More self-powered remote wireless devices are used in extreme environments. Here’s a look at some of the newer battery devices that will help you handle the power needs of these devices in such environments.
Sol Jacobs, VP and General Manager, Tadiran Batteries
We live in an increasingly wireless world, where self-powered remote sensors and communication devices are becoming essential to almost every type of industrial application, such as asset tracking, system control and data automation, and machine-to-machine (M2M), to name a few. This trend will undoubtedly accelerate as the Industrial Internet of Things (IIoT) takes wireless devices to once unimaginable environments.
For battery-powered devices, the effects of extreme environments can compromise data integrity and reliability, add significantly to the total cost of ownership, and place workers who are trying to install and maintain these devices at greater risk.
Make sure the benefits outweigh the costs
The fast-growing market for self-powered wireless devices is being driven largely by economics, since wireless devices can eliminate labor, logistical, and regulatory expenses associated with installing a hard-wired device. Installing something as simple as an electrical switch can cost upwards of $100 or more per foot: an expense that increases in remote, environmentally sensitive locations.
Designing a wireless device to be as small as possible without compromising performance or system functionality is always a challenge. Intelligent power management approaches are required to use battery power efficiently and cost effectively, especially in extreme environments.
Start by thinking small
A common practice is to specify batteries that are overly large or that deliver unneeded capacity to achieve the required battery operating life. But selecting an oversized battery carries hidden costs, including the expense of transporting batteries to remote, hard-to-access locations, where the labor and logistical expenses can be high. Increasingly restrictive UN and IATA shipping regulations can further add to these transportation costs.
A compact, lightweight, long-life power supply can offer tangible benefits. For example, scientists conducting experiments to monitor the changing size and position of ice flows in the Arctic Ocean want the device to be as small and lightweight as possible in order to be deployed by helicopter. Conversely, a utility linemen carrying line fault sensors up and down utility poles seeks a compact, lightweight solution that reduces fatigue.
Of course, specifying an undersized battery to achieve product miniaturization goals could result in excessive battery replacements throughout the life of the device.
Extreme temperatures can impact battery performance
Many primary battery chemistries are available, each offering unique performance features, including varying temperature ranges. These chemistries include alkaline, iron disulfide (LiFeS2), lithium manganese dioxide (LiMNO2), and lithium thionyl chloride (LiSOCl2) batteries (see Table 1).
Exposure to extreme temperatures reduces both battery life and voltage under pulse, especially if the battery already has a limited temperature range. In such situations, an oversized battery may be required to compensate for an expected voltage drop under pulsed load.
One solution is to specify a bobbin-type lithium thionyl chloride (LiSOCl2) battery that features high capacity, high energy density, and can deliver high pulses at extreme temperatures, thus eliminating the need for extra capacity or voltage. Bobbin-type LiSOCl2 cells can also be modified for the extreme temperatures associated with the medical cold chain, where frozen tissue samples, transplant organs, drugs and pharmaceuticals must be continually monitored and maintained at -80°C. Bobbin-type LiSOCl2 batteries can also handle extreme heat, including use in medical asset tracking RFID devices that can withstand autoclave sterilization temperatures of up to 125°C without having to remove the battery.
Since it takes more than twice as many 1.5 V cells to deliver the same voltage as a single 3.6 V cell, using a fewer number of higher voltage cells can effectively minimize size, weight, and cost.
Extreme temperatures can also negatively impact the annual self-discharge rate of a battery, which is an acute problem for battery chemistries already prone to high annual self-discharge. For example, certain conditions can cause an alkaline cell to fully self-discharge in less than a year, thus requiring an oversized power supply to compensate for such energy losses. By contrast, an industrial grade LiSOCl2 battery that features a lower annual self-discharge rate can permit a smaller power supply while also eliminating the need for future battery replacements.
Annual self-discharge rates vary significantly depending on how the battery is manufactured and the quality of the raw materials. For example, a superior grade bobbin-type LiSOCl2 battery can deliver a self-discharge rate as low as 0.7% per year at ambient temperature, and thus retain more than 70% of its original capacity after 40 years. A lesser grade LiSOCL2 battery could have a self-discharge rate of 3% per year at ambient temperature, causing the cell to lose 30% of its original capacity every 10 years, making 40-year battery life impossible, especially at extreme temperatures.
Factoring in high pulse requirements
Remote wireless devices increasingly require high pulses to power advanced two-way communications and remote shut-off capabilities. A prime example is AMR/AMI utility meters, which are often buried in underground pits, then called upon to query and periodically transmit data between a meter transmitter unit (MTU) and a host computer.
Consumer lithium batteries can deliver the high pulses required for AMR/AMI metering applications due to their high rate design. However, consumer grade lithium batteries are completely unacceptable for AMR/AMI applications due to their low voltage (1.5 V), limited temperature range (-0°C to 60°C), and a high self-discharge rate that reduces their life expectancy to as short as 1-2 years, making the AMR/AMI network highly unreliable and costly to maintain.
Supercapacitors are commonly used in consumer products to deliver high pulses while minimizing TMV. However, supercapacitors are ill-suited to many industrial applications due to serious drawbacks such as bulkiness, high self-discharge rates of up to 60% per year, and a limited temperature range. Power supplies made up of multiple supercapacitors also require balancing circuits that draw energy and add to the cost.
Standard bobbin-type LiSOCl2 batteries can deliver the long life required by AMR/AMI applications. However, these cells are not designed to deliver periodic high pulses, as they can experience a temporary drop in voltage when first subjected to this type of pulsed load: a phenomenon known as transient minimum voltage (TMV).
To overcome this challenge, a standard bobbin-type LiSOCl2 battery can be modified to deliver periodic high pulses with the addition of a patented Hybrid Layer Capacitor (HLC). The bobbin-type LiSOCl2 battery and the HLC work in parallel, with the standard battery supplying background current in the 3.6 to 3.9 V nominal range, while the single-unit HLC works like a rechargeable battery to store and deliver high pulses. As an added benefit, the HLC features a unique end-of-life performance curve that allows controllers to be programmed to issue ‘low battery’ status alerts.
Bobbin-type LiSOCl2 batteries are also available that can deliver moderate to high pulses with virtually no voltage delay or TMV, yet do not require an HLC or possibly can use fewer or smaller HLCs. These long-life lithium batteries also operate efficiently, especially in extreme temperatures, which extends their operating life up to 15%.
Make sure the environment is right for rechargeable batteries
Certain remote wireless applications may be well suited for energy harvesting devices that extract energy from solar, wind, thermal and kinetic energy, or RF/EM signals, and then store the energy in rechargeable Lithium-ion (Li-ion) batteries.
Consumer grade Li-ion cells have drawbacks, including a limited temperature range and an inability to deliver high pulses, making them ill-suited for applications that require advanced two-way communications. Consumer grade rechargeable Li-ion batteries also have a limited lifespan of approximately 5 years and 500 full recharge cycles. If the wireless device needs to operate beyond 500 recharge cycles without battery replacement, then extra cells may be required to reduce the average depth of discharge per cell.
Fortunately, an industrial grade rechargeable Li-ion battery offers up to 20-year battery life with up to 5,000 full recharge cycles. These ruggedized cells also feature a temperature range of -40°C to 85°C, and can deliver the high pulses needed to support advanced two-way communications.
Power supply options are growing for self-powered remote wireless devices, especially for extreme environments. Primary and rechargeable lithium batteries are available that help minimize size and weight while reducing the total cost of ownership.
Filed Under: Design World articles, Capacitors, Energy management + harvesting