobbin-type LiSOCl2 batteries are preferred for use in remote wireless applications. These cells deliver higher capacity and energy density, up to a 40-year operating life, and the widest possible temperature range, which is ideal for hard-to-access locations and extreme environments.Among these primary chemistries, bobbin-type LiSOCl2 (Figure 2) is overwhelmingly preferred for long-term deployments in remote locations due to its higher capacity and energy density, wider temperature range, and an incredibly low annual self-discharge rate of less than 1% per year for certain cells.

IIoT-connected devices utilize two-way wireless communications, thus demanding specialized power management solutions. To maximize battery life, these devices must be engineered to conserve energy by employing a variety of energy-saving techniques, including the use of a low power communications protocol (WirelessHART, ZigBee, LoRa, etc.), low-power chipsets, and proprietary techniques designed to minimize energy consumption when the device is in "active" mode. While extremely useful, these energy-saving techniques are often dwarfed by the energy losses associated with annual self-discharge.Self-discharge is common to all batteries, as chemical reactions occur even when a cell is disconnected or in storage. The annual selfdischarge rate of a battery can vary considerably based on its chemistry, the design of the cell, the current discharge potential, the quality and purity of the raw materials, and, most important, the ability to harness the passivation effect.Unique to LiSOCl2 batteries, passivation involves a thin film of lithium chloride (LiCl) that forms on the surface of the lithium anode to limit reactivity while not in use. LiSOCl2 cells can be constructed two ways: bobbin-type cells feature less reactive surface area, which is ideal for reducing self-discharge. However, the trade-off is an inability to deliver high rate energy. LiSOCl2 batteries can also be made with spiral wound construction, which permits a higher rate of energy flow, with the trade-off being shorter operating life due to higher self-discharge.Whenever a load is placed on the cell, the passivation layer causes initial high resistance and a temporary dip in voltage until the discharge reaction begins to dissipate the LiCl layer, a process that repeats each time the load is removed. The cell's ability to harness the passivation effect can be influenced by its current capacity; length of storage; storage temperature; discharge temperature; and prior discharge conditions, as removing the load from a partially discharged cell increases the level of passivation relative to when it was new.Experienced battery manufacturers can optimize the passivation effect through the use of higher quality raw materials and by employing proprietary manufacturing techniques. While passivation can be highly beneficial to reducing the annual self-discharge rate, this process needs to be carefully harnessed to avoid over-restricting energy flow.

While standard bobbin-type LiSOCl2 cells are ideal for harnessing the passivation effect, they are unable to generate the high pulses required for two-way wireless communications due to their low-rate design. This challenge can be overcome with a hybrid solution, where the standard bobbin-type LiSOCl2 cell is used to deliver low-level background current while being augmented by a hybrid layer capacitor (HLC) that stores and delivers high pulses (Figure 3).

Major differences can exist between seemingly identical bobbin-type LiSOCl2 cells. For example, a superior quality bobbin-type LiSOCl2 battery can feature a self-discharge rate as low as 0.7% per year and is able to retain 70% of its original capacity after 40 years. By contrast, an inferior quality cell can have a higher self-discharge rate of up to 3% per year and lose 30% of its capacity every 10 years, making 40-year battery life unattainable.Choosing the ideal battery can be difficult, in part because the annual energy losses associated with higher self-discharge can take years to become fully apparent, and the predictive models used to estimate expected battery life tend to underestimate the passivation effect as well as the impact of long-term exposure to extreme temperatures. Various testing procedures are available to approximate expected battery life, with the best source being historical test data taken from cells being used in the field.When extended battery life is essential to maximizing your return on investment (ROI), it pays to perform some added due diligence by demanding fully documented long-term test results along with historical in-field test data involving comparable devices under similar loads and environmental conditions. By paying more careful attention when evaluating competing batteries, you can achieve significant longterm savings by increasing the reliability and service life of your device.This feature originally appeared in the IIoT & Industry 4.0 edition of Automation 2023.

Sol Jacobs is the vice president and general manager of Tadiran Batteries. He has more than 30 years of experience in powering remote devices. His educational background includes a BS in engineering and an MBA.

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