We are seeing a convergence of IIoT-connected devices, including rapid growth in two-way wireless communications. Common applications include asset tracking, supervisory control and data acquisition, environmental monitoring, artificial intelligence, machine to machine communications, and machine learning, to name a few.
As demand grows for battery-powered solutions in challenging environments and hard-to-access locations, industrial grade lithium batteries are gaining attention. Simple math tells you that the higher initial cost of an industrial grade battery is soon overtaken by the added value of higher reliability and a lower cost of ownership.
Low-power wireless devices
There are two main types of industrial grade low-power devices: 1) Those that draw an average current in microamps, typically requiring the use of an industrial grade primary (non-rechargeable) lithium battery; and 2) Devices that draw average current in milliamps, enough to prematurely exhaust a primary battery, typically requiring the use of an energy harvesting device in combination with an industrial grade rechargeable Lithium-ion (Li-ion) battery to store the harvested energy.
Specifying a battery involves numerous considerations, including: the amount of current consumed during active mode (including the size, duration, and frequency of pulses); energy consumed during stand-by mode (the base current); storage time (as normal self-discharge during storage diminishes capacity); thermal environments (including storage and in-field operation); and equipment cut-off voltage, which drops as cell capacity is exhausted or during prolonged exposure to extreme temperatures. Criticaly important is the battery’s annual self-discharge rate, which often exceeds the energy consumed to operate the device.
The vast majority of remote wireless devices are powered by primary (non-rechargeable) chemistries, including iron disulfate (LiFeS2), lithium manganese dioxide (LiMNO2), lithium thionyl chloride (LiSOCl2), alkaline, and lithium metal oxide chemistry.
As the lightest non-gaseous metal, with a high intrinsic negative potential that exceeds all others, lithium offers the highest specific energy (energy per unit weight) and energy density (energy per unit volume) of all commercially available chemistries. Lithium cells operate within a normal operating current voltage range of 2.7 to 3.6V. This chemistry is non-aqueous, thus less likely to freeze in extremely cold temperatures.
When extended battery life is required, the preferred choice is lithium thionyl chloride (LiSOCl2) chemistry, which can be constructed in two ways: bobbin-type or spiral wound. Spiral wound cells provide higher energy flow while bobbin-type LiSOCl2 batteries are ideal for low-power applications, delivering high capacity, high energy density, and a wider temperature range (-80° to 125° C). Bobbin-type LiSOCl2 cells also feature exceptionally low self-discharge (less than 1% per year for certain cells), which permits up to 40-year battery life.
Understanding battery self-discharge
Self-discharge is common to all batteries, as chemical reactions rob energy even when the cell is unused or in storage. Self-discharge can be greatly reduced by harnessing the passivation effect.
Passivation is unique to LiSOCl2 batteries, resulting from a thin film of lithium chloride (LiCl) that forms on the surface of the lithium anode, separating the anode from the electrode, thus limiting the chemical reactions that cause self-discharge. When a load is placed on the cell, the passivation layer causes initial high resistance along with a temporary voltage dip until the discharge reaction slowly dissipates the passivation layer. This process keeps repeating every time the load is removed.
Numerous factors can influence cell passivation, including 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 increases the amount of passivation relative to when the cell was new. While passivation serves to minimize battery self-discharge, too much of it can restrict energy flow.
Self-discharge is also affected by the cell’s current discharge potential, the method of manufacturing, and the quality of the raw materials. For example, a superior quality bobbin-type LiSOCl2 cell can feature a self-discharge rate as low as 0.7% per year, retaining 70% of its original capacity after 40 years. By contrast, a lower quality bobbin-type LiSOCl2 cell can experience a self-discharge rate of up to 3% per year, losing 30% of its initial capacity every 10 years, making 40-year battery life impossible.
Unfortunately, battery self-discharge can take several years to be fully measurable, and theoretical test data is often highly unreliable, so thorough due diligence is required when evaluating potential battery suppliers.
Two-way wireless connectivity demands high pulses
Increasingly, remote wireless devices require high pulses to power two-way wireless communications. To conserve energy, intelligently designed devices incorporate a low power communications protocol (e.g., WirelessHART, ZigBee, or LoRa), along with a low-power chipset, and proprietary energy-saving techniques.
Standard bobbin-type LiSOCl2 cells are not designed to deliver the high pulses required for two-way communications: a challenge overcome with the addition of a patented hybrid layer capacitor (HLC). The bobbin-type LiSOCl2 cell delivers background current during standby mode while the HLC works like a rechargeable battery to generate high pulses up to 15A. The HLC also features an end-of-life voltage plateau that can be interpreted to communicate ‘low battery’ status alerts for scheduled replacement.
Supercapacitors perform a similar function in consumer electronics but are rarely used in industrial applications due to inherent limitations, including: short-duration power; linear discharge qualities that do not permit full discharge of available energy; low capacity; low energy density; and a very high self-discharge rate of up to 60% per year. Supercapacitors linked in series require cell-balancing circuits that are bulky, expensive, and draw additional energy to further shorten their operating life.
Following are examples involving bobbin-type LiSOCl2 batteries:
Oceantronics: To simplify the transport of scientific equipment across the Artic, Oceantronics redesigned the battery pack for its GPS/ice buoy by replacing a huge battery pack consisting of 380 alkaline D cells with a more compact, lighter, and cost-efficient solution using 32 bobbin-type LiSOCl2 cells and 4 HLCs. They achieved a 90% reduction in size and weight (54 kg down to 3.2 kg), enabling the GPS/ice buoy to be more easily transported by helicopter. Converting from alkaline to LiSOCl2 chemistry also multiplied the device’s operating life many fold.
Cryoegg: Researchers studying the relationship between climate change, rising sea levels, and deep water channels beneath glaciers in Greenland and Antarctica utilize Cryoegg, a remote wireless sensor that continuously monitors temperature, pressure, and electrical connectivity. Cryoegg eliminates the need for bulky and expensive cables that can be easily damaged by glacial movement. Bobbin-type LiSOCl2 cells were specified due to their high capacity, high energy density, wide temperature range, and high pulse capabilities.
Cryoegg utilizes the same 169 MHz Wireless M-Bus radio waves found in AMR/AMI water and gas utility meter transmitter units (MTUs). Bobbin-type LiSOCL2 batteries lower the cost of ownership of a water or gas MTU by preventing wide-scale battery failures that can disrupt billing systems and disable remote start-up/shut-off capabilities.
Southwire: Reducing size and weight is highly beneficial to utility line crews installing line/connector sensors that monitor temperature, catenary, and line current on utility power lines to warn if a transmission line goes down. Use of bobbin-type LiSOCl2 cells enable a more compact and lightweight (3.5 lbs.) solution that can handle extreme temperatures (-40° to 50° C), providing months of back-up power if no line current is detected.
When energy harvesting is required
If an application draws milliamps of current, it could quickly exhaust a primary lithium battery. This may require the use of an energy harvesting device in tandem with an industrial grade rechargeable Lithium-ion (Li-ion) battery.
One prime example is Cattlewatch, which combines small solar (PV) panels and Li-ion batteries to create mesh networks that track the location, health, and safety of animal herds. Solar/Li-ion hybrids also power smart parking meter fee collection systems that are equipped with AI-enabled sensors to identify open parking spots.
Low-cost, consumer-grade rechargeable Li-ion cells have a relatively short service life (5 years and 500 recharge cycles), a limited temperature range (0-40° C), and are unable to deliver high pulses. By contrast, industrial grade Li-ion batteries can operate up to 20 years and 5,000 full recharge cycles, along with an expanded temperature range (-40° to 85° C) and the capability to deliver high pulses to power two-way wireless communications.
If your battery needs to last as long as the device, then do your due diligence when evaluating potential battery suppliers. Start by requesting well-documented long-term test results, actual in-field performance data under similar environmental conditions, and numerous customer references. Specifying a more rugged battery often reduces your cost of ownership.