Picture credit: Tadiran Batteries
Advanced primary lithium batteries will support the Industrial Internet of Things (IIoT) by enabling remote wireless devices to operate maintenance-free for up to 40 years.
Wireless connectivity is key to permitting greater convergence and interoperability between ‘smart’ devices that will soon be connected to the burgeoning IIoT.
Closely tied to the development of low-power communications protocols such as ZigBee, WirelessHART, and LoRa, remote wireless technology has grown exponentially, impacting everything from manufacturing and distribution to transportation infrastructure, energy production, environmental monitoring, healthcare, smart metering, process control, asset tracking, safety systems, machine-to-machine (M2M), and system control and data automation (SCADA) applications, to name a few.
Wireless technology is exploding in part because it eliminates the need for hard-wiring, which can cost $100 or more per linear foot, especially in remote, environmentally sensitive locations, where logistical, regulatory, and permitting hurdles make wireless connectivity a virtual necessity.
Choosing among primary batteries
Numerous factors need to be considered when selecting a primary (non-rechargeable) battery, 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 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 lifetime cost considerations, taking into account future battery replacement costs, where applicable.
Inexpensive consumer-grade alkaline batteries can be ideal if the device is easy to access and operates within a moderate temperature range (i.e. indoor thermostats and wireless light switches). However, alkaline batteries do have some serious drawbacks, including low voltage (1.5 V or lower), a limited temperature range (0°C to 60°C), a high self-discharge rate that reduces life expectancy to as little as one to two years, and crimped seals that may leak. The low cost of an alkaline battery can also be highly misleading once you factor in the need for future battery replacements.
Comparing lithium chemistries
Lithium is preferred for long-life battery applications because its intrinsic negative potential exceeds that of all other metals. Lithium is also the lightest non-gaseous metal and offers the highest specific energy (energy per unit weight) and energy density (energy per unit volume) of all, which serves to reduce battery size and weight. Lithium cells also feature a normal operating current voltage (OCV) ranging between 2.7 and 3.6, and the electrolyte is non-aqueous, which extends the temperature range.
Numerous primary lithium battery chemistries are commercially available, including iron disulfate (LiFeS2), lithium manganese dioxide (LiMNO2), and lithium thionyl chloride (LiSOCl2) chemistry (see table below):
Consumer grade lithium iron disulfate (LiFeS2) cells are inexpensive and can deliver high pulses, but feature a narrow temperature range (-20°C to 60°C), a high annual self-discharge rate, and crimped seals that may leak.
Lithium Manganese Dioxide (LiMNO2) cells, including the popular CR123A, offer a space-saving solution, as one 3V LiMNO2 cell can replace two 1.5V alkaline cells. LiMNO2 batteries deliver moderate pulses, but suffer from low initial voltage, a narrow temperature range, a high annual self-discharge rate, and crimped seals.
Bobbin-type lithium thionyl chloride (LiSOCl2) batteries are overwhelmingly preferred for long-life applications that draw low average daily current. Bobbin-type LiSOCl2 batteries feature the highest capacity and highest energy density of all lithium cells, along with the widest temperature range (-80°C to 125°C) and a glass-to-metal hermetic seal.
A superior quality bobbin-type LiSOCl2 cell can have a self-discharge rate as low as 0.7% per year, thus retaining nearly 70% of its original capacity after 40 years. By contrast, a lesser quality bobbin-type LiSOCl2 cell can have a self-discharge rate as high as 3% per year, thus losing nearly 30% of its available capacity every 10 years.
Batteries that deliver long life – even in extreme environments
Bobbin-type LiSOCl2 batteries power virtually all wireless meter transmitter units (MTUs) in AMI/AMR gas and water utility metering applications. The MTUs are typically buried in underground pits or mounted on building exteriors and exposed to extreme temperatures. It is critical for the batteries to deliver long operating life with high reliability so as to avoid the risk of large-scale system wide battery failures that can disrupt normal billing cycles and create customer service nightmares that drive up the total cost of ownership.
The fact that bobbin-type LiSOCl2 chemistry can endure extreme temperature cycling makes them ideal for use in automotive electronic toll tags, where where heat soak on an automotive windshield can hit 113°C (according to SAE) when parked, cooling down rapidly to room temperature. Conversely, in extremely frigid weather, the battery must be able to handle cold soak and a rapid temperature rise.
Within the medical field, bobbin-type LiSOCl2 cells are being used to power RFID asset tracking devices that can withstand high temperature autoclave sterilisation. Specially modified bobbin-type LiSOCl2 cells are also being deployed in the medical cold chain, where frozen pharmaceuticals, tissue samples and transplant organs must be kept at controlled temperatures as low as -80°C, with certain cells able to survive prolonged testing at -100°C.
Adapting lithium batteries to deliver high pulses
Due to its low rate design, a standard bobbin-type LiSOCl2 cell cannot deliver the high pulses required to power advanced, two-way communications. This challenge can be overcome by combining a standard bobbin-type LiSOCl2 cell with a patented hybrid layer capacitor (HLC). The standard cell delivers low background current while the HLC works like a rechargeable battery to deliver the high pulses required to initiate data interrogation and transmission.
Supercapacitors can also be used to store high pulses for certain consumer applications, but are generally not recommended for industrial applications due to inherent limitations, including an inability to provide long-term power, linear discharge qualities that do not allow for use of all the available energy, low capacity, low energy density, and high annual self-discharge rates (up to 60% per year). Supercapacitors linked in series also require the use of cell-balancing circuits that draw additional current.
Lithium batteries will continue to evolve to serve the emerging IIoT, bringing reliable low-cost power management solutions to remote locations and extreme environments.
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