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RFID solutions

2016-05-04

RFID solutions use non-rechargeable batteries that are also known as primary batteries. The battery requirements for RFID depend on the power requirements of the application, the operating environment, portability, size, weight and battery cost. 
The most significant battery characteristics to consider when comparing primary battery types for RFID are - the specific energy, energy density, voltage across the battery terminals when it is at full capacity and not supplying any current (i.e., open circuit voltage), lifespan of the battery, its continuous and pulse current capacity and the temperature range in which the battery can operate. 
Batteries used in RFID tags must have a long shelf life, low self-discharge and should be operationally safe. Additionally these batteries should be small and lightweight so that the RFID tag can be compact. For example, in the RFID tag used for toll payment the battery is slim and flat to complement the card shaped RFID tag.

Types of Primary RFID Batteries

Lithium has been the most popular choice of material for primary RFID batteries because it is very light, non-gaseous and has a high negative potential. Lithium based batteries have the highest specific energy and energy density in comparison with other types of primary batteries. Additionally, lithium batteries have a very wide operating temperature range. 
The most common primary lithium batteries used for RFID tags today are Lithium/Manganese Dioxide, Lithium/Thionyl chloride, Lithium/Polycarbon Monofluoride and Lithium/CFx.

Lithium/Manganese Dioxide (Li/MnO2)

The low cost Li/MnO2 batteries have an operating temperature range of -20°C to 60°C. They are environmentally safe, have a high energy density and an open circuit voltage of about 3V. These batteries can maintain a fairly constant discharge voltage and can be stored safely for 5-10 years as they have low self-discharge. Since the minimum operating voltage of the electronics in RFID tags is 3V, at least two Li/MnO2 cells must be connected in series to ensure reliable operation. This adds to the weight and cost of the RFID tag and also impacts its reliability. 
Li/MnO2 batteries are not suitable for RFID applications that require spiral shaped cathodes, because the adjacent layers of the cathode can come in contact with each other in the event of shocks or vibrations, and render the battery unusable.

Lithium/Thionyl Chloride (Li/SOCl2)

Li/SOCl2 batteries have an operating temperature range between -55°C to 150°C, high pulse power capability and a long shelf life of 10 to 15 years. They have a high open circuit voltage of 3.6V. For most RFID tag applications a single Li/SOCl2 battery is sufficient as long as it can maintain the voltage above 3.0V. Additionally, Li/SOCl2 batteries can deliver low continuous current as well as moderate pulse currents that RFID tag applications require. However, this type of battery suffers from passivation, generates toxic waste and is unsafe because of pressure build up during sustained high discharge. 
Passivation is the phenomenon in lithium batteries wherein a layer of lithium chloride forms on the surface of the lithium anode. This layer prevents the battery from self-discharging when it is unused. However, when this type of battery is used in a circuit, the layer of lithium chloride causes the battery voltage to dip before rising to a peak value and then stabilizing. The dip in battery voltage can significantly impact the performance of the RFID tag.

 

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