Active RF transponder tags have expanded into a wide range of applications - from asset tracking to automatic toll-paying to wildlife research. While users are - and should be - primarily concerned with the operating performance of system hardware and software, the system manufacturer's choice of battery technology has a direct effect on how long and how reliably that performance is delivered. Each application class has its special requirements for batteries, based on electrical, mechanical, physical (i.e., weight and size) and environmental considerations as well as cost and accessability for replacement. While the choice of battery technology is often invisible to the end user, a bit of education on battery choices can be beneficial.
First, some battery basics: Batteries are divided into two main categories: primary (nonrechargeable) and secondary (rechargeable). As the batteries employed in RF ID tags are exclusively of the primary type, this discussion will include only nonrechargeable type.
Primary battery types are themselves classified according to the chemical system by which they store and release electrical energy. The major classes are LeClanche (the traditional zinc/carbon/ammonium chloride "dry cell" type), alkaline (widely used in consumer applications), zinc-air (for hearing aids and similar applications) and lithium.
The most important characteristics by which these battery types are compared are specific energy (amount of energy available per unit mass, expressed in watt-hours per kilogram), energy density (energy per unit volume, expressed in watt-hours per cubic centimeter), open circuit voltage (the voltage available at the battery terminals when the battery is at full capacity and not supplying current, expressed in volts), service lifetime (the time, in months or years, that the battery can be expected to perform as specified) and operating temperature range.
For RF ID tag applications, even the most efficient alkaline cells cannot come close to matching the two most popular lithium cell types in the characteristics just described. Of all the primary battery chemistries, lithium has stirred the most interest in the electronics industry at large. Lithium is an ideal material for battery anodes because of its high intrinsic negative potential, the greatest of all metals. Lithium is also the lightest nongaseous metal.
Batteries based on lithium chemistries have the highest specific energy and energy density of all types. The high energy density is a result of lithium's high negative potential and the fact that lithium reacts strongly with water. The latter characteristic precludes the use of any aqueous (water-containing) electrolyte, but this turns out to be a benefit. Because the oxygen and hydrogen in water dissociate in the presence of a potential above 2 volts, cells using aqueous electrolytes (such as alkaline cells) are limited in voltage. Lithium cells, all of which use a nonaqueous electrolyte, have nominal open circuit voltages (OCVs) of between 2.7 and 3.6 volts. Lithium batteries also have extended operating temperature ranges, enabled by the absence of water and the chemical and physical stability of the materials. Some lithium-based systems, including Tadiran's inorganic thionyl chloride system, can operate at temperatures as low as -55C and as high as +150C.
Under the broad category of primary lithium battery types there are several chemical systems in "mainstream" use, each with its own set of performance and safety characteristics. However, only two have characteristics that allow them to be considered for use in RF ID tag systems. These are lithium/manganese dioxide or Li/MnO2 and lithium/thionyl chloride or Li/SOCl2.
Lithium/manganese dioxide cells have an OCV of 3.1 V and moderately high energy density. They are best suited to applications having relatively high continuous or pulse current requirements. However, because most electronic components used in RF ID tags require a minimum operating voltage of 3 V, at least two lithium/manganese dioxide cells must be connected in series to ensure a proper margin of safety for reliable system operation. This requirement adds weight and cost while potentially decreasing reliability due to increased part count.
Lithium/thionyl chloride cells have the highest energy density of all lithium types and they also have the highest open-circuit voltage, 3.6 V. Thus, in most applications, only one cell is required to maintain sufficient operating voltage, so long as one cell is sufficient to supply the current necessary for the required operating lifetime. Service life is an unmatched 15-20 years and holds for all case types - cylindrical and coin or wafer. Lithium/thionyl chloride cells are best suited for applications having very low continuous current and moderate pulse current requirements - a description that fits most RF ID tag applications. Their extremely long service life and low self-discharge rate (the rate at which a cell loses energy while not in use) make them ideal for applications where physical access is limited, where it is desired to have a very long time to battery replacement or where replacement is not desired during the service life of the device being powered. Again, these conditions are often encountered in RF ID systems.
While the range of RF ID tag applications is broad, battery requirements are similar. For toll tags, expected service life is ten years, although systems-makers specifications often call for a minimum of six years. The battery must be small and flat so that the tag can fit under a sun visor or on a license plate or in a similar location. The expected temperature range is -40C to +80C for exterior tags and -40C to 113C as specified in SAE Paper J1211 for interior tags. It is also desirable to employ a UL-approved battery.
Asset-tracking tags have similar environmental requirements for batteries, but some require slightly higher pulse currents because the transmitter portion of the tag has greater output power for reaching somewhat more distant readers than may be the case in toll-tag applications.
In wildlife-tracking applications, the batteries must be able to survive a wide temperature range, but the primary considerations are compactness, light weight and service life.
According to a leading maker of toll-tag systems, in early toll tag versions, a nonreplaceable button cell was sufficient to supply current for the required minimum of six years. However, as toll tag designs have progressed, new features require more current. These new features include LEDs and audible warning devices for signaling the user that the allotted funds are about to be depleted. For these newer systems, the tags are powered by cylindrical, AA-size ("penlight") cells.
While the two lithium battery chemistries described above differ in important electrical and lifetime characteristics (see Table I for quantitative information), the type of construction used in cylindrical cells separates the two types even more. As RF ID tag systems migrate to more feature-rich and current-hungry designs, it is logical to assume that cylindrical cells will find wider use. Thus, the differences between the two major types in cylindrical form becomes increasingly important.
Lithium/manganese dioxide cells are manufactured with a spiral-shaped cathode ("jelly roll" construction) and crimped, nonhermetic elastomer seals. Although generally safe, under extreme conditions the elastomer seals can fail before the case fails, thus allowing the cell constituents to escape. Another potential problem is that the electrolyte can, over time, migrate through the elastomer seals, shortening battery life. Storage or operation at higher temperatures accelerates this process.
Several leading makers of various types of RF ID systems - including asset tags, toll tags, wildlife-tracking systems and house-arrest monitoring systems - have found that the spiral cathode construction of cylindrical lithium/manganese dioxide cells can cause other problems. Specifically, if the cells are subjected to shock or vibration of sufficient magnitude in a direction perpendicular to the long axis of the cell, adjacent layers of the jelly-roll cathode can come into contact with each other, causing the battery to perform intermittently or become completely unusable. As many RF ID tag applications have significant shock and vibration survivability requirements, the use of spiral-electrode cells can adversely affect system reliability.
Lithium/thionyl chloride cells are manufactured in welded, hermetically-sealed cases using a "bobbin" construction, in which the electrodes are a central rod and a surrounding "can". The electrodes are well-separated and cannot come into contact with each other unless the cell is nearly completely crushed.
Another significant difference between lithium/manganese dioxide and lithium/thionyl chloride is that the former has a limited operating and storage temperature range. A leading maker of wildlife-tracking systems has reported that lithium/manganese dioxide cells, when exposed to temperatures often encountered during winters in the northern U.S. ("lower 48", not Alaska), failed to deliver specified voltage when installed and could not be made to do so. This maker switched to lithium/thionyl chloride and the problem was not encountered again.
At the other end of the temperature scale, tags used to track pallets or work pieces through production processes may be subjected to temperatures in excess of 100C. [Example: The Sony Electronics San Diego Manufacturing Center in Rancho Bernardo, California employs RF ID tags on picture tubes and pallets to collect process data on individual tubes. Tubes, pallets and tags are subjected to elevated temperatures at several points in the manufacturing process.] Only lithium/thionyl chloride cells can survive such elevated temperatures and continue to perform reliably.
As can be seen from the foregoing discussion and the data in Table I, the two lithium battery chemistries differ in several important characteristics. Carefully matching these characteristics to the conditions of a particular application is key to safe and reliable operation of the system. In order to ensure success for all involved, system operating requirements and expected environmental conditions must be clearly communicated all the way up the "food chain" - from the end user to the system integrator, to the system manufacturer and finally to the battery maker.
Characteristic Li/MnO2 Li/SOCl2 Nominal OCV 3.1 V 3.6 V Internal construction Spiral Bobbin Hermeticity Nonhermetic Hermetic Hermeticity after temperature cycling Potential leakage Excellent Flammability of electrolyte Flammable Nonflammable Energy density 637 Wh/L 1080 Wh/L Specific energy 319 Wh/Kg 430 Wh/Kg Operating temperature range -20C TO +60C -55C to +85C(1) Shelf life under manufacturer-specified storage conditions 10 years 10 years More than one source Yes Yes (1) Standard - High temperature version to +125C
Tadiran batteries - manufacture of specialised Lithium batteries for RFID applications
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