Ultra-long-life Batteries Take the IIoT to Remote Sites
The Industrial Internet of Things (IIoT) is expanding to increasingly remote locations, with lithium battery-powered remote wireless devices bringing digital connectivity to virtually all industrial applications, including SCADA, process control, industrial robotics, asset tracking, safety systems, environmental monitoring, M2M, AI and wireless mesh networks, to name a few.
Industrial-grade lithium batteries enable remote data to be applied more intelligently to improve operational efficiency, enhance quality control, track assets, promote greater environmental sustainability, optimize supply chains, enhance in-field predictive maintenance programs and more. Use of batteries also eliminates the expense and time-consuming task of having to hard-wire the devices.
With numerous battery chemistries to choose from, the process of identifying the ideal power involves various criteria, including:
- Determining the energy demand
- Identifying the ideal battery chemistry
- Understanding the importance low battery self-discharge
- Adapting the solution for high pulse requirements
- Comparing batteries with similar chemistries
Determining the energy demand
A remote wireless device is only as reliable as its battery. In order to maximize operating life, design engineers must consider numerous factors such as the amount of energy consumed during active mode (including the size, duration and frequency of pulses); the amount of energy consumed while the device is in standby mode (the base current); the length of storage (as normal self-discharge during storage diminishes capacity); the impact of thermal environments (including storage and in-field operation); equipment cut-off voltage (as battery capacity becomes exhausted, or in extreme temperatures, voltage can drop to a point too low for the sensor to operate). Most critically, the design engineer must consider the annual self-discharge rate of the battery, which often exceeds the amount of energy consumed while operating the device.
If the application is easily accessible for battery replacement and the operating environment is relatively moderate, the solution could be as simple as utilizing an inexpensive consumer-grade alkaline or lithium battery. Conversely, if the application involves a long-term deployment in a hard-to-access location or extreme environment where battery replacement is prohibitively expensive or impossible, an industrialgrade lithium battery is generally required.
To conserve energy and extend operating life, low-power IIoT devices operate in a standby state, drawing micro-amps of average current. These devices may also require periodic high pulses in the multi-amp range to power bi-directional wireless communications.
Low-power IIoT devices are predominantly powered by bobbintype lithium thionyl chloride (LiSOCl2 ) batteries that feature very high capacity, high energy density, an extended temperature range and an exceptionally low annual self-discharge rate. Certain niche applications draw higher amounts of average current measurable in milli-amps with pulses in the multi-amp range, which is enough average energy to prematurely exhaust a primary (non-rechargeable) battery. These specialty applications may be better suited for an energy harvesting device in combination with an industrial-grade Lithium-ion (Li-ion) battery to store the harvested energy.
Identifying the ideal battery chemistry
Numerous primary (non-rechargeable) lithium battery chemistries are available (Table 1). At one end of the spectrum are inexpensive alkaline batteries that deliver high continuous energy but suffer from a very high self-discharge rate (which limits battery life), low capacity and energy density (which adds size and bulk), and an inability to operate in extreme temperatures due to the use of water-based constituents. At the opposite end of the spectrum are industrial grade lithium chemistries.
As the lightest non-gaseous metal, lithium features an intrinsic negative potential that exceeds all other metals, delivering the highest specific energy (energy per unit weight), highest energy density (energy per unit volume) and higher voltage (OCV) ranging from 2.7 to 3.6V. Lithium battery chemistries are also non-aqueous, and therefore less likely to freeze in extremely cold temperatures.
Bobbin-type lithium thionyl chloride (LiSOCl2 ) batteries are overwhelmingly preferred for long-term deployments since they deliver the highest capacity and energy density, endure the most extreme temperatures (-80°C to +125°C), and feature an annual self-discharge rate as low as 0.7% per year for certain cells, thereby creating the potential for 40-year battery life. Bobbin-type LiSOCl2 batteries offer the following benefits:
- Higher reliability—This is ideal for remote locations where battery replacement is difficult or impossible, and highly reliable connectivity is required.
- Long operating life—Since the battery’s self-discharge rate often exceeds actual energy usage, high initial capacity and a low selfdischarge rate are highly beneficial.
- The widest temperature range—Bobbin-type LiSOCl2 cells can be modified to operate reliably in extreme temperatures (-80°C to 125°C).
- Smaller size—Higher energy density may permit the use of smaller batteries.
- Higher voltage—This could allow for the use of fewer cells.
- Lower lifetime cost—Can be a major consideration since the manpower and logistical expenses required to replace a battery will far exceed its cost.
Cattlewatch AI-enabled electronics collars allow ranchers to remotely track their cattle herds by providing behavioral information and alerts using an ultra-low-power LoRaWAN network. Select members of the herd are equipped with solar-powered communicators that form a wireless mech network involving the entire herd. Tadiran TLI Series rechargeable li-ion batteries create a lightweight solution that can withstand extreme temperatures, offers up to 20-year operating life and 5,000 full recharge cycles, and generates the high pulses required to power remote wireless communications.
The importance of low battery self-discharge
All batteries experience some amount of self-discharge as chemical reactions draw small amounts of current even while the cell is unused or disconnected.
Self-discharge can be minimized by controlling the passivation effect, whereby a thin film of lithium chloride (LiCl) forms on the surface of the lithium anode to separate it from the electrode to reduce the chemical reactions that cause self-discharge. Whenever a load is placed on the cell, the battery experiences initial high resistance and a temporary drop in voltage until the discharge reaction begins to dissipate the passivation layer: a process that keeps repeating every time a load is applied.
The passivation effect can vary based on 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. While harnessing the passivation effect is essential to reducing selfdischarge, too much it can be problematic if it overly restricts energy flow.
Bobbin-type LiSOCl2 cells vary significantly in terms of their ability to harness the passivation effect. For example, top quality bobbin-type LiSOCl2 batteries can feature a self-discharge rate as low as 0.7% per year, thus retaining nearly 70% of their original capacity after 40 years. Conversely, lower quality LiSOCl2 cells can have a self-discharge rate as high as 3% per year, exhausting nearly 30% of their available capacity every 10 years, which greatly reduces their operating life.
Adapting the solution for high pulse requirements
Low-power remote wireless devices increasingly require periodic pulses up to 15 A to support two-way wireless communications. Standard bobbin-type LiSOCl2 cells are unable to deliver these high pulses due to their low-rate design. This hurdle can be easily overcome with the addition of a patented hybrid layer capacitor (HLC). This hybrid solution utilizes the standard bobbin-type LiSOCl2 cell to deliver nominal background current during standby mode while the HLC delivers high pulses to support data transmission. As an added bonus, the HLC experiences a unique end-of-life voltage plateau that can be interpreted to generate low battery status alerts.
Supercapacitors perform a similar function with consumer electronics but are poorly suited for industrial applications due to serious limitations, including: short-duration power; linear discharge qualities that do not allow for the use of all available energy; low capacity; low energy density; and very high self-discharge rates up to 60% per year. Supercapacitors linked in series also require the use of expensive cell-balancing circuits that add bulk and drain additional current to further shorten their operating life.
Comparing seemingly similar batteries
To maximize return on investment (ROI), the ideal battery-powered solution should last for the entire lifetime of the device to reduce or eliminate the need for costly battery change-outs. However, it can take years to differentiate a higher quality battery from a poorer quality cell since the initial capacity losses are not easily measurable. In addition, the theoretical models and algorithms used to calculate battery life expectancy can be highly unreliable since they tend to underestimate the passivation effect as well as long-term exposure to extreme temperatures.
Careful due diligence is required when specifying an ultra-longlife battery. All potential suppliers should be required to provide fully documented and verifiable test results along with in-field performance data involving similar devices operating under similar loads and environmental conditions. Going the extra mile to carefully compare batteries could pay important dividends by increasing product longevity and lowering the total cost of ownership.
Images courtesy of Tadiran Batteries
This feature comes from the ebook AUTOMATION 2023 Volume 3: IIoT & Industry 4.0.
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