- By Sol Jacobs
- July 19, 2021
Understanding your application can help to miniaturize your battery-powered solution. This article comes from the IIoT & Industry 4.0 eBook.
Choosing a battery for a consumer-grade device is relatively easy compared to powering an industrial-grade device, where applicationspecific requirements present far greater challenges. This is especially true for low-power devices that need to be intelligently designed to conserve energy so as to operate maintenance-free for decades in remote locations and extreme environments.
Selecting a battery that is ill-suited for the application can cause the power source to be unnecessarily large and expensive, especially if it requires more frequent battery replacements, since the added time and labor costs will far exceed the price of the batteries. For this reason, specifying an industrial-grade battery is a critical decision that requires a firm understanding of the various primary (nonrechargeable) chemistries that are commercially available (table 1).
Here are some key considerations common to most low-power applications:
Operating voltage – Simple math dictates that it takes two 1.5 V cells to deliver the same voltage as a single 3.6 V cell. Choosing a battery with higher voltage can reduce the size and weight of the device and potentially lead to even greater savings if fewer cells are required
Cold and hot temperatures – Extreme temperatures reduce battery voltage and capacity under pulse. If a cell with a limited temperature range is deployed in an extreme environment, oversized batteries may be required to compensate for an expected drop in voltage under pulsed load. This can be avoided by using a bobbin-type lithium thionyl chloride (LiSOCl2) battery that features extremely high energy density along with the ability to handle high pulses at extreme temperatures, thus eliminating the need for extra capacity and/or voltage.
Lower self-discharge – Consumer-grade battery technologies suffer from extremely high self-discharge of up to 8 percent per month, thus requiring a larger battery to compensate for expected capacity losses. Extreme temperatures also cause an accelerated selfdischarge rate with most battery chemistries. If the device needs to operate maintenance-free for decades, then it is essential to specify a battery with an exceptionally low self-discharge rate to eliminate future battery replacements and to possibly enable the use of a smaller battery.
For example, a superior-quality bobbin-type LiSOCl2 battery features a self-discharge rate of 0.7 percent per year, retaining over 70 percent of its original capacity after 40 years. By contrast, a lessergrade battery using the exact same chemistry can experience a much higher self-discharge rate of 3 percent per year, exhausting 30 percent of its original capacity every 10 years, making 40-year battery life unachievable
Power is often confused with energy – Demands for battery power (a measure of short-term energy consumed) are often confused with the total amount of energy required (total battery capacity). Certain wireless devices require relatively high amounts of power (high pulses) for short bursts without exhausting a large amount of total energy. Common examples include surgical power tools that may operate for a few minutes, industrial actuation devices that perform sporadically, and certain mil/aero applications.
Most commercially available battery technologies are not designed to deliver a high power-per-energy ratio to satisfy this type of requirement. They need to use more cells to compensate for their low pulse design, which often leads to compromise solutions that add bulk and unnecessary cell capacity.
Handling high pulse requirements – Throughout the Industrial Internet of Things (IIoT) we are seeing growing demand for applications requiring high pulses of energy to power twoway communications and other advanced functionality. Alkaline batteries can deliver high pulses due to their high-rate design, but have major drawbacks, including low voltage (1.5 V), a limited temperature range (0°C to 60°C), a high selfdischarge rate that reduces life expectancy, and crimped seals that may leak. Alkaline batteries often need to be replaced every few months due to their very high selfdischarge rate, causing additional long-term maintenance expenses as well as reliability concerns, which are often critical considerations for remote wireless applications.
Standard bobbin-type LiSOCl2 batteries are overwhelmingly preferred for long-term deployments in remote locations and harsh environments. These cells are not designed to deliver high pulses, experiencing a temporary drop in voltage when first subjected to a pulsed load: a phenomenon known as transient minimum voltage (TMV).
Consumer electronic devices often use supercapacitors to minimize TMV. However, supercapacitors are ill-suited for most industrial applications due to their inherent drawbacks, including bulkiness, a high annual self-discharge rate, and a limited temperature range. Solutions involving multiple supercapacitors also require the use of expensive balancing circuits that draw additional current to further increase self-discharge.
A simpler solution is to combine a standard bobbin-type LiSOCl2 cell with a patented Hybrid Layer Capacitor (HLC). The two technologies work in parallel: the battery supplies low-level background current in the 3.6 to 3.9 V nominal range; while the HLC acts like a rechargeable battery to deliver periodic high pulses. This hybrid solution offers an added bonus in the form of a unique end-of-life voltage curve plateau that can be interpreted to deliver low-battery status alerts.
Long-life rechargeable Li-ion cells – If your application demands enough energy to prematurely exhaust a primary battery, it may require the use of a rechargeable Lithium-ion (Li-ion) battery. However, consumer-grade Li-ion cells have serious performance limitations, including a maximum battery life of approximately three years and 300 full recharge cycles, a relatively narrow temperature range with no ability to discharge or recharge at extremely cold temperatures, and the inability to generate high pulses required to power two-way wireless communications (table 2).
If the rechargeable device needs to operate for more than 300 full recharge cycles, then additional consumer-grade Li-ion cells may be required to reduce the average depth of discharge per cell. Choosing a rechargeable battery with a higher cycle life can often reduce the number of cells required. Industrial-grade rechargeable Li-ion batteries are available that can last for up to 20 years and 5,000 recharge cycles while also delivering the high pulses (15 A pulses and 5 A continuous current) required to power two-way wireless communications. These ruggedized cells also feature an extended temperature range (–40°C to 85°C), allowing them to be charged and discharged at extremely cold temperatures.
Reducing battery size with solar-powered energy harvesting
In some cases, PV panels and their companion rechargeable batteries need to be over-designed to ensure reliable operation under worst-case scenarios, such as five straight days of cloudiness. One solution is to use primary (nonrechargeable) bobbin-type LiSOCl2 cells as a backup solution to recharge the batteries on sunless days, thus enabling the use of smaller PV panels as well as smaller batteries. This type of backup energy supply is especially valuable during months of extended darkness (polar winters) or if the battery requires extended storage.
Cheaper can get more expensive – There are certain situations where a short-term solution is preferred to achieve the lowest possible initial purchase price. By contrast, many industrial applications require long-term solutions to achieve a lower cost of ownership over the lifetime of the device.
If your wireless device is intended for long-term deployment in a highly remote and inaccessible location, then you will most likely need a battery-powered solution that minimizes future battery replacement. The additional cost of labor, loss of data integrity, and downtime will far exceed any initial savings realized by specifying a lower-quality battery.
In addition, you need to be wary of the hidden costs associated with excessive battery size and weight. For example, a compact and lightweight power supply could be especially valuable to research scientists conducting experiments in frigid Artic conditions, as they have limited dexterity and need to conserve energy. Solutions that reduce battery size and weight also serve to minimize transportation costs, as the shipment of lithium batteries has become more expensive due to increasingly restrictive UN and IATA shipping regulations.
As a general rule, if the wireless device is easily accessible and operates at ambient room temperatures, then consider using a consumer-grade battery. However, if your application involves a long-term deployment in a remote location or extreme environment, then pay a little extra for an industrial-grade lithium battery that can operate for the life of your device in order to reduce your total cost of ownership.
This article comes from the IIoT & Industry 4.0 eBook.
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