Power Versus Energy When Designing Smart Wireless Devices

Power Versus Energy When Designing Smart Wireless Devices
Power Versus Energy When Designing Smart Wireless Devices

Battery-powered solutions for industrial-grade wireless devices need to be intelligently designed to minimize size and weight without compromising overall performance. This objective is especially important if the industrial-grade wireless device needs to operate for extended periods in remote locations or extreme environments.

Certain applications permit design engineers to think short term and choose a low-cost battery that reduces the initial purchase price. Conversely, if a remote application requires a long-term power source to reduce the total cost of ownership, you need to apply a far different calculus.

Battery power is a measure of short-term energy consumed. This term should not be confused with the cell’s total amount of energy or nominal capacity. Certain types of devices require high amounts of power (high pulses) for short bursts without exhausting a large amount of total energy. These applications include surgical power tools that operate for minutes, cells that actuate an electromechanical device, and military applications that draw large amounts of energy for limited periods (i.e., guided munitions).

Such specialized applications are not well served by ordinary battery technologies that cannot deliver a high power-per-energy ratio. Such specialized requirements could demand more and larger cells to compensate for their low pulse design. This often leads to a compromise solution of using larger or more cells with unused energy capacity. One alternative is to use lithium metal oxide batteries that have a very high power-per-energy ratio.

High pulse requirements

The Industrial Internet of Things is increasingly reliant on remote wireless devices that require high pulses to power two-way wireless communications. Alkaline batteries are ideal for delivering high pulses due to their high-rate design. However, they have major limitations in industrial applications, including low voltage (1.5 V), a limited temperature range (0°C to 60°C), a very high self-discharge rate that shortens their life expectancy, and crimped seals that may leak. Alkaline batteries often need be replaced every few months, making them totally unsuited for long-term deployment in remote locations.

Standard bobbin-type LiSOCl2 battery chemistry is overwhelmingly preferred for low-power remote wireless devices. The major drawback of this chemistry is its inability to deliver high pulses, as it can experience a temporary drop in voltage when first subjected to a pulsed load—a phenomenon known as transient minimum voltage (TMV).

A way to circumvent TMV is to use a battery that combines a standard bobbin-type LiSOCl2 cell with a hybrid layer capacitor (HLC). The battery and the HLC work in parallel—the battery supplies low-current background power in the 3.6- to 3.9-V nominal range, while the single-unit HLC delivers periodic high pulses to power two-way wireless communications. The HLC also has a bonus: a unique end-of-life voltage curve plateau that can be interpreted to generate low-battery status alerts.

Supercapacitors are commonly used to minimize TMV in consumer electronics but are ill-suited for most industrial applications due to drawbacks like bulkiness, a high annual self-discharge rate, and an extremely limited temperature range. Moreover, when multiple supercapacitors are combined, they require expensive balancing circuits that add expense and draw additional and greater current to further shorten battery life.

Industrial-grade Li-ion rechargeable cells 

If your low-power application draws enough energy to prematurely exhaust a primary (nonrechargeable) lithium battery, then it may need an energy-harvesting device in combination with a rechargeable lithium-ion (Li-ion) battery. Consumer-grade rechargeable Li-ion cells have limitations for industrial applications, including a maximum battery life of roughly five years and 500 full recharge cycles, a narrow temperature range with no ability to discharge or recharge at extremely cold temperatures, and the inability to generate the high pulses needed to power two-way wireless communications.

The feature originally appeared in the April issue of InTech magazine.

About The Author

Sol Jacobs, vice president and general manager of Tadiran Batteries, has more than 30 years of experience in developing solutions for powering remote devices. His educational background includes a BS in engineering and an MBA.

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