Semi-passive tags are also called semi-active, battery-assisted passive (BAP), or battery-assisted tags (BATs). This tag has an onboard battery to power its IC, but, like a passive tag, it does not have an active transmitter. It uses backscatter to communicate with the interrogator. It modulates the reflection of the waves from the interrogator and requires an interrogator to send data. Since no transmitter is present, it also does not contribute to radio noise. This type of tag is used because it can provide a longer read range than passive tag and can accommodate environmental sensors onboard. The sensor on the tag helps record the environmental experience of the object to which the tag is attached.
You may wonder why the semi-passive tag includes a battery. First, the battery provides power to the IC so the tag does not depend on the interrogator to power it. The forward link is used to provide power to the tag and to communicate with the tag, and it has a smaller range than the return link. The return link, which includes a round trip from interrogator to tag and back to interrogator, still has a longer range than the forward link. This means that the limiting factor for a read range of a passive tag is how far away from the antenna a tag can be powered, not how far away from the antenna the signals can be decoded. With that in mind, if we provide a battery to power the IC, we can extend the read range of the passive tag. Therefore, by slightly modifying the passive tag to accept the power from onboard battery, we can increase its read range.
The second reason to have a battery on the tag is to attach an environmental sensor to the tag. Environmental sensors require reliable continuous power to operate, and they require a higher power level than the tag IC. The passive tag cannot have reliable and continuous power because it gets power only when it is in the IZ, which is not the case most of the time. In addition, when it gets the power inside the IZ, it is very limited—not enough to power an onboard sensor. Therefore, if a battery is added to the tag, the sensors and the tag IC can use that power, which is available all the time. The sensors on the tag can collect data and transmit that data along with the identifying number when the tag is interrogated. This is a valuable prospect, because, for example, it allows you to collect the temperature experience of the object automatically as the object moves around during its lifecycle. Therefore, when a case of temperature-sensitive drugs is transported, you can collect a record of how long the case was subjected to the temperatures beyond certain set limits and dynamically calculate its expiry date or its potency. Frozen food storage conditions can be easily monitored using this type of tag. Temperature, pressure, relative humidity, acceleration, vibration, motion, altitude, and chemical sensors can all be placed on a tag.
The extra functionalities of an on-tag battery do not come without cost, however. The battery creates a few problems, such as extra weight, larger size, higher cost, shorter life, and temperature sensitivity. An integrated battery means the tag dies when the battery dies, and a replaceable battery means larger size and weight for the battery compartment.
How do you know when a battery is drained? The interrogator can never detect a tag that does not respond. If the object with the tag is sitting outdoors in below-freezing temperatures, the battery may not work and the tag will not respond. Batteries on tags have a typical maximum shelf life of 2 to 7 years, but when tags are used and interrogated frequently, the batteries can quickly be depleted.