The HART communications protocol continues to provide a critical link to nearly 30 million intelligent field instruments and host systems used in SCADA, process control, asset management, safety systems, M2M (machine to machine), AI (artificial intelligence), wireless mesh networks, and numerous other applications.
HART-driven network control systems have been adopted globally to monitor and control field instruments across process industries and manufacturing facilities, as well as in power generation plants. These industrial control systems consist of a single instrument or a group of instruments that form a single- or multiple-loop network based on their design and deployment.
Amid the wide implementation of HART-enabled devices, millions of these devices remain un-networked due to the exorbitantly high cost of hard-wiring them to the electrical grid, with is especially problematic in remote locations and extreme environments.
To address this problem, Linear/Dust Networks developed WirelessHART, a low-power communications protocol specifically designed for use in tandem with bobbin-type lithium thionyl chloride (LiSOCl2) batteries.
Choosing lithium batteries
Low-power wireless devices extend battery life by operating predominantly in a standby state, awakening only to query or transmit data on a predetermined schedule or if certain data thresholds are exceeded. As a result, more energy is typically expended by annual self-discharge than actual battery use.
Since a wireless device is only as reliable as the batteries that power it, product designers must prioritize between desired attributes, including: energy consumed in active mode (including the size, duration, and frequency of pulses); energy consumed in dormant mode (the base current); storage time (as normal self-discharge during storage diminishes capacity); thermal environments (including storage and in-field operation); equipment cut-off voltage (as battery capacity is exhausted or when operating in extreme temperatures, voltage can drop to a point too low for the sensor to operate); cost considerations; and, most importantly, by the battery’s self-discharge rate (which often exceeds the amount of the current consumed annually by a low-power device).
Lithium battery chemistry supports long-term deployments based on having an intrinsic negative potential that exceeds all other metals. As the lightest non-gaseous metal, lithium delivers the highest specific energy (energy per unit weight), highest energy density (energy per unit volume), along with a normal operating current voltage between 2.7 and 3.6V. All lithium chemistries are non-aqueous, making them better adapted to frigid temperatures.
Numerous primary lithium battery chemistries are available. However, bobbin-type lithium thionyl chloride (LiSOCl2) chemistry is overwhelmingly preferred for use in low-power devices that require periodic high pulses to power two-way wireless communications, including protocols such as WirelessHART, ZigBee, and LoRA, to name a few.
Bobbin-type LiSOCl2 chemistry delivers the highest capacity and highest energy density, along with an extremely low annual self-discharge rate as low as 0.7% per year, enabling certain devices to operate continuously for up to 40 years. Bobbin-type LiSOCL2 batteries deliver the following unique benefits:
- Higher reliability—which is ideal for inaccessible locations where battery replacement is difficult or impossible, and data continuity is essential.
- Long operating life—as the self-discharge rate of the battery often exceeds actual energy usage, requiring high initial capacity.
- The widest possible temperature range—from -80°C to 125°C, providing a more reliable power source in extreme environments.
- Smaller size—a space-saving form factor is possible with higher energy density.
- Higher voltage—can result in the need for fewer cells.
- Lower lifetime costs—considering that the expense to replace a battery far exceeds its original cost.
Bobbin-type LiSOCl2 cells commonly power WirelessHART applications such as tank level monitoring, asset tracking, and environmental sensors that must endure extreme temperature cycling. A prime example of this can be found in the medical cold chain, where wireless data loggers are used to continously monitor the safe transport of frozen pharmaceuticals, tissue samples, and transplant organs at carefully controlled temperatures as low as -80°C.
When specifying a bobbin-type LiSOCl2 battery, be aware that a battery’s annual self-discharge can vary significantly based on how the cell was manufactured and its raw materials, affecting its ability to harness the passivation effect.
Passivation is affected by the cell’s current discharge capacity, the length of storage, storage temperature, discharge temperature, and prior discharge conditions, as partially discharging a cell and then removing the load increases the level of passivation over time. Passivation serves to minimize self-discharge, but too much of it overly restricts energy flow.
Self-discharge occurs with all batteries as chemical reactions draw current even while the cell is unused or disconnected. Self-discharge can be minimized by controlling the passivation effect, as a thin film of lithium chloride (LiCl) forms on the surface of the lithium anode, separating it from the electrode to reduce chemical reactions that cause self-discharge. Whenever a load is placed on the cell, the passivation layer causes initial high resistance and a temporary drop in voltage until the discharge reaction start to dissipate the passivation layer: a process that repeats continuously.
The highest quality bobbin-type LiSOCl2 cell features a self-discharge rate as low as 0.7% annually, retaining nearly 70% of its original capacity after 40 years. By contrast, a lower quality cell can have a self-discharge rate of up to 3% per year, exhausting nearly 30% of available capacity every 10 years solely due to self-discharge, limiting its operating life to a maximum of 10-15 years.
High pulse requirements
WirelessHART-enabled devices demand periodic pulses of up to 15 A to actuate two-way wireless communications. Unfortunately, standard bobbin-type LiSOCl2 cells are not designed to deliver high pulses. This challenge is easily overcome by combining a standard bobbin-type LiSOCl2 cell with a patented hybrid layer capacitor (HLC). The standard LiSOCl2 cell delivers low level background current while in standby mode with the HLC delivering high pulses to support brief periods of data interrogation and transmission.
Supercapacitors perform a similar function within consumer products but are generally ill suited for industrial applications because of inherent limitations, including short-duration power, linear discharge qualities that do not allow for use of all the available energy, low capacity, low-energy density, and very high self-discharge rates of up to 60% per year. In addition, supercapacitors linked in series require the use of bulky cell-balancing circuits that add expense and draw additional current.
Lowering your cost of ownership
Industrial grade applications require self-contained power supplies that can perform reliably even in extreme environments. The future of industrial automation will be driven largely by electronic devices that are truly wireless with bobbin-type LiSOCl2 cells and, in certain instances, industrial grade Li-Ion rechargeable batteries supporting technology convergence and interoperability between a growing number of HART-enabled devices.
When designing for deployment in a remote location or extreme environment, it generally pays to go the extra mile to specify a superior quality battery that can last for the entire lifetime of the device. Unfortunately, choosing among competing batteries can be a challenge, as high self-discharge could take years to become fully apparent and theoretical life cycle models tend to underestimate the effects of both passivation and extreme temperatures.
The key is to gain valuable insight by demanding fully documented, long-term test results along with historic in-field performance data under similar environmental conditions. Gaining this knowledge will serve to reduce your cost of ownership.