The Industrial Internet of Things (IIoT) has created dramatic growth opportunities for battery-powered remote wireless devices and sensors deployed in remote locations and extreme environments.
Application-specific requirements dictate the choice of battery, often eliminating the use of consumer-grade batteries. If a wireless device requires long operating life and has low average daily energy consumption (micro-amp hours), then it will likely be powered by an industrial grade primary (non-rechargeable) lithium battery. If the device draws enough average current to prematurely exhaust a primary battery (milli-amp hours), then it may be better suited for an energy harvesting device in combination with a Lithium-ion (Li-ion) rechargeable battery.
How to Choose an Industrial-Grade Battery
Batteries involve trade-offs, so it is important to prioritize. Common considerations include:
- The amount of current consumed in active mode (including the size, duration, and frequency of pulses);
- Energy consumed in “stand-by” or “sleep” 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 too low for the sensor to operate);
- The annual self-discharge rate of the battery (which can approach the amount of current drawn from actual use).
Other important considerations include:
Long life and reliability. Is the wireless device being deployed in an inaccessible location where battery replacement is difficult or impossible, and loss of data is unacceptable? Is the self-discharge rate of the battery approaching or greater than the energy being consumed?
Miniaturization. Batteries with high capacity and high energy density enable a smaller footprint. Use of higher-voltage batteries may enable the use of fewer cells.
Extended temperatures. Certain chemistries can withstand extreme environments.
Lifetime costs. To properly calculate cost of ownership you must factor in future battery replacements, including the risk of battery failure.
Choices Among Primary Lithium Batteries
Lithium batteries are preferred for long-term deployments due their high intrinsic negative potential, which exceeds all other metals. As the lightest non-gaseous metal, lithium offers the highest specific energy (energy per unit weight) and energy density (energy per unit volume) of all available battery chemistries. Lithium cells operate within a normal operating current voltage (OCV) range of 2.7 to 3.6 V. Lithium batteries are non-aqueous (less prone to freezing).
Numerous primary lithium chemistries are available, including iron disulfate (LiFeS2), lithium manganese dioxide (LiMNO2), lithium thionyl chloride (LiSOCl2), and lithium metal oxide chemistry.
For long-life applications that draw micro-amps of average current, the overwhelming choice is bobbin-type LiSOCl2 batteries. These cells feature higher capacity and higher energy density, along with extremely low annual self-discharge (under 1% per year), enabling up to 40-year battery life. Bobbin-type LiSOCl2 batteries also deliver the widest temperature range (−80°C to 125°C) and a superior glass-to-metal hermetic seal.
Specially modified bobbin-type LiSOCl2 batteries are used in the cold chain to monitor the transport of frozen foods, pharmaceuticals, tissue samples, and transplant organs at temperatures as low as −80°C. Bobbin-type LiSOCl2 batteries also perform at high temperature, including use in RFID tags that track the location and status of medical equipment without having to remove the battery prior to autoclave sterilization at 125°C.
Bobbin-Type LiSOCl2 Batteries Are Not Easy to Differentiate
The annual self-discharge rate of a bobbin-type LiSOCl2 battery varies based on how it’s manufactured and the quality of the raw materials. A top-quality bobbin-type LiSOCl2 cell can have an annual self-discharge rate of 0.7% per year, retaining more than 70% of its original capacity after 40 years. By contrast, a lower quality bobbin-type LiSOCl2 cell can have a self-discharge rate of up to 3% per year, losing up to 30% of its available capacity every 10 years and making 40-year battery life unachievable.
However, higher self-discharge may not become apparent for years, and short-term tests often fail to accurately measure long-term performance. Thorough due diligence is required when evaluating battery suppliers, including the need for fully documented long-term test results, in-field performance data from similar applications, and customer references.
Powering Two-Way Wireless Communications
IIoT-connected remote wireless devices increasingly require high pulses to power advanced two-way wireless communications.
Standard bobbin-type LiSOCl2 batteries can be combined with a patented hybrid layer capacitor (HLC) to deliver periodic high pulses. The standard bobbin-type LiSOCl2 cell delivers low daily background current while the HLC stores high pulses. The HLC features a unique end-of-life voltage plateau that can be interpreted to deliver “low battery” status alerts.
Supercapacitors are used in consumer electronics to store high pulses, but are ill-suited for industrial applications due to various limitations, including short-duration 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, which adds cost and bulkiness, and consumes added energy to reduce shelf-life.
Energy Harvesting Keeps Expanding
Energy harvesting is a growing option for industrial applications that draw milli-amps of average current, enough to prematurely exhaust a primary lithium battery.
Photovoltaic (PV) panels are the most proven and reliable form of energy harvesting. Common examples include solar-powered tracking devices that continuously monitor the location and status of animal herds, and solar-powered parking meters that collect parking fees and identify open parking spots.
Consumer-grade rechargeable Li-ion cells can operate for roughly five years and 500 recharge cycles, where temperatures are moderate (0 to 40°C), and high pulses are not required. Industrial grade Li-ion batteries can operate for up to 20 years and 5,000 full recharge cycles, with a wider temperature range (−40° to 85°C), able to deliver high pulses.
When specifying an industrial grade power source for a remote wireless device, it pays to specify a battery can last as long as the device to minimize your cost of ownership.
Sol Jacobs is vice president and general manager of Tadiran Batteries.