By Sara M. Bradford, Industry Manager

Radio Frequency Identification (RFID) tags send stored information to the reader. In the case of automatic identification toll tags, the reader identifies the tag through the transmission of radio signals and the utilization of its battery source. After the tag is "read," a radio signal is sent to the tag’s receiver. The battery is solely responsible for sending the signal back to the reader.

The RFID market is very diverse, with the technology being utilized in varied application markets such as manufacturing and logistics, security and control, transportation, and other purposes. Although the market has been affected by the economic downturn, particularly in the United States, this industry is set to expand and experience healthy growth in the coming years.

Logistics users include inventory location/control, warehouse management, work-in progress, and labor tracking to name a few. Manufacturing users utilize RFID on a continual basis to monitor many aspects of the daily activities.Though the growth in this section is continual, it is slow. This is due to the high cost of tags and readers. With the introduction of low-cost, high performance tags, RFID systems are now able to compete with bar code systems.

The transportation RFID market consists of electronic toll collection, rail car tagging (becoming a niche market), fleet management, loading docks, waste recycling, and other applications. This is a mature RFID application.

Security and access control applications include access control in restricted areas of the corporate, government or residential areas; tracking of assets and personnel; car immobilizers and access control systems; tracking prisoners; tracking patients; parking garages; and other areas. This market is in the development stage, and is set to grow healthily in the coming years, particularly with the increase in security precautions

Although the primary concern of RFID users is the operating performance of system hardware and software, batteries play a very important role in the mix. Without the energy the battery provides, the tag cannot send the needed information to the system reader. The choice of battery technology definitely has an effect on the overall system performance in the long-term. This effect is through factors including how long and how reliable the tag performance is under all circumstances.

Each RFID application has a different requirement for batteries, typically based on electrical and physical considerations, in addition to environmental concerns. Additionally, the cost of the battery plays an important role in the determination of what type of battery is ultimately used in the RFID tag.

Typically, the choice of battery used is unseen to the final end-user. Primary lithium has been a favorite option in this market, as the chemistry offers several positive factors including high energy density, long life (approximately ten years), and long storage life. Additionally, this chemistry is ideal for RFID tag applications because it is light-weight.

For RFID tag systems, primary lithium/manganese dioxide (Li/MnO2) and lithium/thionyl chloride (Li/SOCL2) are the two types of batteries that are most common. These lithium batteries offer a set of performance and safety characteristics that are optimal for RFID tag applications.

The Li/MnO2 battery cell consists of three components: an anode, a cathode, and an electrolyte. The anode contains the pure lithium metal, while the cathode is made up of manganese dioxide in an electrolytic layer that can contain a variety of agents to enhance conductivity. This variation in agents depends on the design and structure of each battery manufacturer.

The electrolyte is dissolved in an organic mixture typically of propylene carbonate and dimethoxyethane. Li/MnO2 is relatively safe compared to volatile lithium batteries such as lithium/sulfur dioxide (Li/SO2) and lithium/thionyl chloride (Li/SOCL2), and does not develop any gas or pressure during battery operation.

However, one main disadvantage is that a single Li/MnO2 cell cannot operate at voltages greater than 3.0 volts. These are typical in high pulse applications that Li/SO2 and Li/SOCL2 can satisfy. Li/MnO2 cells are best suited for applications that have relatively high continuous or pulse current requirements. However, because most electronic components used in RFID tags require a minimum operating voltage of 3 volts, at least two Li/MnO2 cells must be connected in series to ensure a proper margin of safety for system reliability. This requirement adds weight and cost while potentially decreasing reliability due to increased part count.

Overall, the Li/MnO2 chemistry has a high energy density, and has the ability to maintain a high rate of discharge for long periods of time. It can be stored for a long time (typically between five and ten years) due to its low self-discharge rates. It also has the capability to supply both pulse loads and maintain a constant discharge voltage.

Li/MnO2 cells can operate in temperatures ranging from -20 degrees to +70 degrees Celsius (C), although storage in temperatures exceeding +55 degrees C is not highly recommended, and operation will be below full energy capacity at low temperatures. Their nominal voltage is typically 3.0 volts, which is two times the amount of that found in alkaline manganese batteries.

The Li/SOCL2 battery comprises three parts: a negative-charged pure lithium metallic anode or alloy anode, a positive-charged carbon cathode, and an electrolyte made of thionyl chloride and an electrolyte salt. Two types of batteries exist in the thionyl chloride battery market: low rate and high rate. Low rate cells are of bobbin constructions that are utilized in applications requiring low discharge rates. With the usage of a thick electrode, the bobbin construction is applied to the design of this battery to maximize the amount of energy density. The high rate battery cells are spiral wound and are designed for use in applications requiring high discharge rates, high rate operation, or high current pulse capacities.

Li/SOCL2 cells function through the oxidation of the lithium anode and the decrease in the amount of thionyl chloride at the cathode. The cells use a lithium anode and a gaseous cathode that is dissolved in an inorganic electrolyte. This electrolyte is made up of a soluble lithium aluminum chloride that is dissolved in thionyl chloride. In order for the battery to operate, a layer of lithium chloride forms on the lithium anode when the electrolyte interacts with the cell. This layer becomes a problem when it begins to thicken excessively and is insoluble, which is caused by the interaction of lithium and thionyl chloride.

When the battery cell is first placed into a circuit, its voltage drops from the open circuit to an operating voltage, resulting in a delay. This hesitation typically occurs when a load is initially put on the battery cell. This delay is a function of the discharge current; the higher the current, the longer the delay time. This delay from passivation prevents the self discharge of the battery when it is in storage.

Li/SOCL2 is a low-pressure system that is considered superior to lithium/sulfur dioxide systems in terms of high-temperature and/or unusual form factor applications. Due to its low self-discharge rate, Li/SOCL2 has a shelf life of a maximum of ten to fifteen years. This service life is the same for all construction, whether being cylindrical, coin, or wafer. This chemistry also has the highest open-circuit voltage of 3.6 V.

For most applications, only one cell of Li/SOCL2 is required to maintain sufficient operating voltage. This is true as long as one cell can provide enough current to uphold the operating lifetime. RFID tag applications require very low continuous current and moderate pulse current, which Li/SOCL2 batteries can easily provide.

The Li/MnO2 chemistry has a few design issues that may make it unsuitable for certain RFID applications. These include problems with the spiral-shaped cathode and seals. Under extreme conditions, the seals may fail prior to the case failing, allowing the cell elements to escape. Additionally, over time, the electrolyte can wear through the seal, which can ultimately shorten the battery life. With high temperatures, these issues become apparent at a higher rate.

On the other hand, Li/SOCL2 cells are manufactured in hermetically sealed and welded cases in a bobbin construction. The electrodes are well divided and thus cannot come into contact with each other unless the cell is utterly crushed.

The RFID battery market is dynamic, with high and low growth periods experienced in the past couple of years. However, as RFID system applications continue to expand and strengthen, the battery market will be affected in parallel. Vendors including Tadiran and others are posed for a healthy growth in coming years.