Battery-powered solutions are expanding the realm of industrial automation to virtually all external environments, enabling remote wireless devices to thrive throughout the Industrial Internet of Things (IIoT).
Industrial automation no longer is constrained to the factory floor. With the help of wireless communications and advanced lithium battery technology, the landscape is expanding rapidly to incorporate increasingly remote and hostile environments.
The explosion of wireless technology has fueled rapid expansion of the Industrial Internet of Things (IIoT), allowing billions of wireless devices to become seamlessly networked and integrated while being liberated from the power grid. Battery-powered devices have brought wireless connectivity to virtually all industrial sectors, including process control, asset management, machine-to-machine, systems and systems control and data automation, transportation infrastructure, energy production, environmental monitoring, manufacturing, distribution, health care, and smart buildings, to name a few.
Critical to this growth surge has been the evolution of low-power communications protocols, such as ZigBee, WirelessHART, and LoRa (a long range, low power wireless platform), and related technologies that permit two-way wireless communications while also extending battery life.
For example, the highway addressable remote transducer (HART) communications protocol has been providing a critical link between intelligent field instruments and host systems for decades, employing the same the caller ID technology found in analog telephony and operating via traditional 4-20 mA analog wiring. However, in the past, requirements for hard-wiring severely restricted the deployment of HART-enabled devices due to high initial expense, as it costs roughly $100 per foot to install any wired connection, even a basic electrical switch. This cost barrier becomes far more problematic in remote, environmentally sensitive locations, where complex logistical, regulatory, and permitting requirements cause expenses to skyrocket. Development of the WirelessHART protocol has eliminated all these constraints.
Choosing the ideal power source
The vast majority of remote wireless devices are powered by primary (non-rechargeable) lithium batteries. In addition, certain applications are well-suited to be powered by an energy harvesting device in conjunction with a rechargeable lithium-ion (Li-ion) battery to store the harvested energy.
The more remote the application, the more likely the need for industrial-grade lithium batteries. Inexpensive consumer-grade batteries may suffice if the device is easily accessible and operates within a moderate temperature range. However, the cost of replacing a consumer-grade battery can far exceed the price of the battery itself, causing the total cost of ownership to rise dramatically. For example, imagine having to replace a battery in a seismic monitoring system sitting on the ocean floor or in a stress sensor attached to a bridge abutment.
Specifying an industrial-grade battery involves multiple parameters, such as 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 cutoff voltage (as battery capacity is exhausted, or in extreme temperatures, voltage can drop to a point too low for the sensor to operate); battery self-discharge rate (which can be higher than the current draw from average sensor use); and cost considerations. Industrial-grade lithium batteries most commonly are recommended for applications that demand the following:
- Reliability: The remote sensor is in a hard-to-reach location where battery replacement is difficult or impossible, and data links cannot be interrupted by bad batteries.
- Long operating life: The self-discharge rate of the battery can be more than the device usage of the battery, so initial battery capacity must be as high as possible.
- Wide operating temperatures: Especially critical for extremely hot or cold environments.
- Small size: When a small form factor is required, the battery’s energy density must be as high as possible.
- Voltage: Higher voltage requires fewer cells.
- Lifetime costs: Replacement costs over time must be taken into account.
Tradeoffs often are inevitable, so it is important to prioritize your list of desired battery performance attributes.
Choosing among primary lithium batteries
Lithium battery chemistry is preferred for long-term deployments due its intrinsic negative potential, which exceeds that of all other metals. Lithium is the lightest non-gaseous metal, and offers the highest specific energy (energy per unit weight) and energy density (energy per unit volume) of all available battery chemistries. Lithium cells, all of which use a non-aqueous electrolyte, with a normal operating current voltage ranging between 2.7 and 3.6 V. The absence of water allows lithium batteries to endure more extreme temperatures.
Numerous primary lithium chemistries are available including lithium iron disulfate (LiFeS2), lithium manganese dioxide (LiMnO2), lithium thionyl chloride (Li-SOCl2), and lithium metal oxide chemistry (see Table 1: Primary lithium chemistry comparisons).
Table 1: Primary lithium chemistry comparisons
|Li-SOCl2||Li-SOCl2||Li metal oxide||Li metal oxide||Alkaline||LiFeS2||LiMnO2|
|Primary cell||Bobbin-type with hybrid layer capacitor||Bobbin-type||Modified for high capacity||Modified for high power||Lithium iron disulfate||CR123A|
|Energy density (Wh/1)||۱,۴۲۰||۱,۴۲۰||۳۷۰||۱۸۵||۶۰۰||۶۵۰||۶۵۰|
|Power||Very high||Low||Very high||Very high||Low||Low||Moderate|
|Voltage||۳.۶ to 3.9 V||۳.۶ V||۴.۱ V||۴.۱ V||۱.۵ V||۱.۵ V||۳.۰ V|
|Pulse amplitude||Excellent||Small||High||Very high||Low||Moderate||Moderate|
|Performance at elevated temperature||Excellent||Fair||Excellent||Excellent||Low||Moderate||Fair|
|Performance at low temperature||Excellent||Fair||Moderate||Excellent||Low||Moderate||Poor|
|Self-discharge rate||Very low||Very low||Very low||Very low||Very high||Moderate||High|
|Operative temperature||-۶۷ to 185°F; can be extended to 221°F for a short time||-۱۱۲ to 257°F||-۴۹ to 185°F||-۴۹ to 185°F||۳۲ to 140°F||-۴ to 140°F||۳۲ to 140°F|
Source: Tadiran Batteries
Consumer grade LiFeS2 cells are relatively inexpensive, and can deliver the high pulses required to power a camera flash. These batteries have limitations, including a narrow temperature range of -4 to 140°F, a high annual self-discharge rate, and crimped seals that may leak.
LiMnO2 cells, including the popular CR123A, provide a space-saving solution for cameras and toys, as one 3-V LiMnO2 cell can replace two 1.5-V alkaline cells. LiMnO2 batteries can deliver moderate pulses, but suffer from low initial voltage, a narrow temperature range, a high self-discharge rate, and crimped seals.
Li-SOCl2 batteries are manufactured two ways: spirally wound or bobbin-type construction (see Figure 1). Of the two, bobbin-type Li-SOCl2 batteries are better suited for long-life applications that draw low average daily current, such as tank level monitoring, asset tracking, and environmental sensors that must endure extreme temperature cycling.
Bobbin-type Li-SOCl2 batteries feature the highest capacity and highest energy density of any lithium cell, along with an extremely low annual self-discharge rate-less than 1% per year, enabling certain cells to operate maintenance-free for up to 40 years. Bobbin-type Li-SOCl2 batteries also feature a glass-to-metal hermetic seal, and deliver the widest possible temperature range (-112 to 257°F).
A prime example is the medical cold chain, where wireless sensors are used monitor the transport of frozen pharmaceuticals, tissue samples, and transplant organs at carefully controlled temperatures as low as -112°F. Certain bobbin-type Li-SOCl2 batteries have been demonstrated to operate successfully under prolonged test conditions at -148°F, which far exceeds the maximum temperature range of alkaline cells and consumer-grade lithium batteries.
Bobbin-type Li-SOCl2 batteries also are deployed in virtually all meter transmitter units (MTUs) used in AMI/AMR metering applications for the water and gas utility industry. The extended battery life of a bobbin-type Li-SOCl2 cell is essential to AMI/AMR metering applications because large-scale system-wide battery failures can create potential chaos by disrupting billing and customer service operations. Bobbin-type Li-SOCl2 batteries installed in MTU units during the mid-1980s were tested nearly 30 years later and shown to have plenty of remaining available capacity.
Battery operating life is largely influenced by the cell’s annual energy usage along with its annual self-discharge rate. Battery operating life can be extended further by operating the device in a standby mode that draws little or no current, then periodically querying to data to awaken only if certain preset data thresholds are exceeded. If properly conserved, it is not uncommon for more energy to be lost through annual battery self-discharge than through actual battery use.
When specifying a bobbin-type Li-SOCl2 battery, be aware that actual operating life can vary significantly based on how the cell was manufactured and the quality of its raw materials. For example, the highest quality bobbin-type Li-SOCl2 cells can feature a self-discharge rate as low as 0.7% annually, thus retaining nearly 70% of their original capacity after 40 years. By contrast, a lesser quality bobbin-type Li-SOCl2 cell can have a self-discharge rate of up to 3% per year, causing nearly 30% of available capacity to be lost every 10 years due to annual self-discharge.
Though bobbin-type Li-SOCl2 batteries are not created equal, performance differences may not become apparent for years. Thus, due diligence is required when specifying a battery for long-term deployment in remote applications. Engineers must look beyond theoretical data to demand fully documented long-term test results along with actual performance data from the field.
Sol Jacobs, Tadiran Batteries