Choosing the right battery for a remote IIoT application is essential to reducing ongoing maintenance costs and the replacement of failing batteries.

The choice of power source is increasingly complex for remote wireless devices operating in hard-to-access locations, where extreme temperatures can reduce battery performance and hard-wired power is either inaccessible or cost prohibitive. Common applications include industrial robotics, SCADA, process control, asset tracking, safety systems, tank level and flow measuring, environmental monitoring, AI, M2M, and wireless mesh networks, to name a few.

Ultra-long-life lithium batteries enable low-power devices to improve operational efficiencies, track assets, optimize supply chains, reduce environmental impact, initiate cost-saving predictive maintenance programs, and more. The vast majority of these devices draw average current measurable in micro-amps with pulses in the multi-amp range, making them ideally suited for primary (non-rechargeable) lithium batteries. There are also niche applications that draw higher amounts of average current measurable in milli-amps with pulses in the multi-amp range, often better suited for energy harvesting in conjunction with an industrial grade rechargeable lithium-ion (Li-ion) battery to store the harvested energy.

To address the unique requirements of energy harvesting applications, an industrial grade Li-ion battery has been developed that can last up to 20 years, endure 5,000 full recharge cycles, and be recharged and discharged at temperatures down to –40°C.

Ultra-long-life lithium batteries

To achieve extended battery life, remote wireless devices must be designed to conserve energy wherever possible, mainly using low-power chipsets, low-power communications protocols (e.g., WirelessHART, ZigBee, LoRa), and proprietary techniques to minimize energy consumption during ‘active’ mode. While valuable, these energy-saving techniques are often far less important than the choice of battery.

Numerous primary (non-rechargeable) battery chemistries are available for low-power devices. Least expensive is the ubiquitous alkaline cell, which delivers high rates of continuous current with the trade-off being a high self-discharge rate of up to 60% per year, making them ill-suited for long-term deployments. Alkaline cells have very low capacity and low energy density, which may result in added size and bulk. In addition, alkaline cells use a water-based chemistry that is prone to freezing.

On the opposite side of the spectrum are the lithium-based chemistries used in industrial applications. As the lightest non-gaseous metal, lithium features an intrinsic negative potential that exceeds all other metals, delivering the highest specific energy (energy per unit weight), highest energy density (energy per unit volume), and higher voltage (OCV) ranging from 2.7V to 3.6V. Lithium cells are also non-aqueous and less prone to freezing than alkaline cells. 

Bobbin-type LiSOCl2 chemistry

Among primary lithium chemistries, bobbin-type lithium thionyl chloride (LiSOCl2) chemistry is widely preferred for remote wireless applications because it delivers the highest capacity, highest energy density, and widest temperature range of all (–80°C to +125°C). Bobbin-type LiSOCl2 batteries also feature an extremely low self-discharge rate as low as 0.7% per year, enabling certain cells to last up to 40 years.

Achieving lower self-discharge

Self-discharge is common to all batteries. Chemical reactions reduce the energy stored in the cell without any connection between the electrodes and any external circuit. Remote wireless devices often lose more energy annually to self-discharge than is required to operate the device.

Bobbin-type LiSOCl2 cells can minimize self-discharge by harnessing the passivation effect, whereby a thin film of lithium chloride (LiCl) forms on the surface of the anode to separate it from the electrode, thus limiting the chemical reactions that cause self-discharge. When a current load is applied to the cell, the passivation layer causes initial high resistance and a drop in voltage until the discharge reaction begins to dissipate the passivation layer: a process that repeats each time a load is applied.

The amount of passivation can vary based on numerous factors, including cell construction, current discharge capacity, the length of storage and storage temperature, discharge temperature, and prior discharge conditions, as partially discharging a cell and then removing the load will decrease the passivation effect over time.

Experienced battery manufacturers harness the passivation effect through proprietary cell design and construction, including the use of higher quality raw materials. A superior grade bobbin-type LiSOCl2 battery can achieve a self-discharge rate as low as 0.7% per year, able to retain 70% of its original capacity after 40 years. By contrast, an inferior quality bobbin-type LiSOCl2 cell can have a self-discharge rate as high as 3% per year, exhausting 30% of its available capacity every 10 years, making 40-year battery life impossible.

A hybrid approach

High pulses of up to 15A are required to initiate two-way wireless communications. Standard bobbin-type LiSOCl2 cells cannot deliver such high pulses due to their low-rate design but can be modified with the addition of a patented hybrid layer capacitor (HLC). This hybrid approach uses the bobbin-type LiSOCl2 cell to deliver low-level background current during ‘standby’ mode, while the HLC delivers the high pulses required during ‘active’ mode. The patented HLC features a unique end-of-life voltage plateau that can be interpreted to deliver ‘low battery’ status alerts for predictive maintenance programs.

Supercapacitors can also store high pulses but are mainly limited to consumer electronics due to serious drawbacks that do not allow for the use of all available energy, low capacity, low energy density, and high self-discharge rates of up to 60% per year. Supercapacitors linked in series require the use of bulky cell-balancing circuits, which adds expense and drains additional current to further shorten their operating life. However, supercapacitors can be utilized in conjunction with bobbin-type LiSOCL2 cells to enhance voltage response.

What to look for

The ideal power source should last for the entire lifetime of the device, thus reducing or eliminating the need for costly battery changeouts. Unfortunately, a superior grade battery can be difficult to distinguish from an inferior grade cell because annual capacity losses may take years to become fully measurable. Additionally, the algorithms and theoretical models used to calculate battery life expectancy tend to underestimate the passivation effect as well as long-term exposure to extreme temperatures.

Since theoretical models tend to inaccurately predict expected battery life based on short-term data, careful due diligence is required to specify the ideal battery. To properly evaluate competing battery brands, potential suppliers should be required to provide fully documented test reports, including theoretical models verified by historic data, as well as in-field performance data from similar devices operating under comparable loads and temperatures.

Identifying the ideal battery based on application-specific requirements will serve to maximize product performance while reducing the long-term cost of ownership.