by SOL JACOBS, Tadiran Batteries
Many devices on the Industrial IoT will need rugged primary and rechargeable lithium batteries to provide reliable, long-term power.
No question there will be a lot of remote wireless devices on the IoT. Many of them will be powered either by primary lithium batteries or energy harvesting devices combined with rechargeable batteries or supercapacitors. Here are a few ideas about battery chemistries that make sense for power scenarios likely to arise in industrial IoT applications.
A wireless device intended for long-term deployment and drawing a low average daily current could be a candidate for primary bobbin-type lithium thionyl chloride (LiSOCL2) batteries. LiSOCL2 chemistry is the predominant choice for remote wireless applications because of its exceptionally high energy density (1,420 Wh/l volumetric densities are widely available, compared to about 100 Wh/l for lead acid), high capacity, wide temperature range, and low annual self-discharge rate.
Certain bobbin-type LiSOCL2 batteries can deliver a self-discharge rate of less than 1% per year; batteries can operate for up to 40 years in situations where the annual self-discharge of the battery exceeds the annual power consumption of the device. The smart grid is a prime example of where bobbin-type LiSOCL2 batteries have been deployed in an industrial IoT environment. For nearly 30 years these batteries have powered endpoint terminals of metering devices that communicate to central databases. Power meters are increasingly becoming smart meters. They now interface with the IoT to provide real-time information and alerts about consumption patterns. To conserve energy, these wireless devices operate mainly in a dormant state, drawing little or no energy. They periodically take data, but only awaken if they note certain data parameters. Careful control of energy consumption lets these wireless devices operate maintenance-free for decades.
The main limitation of standard LiSOCL2 chemistry is high passivation arising from a low-rate design. In LiSOCL2 cells, thionyl chloride is a liquid. Metal lithium touches the thionyl chloride and will slowly oxidize out lithium chloride. The lithium chloride layer produced on the surface of the metal lithium tends to prevent lithium from reacting with thionyl chloride. This phenomenon is passivation. The passivation takes place slowly, but the speed of passivation is higher at higher temperatures and is more pronounced over longer time periods.
The passivation prohibits these cells from delivering high current pulses. This issue can be addressed by combining a standard LiSOCL2 cell with a patented hybrid layer capacitor (HLC). The standard LiSOCL2 cell delivers low background current to power the device in its standby mode, while the HLC stores and delivers the high pulses required when the device is in its active mode of data interrogation and transmission.
An alternative involves the use of supercapacitors, also known as ultracapacitors or electric double layer capacitors (EDLCs), which store energy in an electrostatic field rather than in a chemical state. Supercapacitors are primarily used to provide memory back-up power for mobile phones, laptops and digital cameras. This technology has certain inherent drawbacks, including short-duration power, linear discharge characteristics that do not allow for use of all the available energy, low capacity, low energy density, high self-discharge (up to 60% per year), and the need for cell balancing when supercapacitors link in series. Supercapacitors also have crimped seals that may leak and have not been proven to deliver long life.
Consumer grade versus industrial grade
Some industrial IoT applications may be well suited for energy harvesting. Energy harvesting (also called energy scavenging) refers to the process of deriving energy from external sources (such as solar power, thermal energy, wind energy, salinity gradients and kinetic energy). Harvested energy is usually used to power wireless autonomous devices. The decision to use an energy harvesting device depends on factors that include the reliability of the device and its energy source; the expected operating life of the device; environmental requirements; size and weight considerations; and total cost of ownership.
An energy harvesting device generally contains five basic components: sensor, transducer, energy processor, microcontroller and optional radio link. The sensor detects and measures environmental parameters such as motion, proximity, temperature, humidity, pressure, light, strain vibration and pH. The transducer and energy processor work in tandem to convert, collect and store the electrical energy in either a rechargeable lithium battery or a supercapacitor. The microcontroller collects and processes the data. The radio link communicates with a host receiver or data collection point. The energy harvested is often relatively small, especially for devices that draw only a few microamps of current daily.
Energy harvesting devices are typically paired with rechargeable lithium-ion (Li-ion) batteries that store harvested energy. Consumer grade Li-ion cells are reasonably inexpensive and widely available, but have a life expectancy of less than five years and 500 recharge cycles. They also only work over a moderate temperature range of -10 to 60° C, so they don’t work well for long-term deployment in extreme environments.
Industrial grade Li-ion batteries are a better choice if the wireless device is intended for use in remote, inaccessible locations. Industrial Li-ion cells can operate for up to 20 years and handle 5,000 full recharge cycles. They also work over a temperature range of -40 to 85° C and can deliver high current pulses (5 A for an AA-size cell). These industrial grade Li-ion cells also feature glass-to-metal hermetic seals, whereas consumer rechargeable batteries use crimped seals more prone to leak.
As a general rule, industrial grade Li-ion batteries make sense where the expense of battery replacement far exceeds the cost of the battery itself. This can be confirmed by calculating the total lifetime cost of the industrial grade Li-ion battery versus a consumer grade Li-ion battery.
For an example application, consider wireless solar-powered parking meters. Made by the IPS Group, they incorporate state-of-the-art features that include multiple payment system options, access to real-time data, integration to vehicle detection sensors, user guidance and enforcement modules, and connections to a comprehensive web-based management system.
PV panels in the meter gather solar energy, which then gets stored in an industrial grade rechargeable Li-ion battery. The rechargeable battery can deliver the high pulses required to initiate two-way wireless communications.
Another example of an industrial IoT application is CattleWatch, which places solar-powered hub collars and solar-powered collar units on cattle. All collars communicate with the hub collars through a wireless mesh network. Hub collars communicate to the cloud through Iridium satellites. Ranchers get real-time updates on daily animal behavior, including herd location, walking time, grazing time, resting time, water consumption, in-heat condition and other health events. Ranchers also receive instant notification if potential threats arise from predatory animals or poachers.
Energy harvested by PV panels in CattleWatch units gets stored in industrial grade Li-ion rechargeable batteries. The batteries were chosen over supercapacitors because they were significantly lighter and smaller, and thus more comfortable for the animals to wear.
Every application is special and specific requirements dictate the best power supply. When long-term reliability is essential, an industrial grade battery generally makes more economic sense than a consumer grade one.