Considerable confusion exists regarding the choice of available battery technologies.
This general lack of knowledge stems partly from the fact that consumer batteries have become so ubiquitous, and their use so often taken for granted, as billions of rechargeable and primary (non-rechargeable) batteries are manufactured annually for use in all types of devices.
Batteries found in handheld devices are usually designed to be easily recharged or easily replaceable, and operate at moderate temperatures. Since most cell phone contracts allow for an equipment upgrade every two years, and laptop/tablet devices become rapidly obsolete, the typical consumer rechargeable battery only needs to work for about five years and approximately 500 full recharge cycles.
The inexpensive over-the-counter primary lithium batteries that are used to power flashlights, remotes, and toys, operate within a narrow temperature range (-0° to 40°C) and have a short life expectancy (two to three years).
Meanwhile, we are seeing dramatic growth in industrial wireless applications intended for remote, off-the-grid locations, where the wireless device needs to be self-powered for its entire operating life, which can be up to 40 years.
These industrial applications include automotive toll tags, GPS tracking devices, oceanographic instruments, automated utility meters, process control, and monitoring devices.
These applications also involve extreme temperatures, ranging from cold chain temperatures as low as -80°C, to extremely high temperatures of up to 150°C. Consumer-grade batteries are not designed to operate in these extreme environments.
Primary Cells vs. The Market
Alkaline cells are readily available and extremely inexpensive, but have inherent drawbacks, including low voltage (1.5 V), high annual self-discharge rate, and a limited temperature range.
Alkaline batteries are also constructed with crimped seals that may be prone to leakage and corrosion, and are not well suited for delivering extended life in extreme environments.
Primary lithium cells (1.5 or 3 V) were designed to deliver the high pulses required for camera flashes. However, these batteries have a narrow temperature range (-20° to 60°C), a high annual self-discharge rate, and crimped seals, which make them unsuited for long-life in extreme environments.
For such challenging applications, the preferred choice is bobbin-type lithium thionyl chloride (LiSOCL2) chemistry, which features the highest capacity and highest energy density of any lithium chemistry, along with an extremely low annual self-discharge rate, the widest possible operating temperature range, and a glass-to-metal hermetic seal to help prevent battery leakage.
Design engineers need to be aware that bobbin-type LiSOCL2 batteries are not created equal. For example, an inferior quality LiSOCL2 battery may only deliver a 10-year operating life with an annual self-discharge rate of two to three percent per year, while a superior grade LiSOCL2 battery can have a much lower annual self-discharge rate of just 0.7 percent per year, permitting maintenance-free operation for up to 40 years.
The Evolution of Consumer Grade Rechargeable Batteries
The first consumer rechargeable batteries were developed back in the mid-nineteenth century using Nickel Cadmium (NiCad) chemistry. These batteries are large, have low energy density, and suffer from memory effect, whereby the battery does not fully recharge if it is not fully depleted.
The next major development was the Nickel-Metal Hydride (NiMH) battery, which eliminated the memory effect problem, but still suffered from a high annual self-discharge rate, making them incapable of delivering extended storage life.
Another key breakthrough was the consumer grade lithium ion (Li-ion) battery, which features high efficiency and high power output. Best known is the ubiquitous 18650 Li-ion cell, which was developed by laptop computer manufacturers for use in their own products.
Since these laptop computers were intended for planned obsolescence,18650 cells only had to operate for approximately five years and 500 full recharge cycles, with a temperature range from -20° to 60°C.
Consumer grade Li-ion cells also experience a gradual degradation of the cathode, making the battery less receptive to future recharging, further reducing battery operating life.
The introduction of low profile consumer devices, such as the smartphone and tablet computers, led to the development of lithium polymer batteries, also referred to as laminate cells.
These cells use sheets of flexible material that can be rolled or stacked like a deck of cards, with their positive and negative terminals protruding from the cell as tabs. As a result, lithium polymer batteries can be very thin, or quite large, depending on the application.
Unlike standard batteries, which are constructed with a protective steel or aluminum outer can, lithium polymer cells have no outer protective layer, so they are prone to puncture, which can result in an internal short circuit or premature self-discharge. A lithium polymer battery can also swell in size.
Fitting a Growing Need
To meet the growing need for industrial grade rechargeable batteries, companies are working to develop products that can provide up to 20 years of service life with a low annual self-discharge rate, and increased durability.
These advances will help fill the performance gap between short-lived batteries intended for consumer applications and those that can deliver the ruggedness and long-term reliability required to power industrial grade remote wireless devices.
This article originally appeared in the November/December 2014 print edition of PD&D.
