Achieving long battery life with remote wireless devices
A lower self-discharge rate leads to longer operating life for batteries and a lower cost of ownership for remote wireless devices.
The Industrial Internet of Things (IIoT) has created growth opportunities for battery-powered devices for applications including supervisory control and data acquisition (SCADA), automated process control, quality assurance, asset management, safety systems, machine-to-machine (M2M) interfaces and others.
Long-life lithium batteries serve to reduce the total cost of ownership (TCO) by enabling certain low-power devices to operate maintenance-free for up to 40 years. Extended battery life involves various energy-saving techniques, with annual self-discharge being the most important consideration.
The IIoT is extending wireless connectivity to remote locations and challenging environments. A prime example is the modern oilfield, which uses an average of 30,000 sensors, many of which can be networked using various low-power communication protocols, such as IEEE 802.15.4e, LoRA, WirelessHART, WiFi and cellular technologies.
In earlier times, industrial automation often relied on the HART protocol. Many HART-enabled devices were not fully networked and integrated due to the high cost of hard-wiring, estimated at roughly $100 per foot, and exponentially higher for remote locations and extreme environments. Advances in remote wireless technology have eliminated this hurdle.
If a remote wireless device requires long operating life and has low daily energy consumption (micro-amps), it will likely be powered by an industrial grade primary (non-rechargeable) lithium battery. If the device draws enough average daily current to exhaust a primary battery (milli-amps), then it may be well suited for energy harvesting in combination with a lithium-ion (Li-ion) rechargeable battery.
How to choose an industrial grade battery
Specifying an industrial-grade lithium battery involves technical considerations such as the amount of current consumed in active mode, which includes the size, duration and pulse frequency. Users also need to think about storage time, thermal environments and the equipment cut-off voltage because it might drop to a point too low for the sensor to operate in extreme environments.
Other considerations include:
Reliability – Does the location make battery replacement difficult or impossible?
Long operating life – If the self-discharge rate of the battery is almost equal to or greater than average daily energy consumption, then a high capacity battery is likely required.
Miniaturization – Batteries with high capacity and high energy density could enable a smaller footprint.
Extended temperature range – Extreme temperatures can affect battery self-discharge.
Higher voltage – Use of higher voltage batteries may enable fewer cells.
Lifetime costs – Cost of ownership calculations must account for expenses associated with future battery replacements and the risk of battery failure.
Lithium battery choices
Lithium chemistry is preferred for long-term deployments due to its 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 3.6V.
Numerous primary lithium chemistries are available, including iron disulfate (LiFeS2), lithium manganese dioxide (LiMNO2), lithium thionyl chloride (LiSOCl2) and lithium metal oxide chemistry.
Lithium iron disulfate (LiFeS2) cells are often used to deliver high pulses to power a camera flash. They feature a narrow temperature range (-20 to 60°C), a high annual self-discharge rate, and crimped seals that may leak.
Lithium manganese dioxide (LiMNO2) cells power cameras and toys. One space-saving 3V LiMNO2 cell can replace two 1.5V alkaline cells, delivering moderate pulses, but suffering from low initial voltage, a narrow temperature range, a high self-discharge rate, and crimped seals.
Lithium thionyl chloride (LiSOCl2) batteries are constructed two ways: bobbin-type and spiral wound. Bobbin-type LiSOCl2 batteries feature very low annual self-discharge, which is ideal for automatic meter reading (AMR), M2M, SCADA, tank level monitoring, asset tracking, environmental sensors, and other remote applications.
Bobbin-type LiSOCl2 batteries feature the highest capacity and highest energy density of any lithium cell, along with a low annual self-discharge (0.7% per year for certain cells), permitting up to 40-year battery life. These cells deliver a wide temperature range (-80 to 125°C), which is adaptable to the cold chain to monitor the transport of frozen foods, pharmaceuticals, tissue samples, and transplant organs at -80°C. These batteries also can be adapted to high temperatures, including medical autoclave sterilization (125°C).
Power two-way wireless communications
Remote wireless applications increasingly demand periodic high pulses to power advanced two-way wireless communications. Standard bobbin-type LiSOCl2 batteries cannot deliver high pulses due to their low rate design. However, they can be combined with a hybrid layer capacitor (HLC), which works like a rechargeable battery, to deliver periodic high pulses. The HLC also features an end-of-life voltage plateau, which enables “low battery” status alerts.
Supercapacitors perform a similar function in consumer products but are generally not recommended for industrial applications.
Energy harvesting is a growing niche
Applications that draw enough average daily current to prematurely exhaust a primary lithium battery (milli-amps) may be well suited for energy harvesting. Photovoltaic (PV) panels are the most common form of energy harvesting.
One example is a solar-powered tracking device monitoring the health and status of animal herds. Another example is a solar-powered parking meter collecting fees and identifyting open parking spots to reduce pollution and traffic congestion.
Energy harvesting devices often work in tandem with rechargeable Li-ion batteries to store the harvested energy.
Consumer-grade Li-ion cells have a limited operating life of 5 years and 500 recharge cycles, with a moderate temperature range (0 to 40°C) and no high pulse capability. Long-term deployments often require the use of an industrial grade rechargeable Li-ion battery (see Table) that can operate for up to 20 years and 5,000 full recharge cycles, with an expanded temperature range of -40° to 85°C, and the ability to deliver periodic high pulses for two-way wireless communications. These rugged cells also feature a hermetic seal.
Choosing the right battery
Not all batteries are created equal. For example, the annual self-discharge rate of a bobbin-type LiSOCl2 cell vary significantly based on how it’s manufactured. A top-quality bobbin-type LiSOCl2 cell can feature an annual self-discharge rate as low as 0.7%, retaining over 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 30% of its available capacity every 10 years, making 40-year battery life impossible.
When specifying a battery, it often makes economic sense to choose a power supply that can last as long as the device, thereby eliminating the need for future battery replacements and minimizing the TCO.
However, the impact of higher self-discharge rate may not become apparent for years and may not be documented by theoretical test data. When evaluating suppliers, it is important to demand fully documented long-term test results, along with customer testimonials and in-field performance data from equivalent devices operating in similar environments to make an informed decision.