Get Charged Up About RFID Battery Options

There's no shortage of battery choices for RFID tags. Sol Jacobs, VP of Tadiran Electronic Industries, maps out terrain.

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By Sol Jacobs, Vice President & General Manager
Battery Division, Tadiran Electronic Industries Inc.

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 accessibility 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 RFID 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 RFID 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 -55° C and as high as +150° C.

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 RFID 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 RFID 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 RFID systems.

While the range of RFID 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.

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