The remote wireless devices used throughout the Industrial Internet of Things (IIoT) should use long-life lithium batteries to deliver reliable performance that reduces the total cost of ownership. Using long-life lithium batteries can eliminate the need for hard-wiring devices to AC power, which can cost as much as $100/ft, or even more in remote locations.
Long-life batteries are especially beneficial for powering wireless devices that draw average currents measurable in microamps, an essential prerequisite for choosing an industrial-grade primary (non-rechargeable) battery. Conversely, if a device draws average current measurable in milliamps, then you should consider using some form of energy harvesting device (typically a small photovoltaic panel) in conjunction with an industrial grade Lithium-ion (Li-ion) rechargeable battery.
Lithium Thionyl Chloride (LiSOCl2) Batteries Last Longer
Lithium batteries are generally preferred because they feature a high intrinsic negative potential that exceeds all other metals. The lightest non-gaseous metal, lithium, offers the highest specific energy (energy per unit weight) and energy density (energy per unit volume) of all commercial battery chemistries. Lithium batteries operate with an output ranging from 2.7 volts to 3.6 volts, whereas other batteries operate with a lower voltage. Alkaline, for instance, is 1.5 volts, requiring larger batteries to deliver equivalent power. One key advantage of higher voltage is that it aids with product miniaturization. Also, being non-aqueous, this chemistry is less likely to freeze, which is ideal for applications involving extreme environments.
Various primary lithium chemistries are available, including iron disulfate (LiFeSg), lithium manganese dioxide (LiMnOg), lithium thionyl chloride (LiSOClg), and lithium metal-oxide (See Table). Of these, LiSOClg is widely preferred for long-term deployment in extreme environments, including applications such as AMR/AMI metering, M2M, SCADA, RFID, tank-level monitoring, asset tracking, animal tracking, RFID, and environmental monitoring, to name a few.
Bobbin-type LiSOClg batteries deliver the highest capacity and highest energy density of all lithium cells, which is an important factor for product miniaturization. This chemistry also features quite low self-discharge (less than 1% per year for certain cells), enabling up to 40-year battery life. In addition, bobbin-type LiSOClg batteries can operate over a temperature range of -80 to 125°C, which is ideal for remote wireless applications.
For example, specially modified batteries are used in cold chains to continuously monitor the status of frozen foods, pharmaceuticals, tissue samples, and transplant organs at minus 80°C. Bobbin-type LiSOClg batteries can also be modified for use in high temperatures. For instance, they can enable active RFID tags to track the location and status of medical equipment to endure multiple cycles of autoclave sterilization at 125°C without having to remove the battery.
The Link Between Self-Discharge and Passivation
Self-discharge is a natural phenomenon that affects all batteries, as chemical reactions reduce energy capacity even while a battery is idle. The self-discharge rate varies based on several factors, including current discharge potential, the purity and quality of raw materials, and controlling the cell passivation to effectively limit the chemical reactions that lead to self-discharge.
Current discharge potential can vary based on cell design. For example, Tadiran manufactures two different bobbin-type LiSOClg cells using the exact same chemistry. One version is designed, by reducing the passivation layer, to deliver higher current. But the trade-off is significantly shorter battery life (10 years) due to higher self-discharge. The same chemistry is used in 40-year batteries, but with much lower self-discharge (0.7% per year) due to greater passivation. The trade-off, however, is delayed voltage response, which can be overcome with a hybrid layer capacitor (HLC) that works like a rechargeable battery to store pulses and eliminate the problem of voltage delays and drops. The HLC comes into play mostly with devices that require periodic spikes for two-way communication, remote shut-off, and other advanced functionality.
The passivation effect occurs when a thin film of lithium chloride (LiCl) forms on the surface of the lithium anode, which naturally limits chemical reactions. When a load is placed on the cell, the higher resistance of the passivation layer causes voltage to dip temporarily until the discharge reaction removes the passivation layer — a process that keeps repeating whenever the load is removed.
Passivation is affected by factors such as the current capacity of the cell, length of storage, storage temperature, discharge temperature, and prior discharge conditions. Removing the load from a partially discharged cell can impact passivation relatively more than a new cell. Passivation is essential for limiting self-discharge, but too much of it can also restrict energy flow when it's needed most. Reduced passivation permits a greater energy flow, but there is a trade-off in the form of higher self-discharge and shorter operating life.
The Bottle Analogy
The effects of passivation on self-discharge and energy flow is similar to comparing bottles of fluid with different size openings:
The volume of a glass/bottle is equivalent to battery capacity
Evaporation/self-discharge is equivalent to capacity loss
Flow volume is equal to discharge/ energy flow
Low liquid/electrolyte quality can clog the bottle opening, causing a stoppage to flow/passivation
Low liquid/electrolyte quality can cause evaporation/self-discharge
Large openings are good for fast flow/discharge but not for storing fluids for a long time
Long-life demands a smaller opening for low evaporation/self-discharge
Bobbin-type LiSOClg batteries have very “small openings” (low flow rate with slower evaporation/self-discharge), while LiMnO2 and Alkaline cells have “larger openings” (higher flow rates with faster evaporation/self-discharge). Too large an opening can cause excessive evaporation/self-discharge. Too small an opening can cause the opening to get clogged (excess passivation), especially with chemical impurities.
Combining Low Self-Discharge with High Pulses
Increasingly, remote wireless devices require periodic high pulses to power two-way communications. Standard bobbin-type LiSOClg batteries can deliver low background current but not high pulses, due to their low rate design. This can be overcome with the Tadiran patented hybrid layer capacitor (HLC) that works like a rechargeable battery to deliver periodic high pulses. The HLC also features a unique end-of-life voltage plateau that can be used for low-battery status alerts.
Supercapacitors work similarly for consumer applications but are rarely used in industrial applications due to inherent drawbacks such as short-duration power, linear discharge qualities that prevent use of all the available energy, low capacity, low energy density, and high annual self-discharge rates (up to 60% per year). In addition, supercapacitors are connected in series, requiring the use of cell-balancing circuits that add expense, bulkiness, and accelerated self-discharge due to added energy consumption.
Things to Consider When Choosing an Industrial-Grade Battery
The cumulative impact of annual self-discharge often does not become apparent for years, and efforts to predict actual battery life typically under-represent the effects of passivation and extreme temperatures. Battery suppliers for critical applications that require long-life reliability should provide fully documented test results, in-field performance data under similar conditions, and multiple customer references.
Specifying a bobbin-type LiSOClg battery with a self-discharge rate as low as 0.7% per year can enable certain low power devices to operate maintenance-free for up to 40 years, significantly reducing the total cost of ownership. To properly calculate the total cost of ownership, all costs need to be anticipated, including the labor and materials that would be required for future battery replacements and all the hidden costs associated with premature battery failure.
When seeking a long-life power solution, it pays to understand how passivation and extreme temperatures can affect battery self-discharge.