I am not sure if it's a matter of NFC (Near Field Communication) standard or implementation of an NFC reader, but if a passive NFC tag is left attached to a reader, does that constantly drain power from it or not?
I've noticed that when I try scanning NFC tag (NFC Forum Type 2 Tags) with a mobile phone, they scan happens just once, so some de-duplication is built in, but I am still not sure if it silently re-scans all the time or not.
The answer is yes. The reader (phone) has to scan continuously to see if it is there. Even if this does not happen at the hardware level, it would be possible to continuously "ping" the reader coil via firmware, so again the answer is "yes" - this will continuously drain the battery.
To open up my answer I will use information from the Android developer page, first sentence from sub heading Tag dispatch system:
Android-powered devices are usually looking for NFC tags when the screen is unlocked, unless NFC is disabled in the device's Settings menu.
This indicates that the device is using current to power the antenna continuously, waiting for a tag to come into the field, thus using small amounts of current, using <100mA to power the TX circuitry for reading, when writing it uses more than 100mA in some instances. This information is from the PN532 datasheet, this is the short one and used in a few small open projects.
Another reference showing the low consumption of current ICs, an integrated bluetooth and NFC chip for these smart watches and health monitors, from the release article:
The device achieves peak power consumption of 5.9mA for Bluetooth communication (@3.3V, -4dBm transmitter output power or receiver operation) and just 600µA or lower for NFC Tag communication (@3.3V).
Reading through, cowboydan's link to the Application note, I wouldn't be surprised if that is implemented in some way on NFC phones, but I don't think you are going to get a designer from some well known company saying: 'Hey, here's the circuit and firmware we use with the NFC apps.' :)
An NFC sensor sell sheet from AMS, the sensor is said to harvest 4mA at 3.3V typical, this will obviously have little affect onthe reader, also from reading the datasheets of common NFC tag type 2 chips the consumption is extremely low.
The are several ways they may have implemented a method of not reading a Tag twice,
The firmware may store the UID from the Tag once it has performed its operation within the desired application and puts the reader into a sleep mode and waits x time, after which it restarts and reads the tag again and if it has changed, it runs the appropriate stuff again, else it waits again etc. Or
The firmware can read the tag and initiate the dispatch sequence and such used by the android apps, then store the Tag UID, and ignore that tag for a specific time before it sees it as another instance, without a sleep mode, or
The hardware and firmware implementation used on the Texas app note could be implemented to reduce current usage.
So to conclude the tags will vary with there current draw, 3<mA<30, these will not harvest much from the readers powering circuit when running at the typical ranges.
Also there will defiantly be firmware to 'silently' re-scan the tag to see if it is a different one, but this will probably be done so that it uses the least amount of power.
Look at this video...Explans NFC in Video Format
and sources...Tells More
"Passive NFC devices include tags, and other small transmitters, that can send information to other NFC devices without the need for a power source of their own. However, they don’t really process any information sent from other sources, and can’t connect to other passive components. These often take the form of interactive signs on walls or advertisements."
See in context this is saying that the active one's actually are electronically powered by the device being used. They have a transmitter and a receiver, But in these usually only have one transmitter and one receiver. This also means that Passive NFC's cannot take charge because how they are powered in the quote and diagram below. (Just in case you may not have known, you can send data over inductive currents.)
This marks the one major difference between NFC and Bluetooth/WiFi, as it can be used to induce electric currents within passive components as well as just send data. This means that passive devices don’t require their own power supply, and can instead be powered by the electromagnetic field produced by an active NFC component when it comes into range, but we’ll talk about that in greater detail some other time. Unfortunately, NFC technology does not command enough inductance to be used to charge our smartphones, but QI charging is based on the same principle.
The Reader can detect the magnetic field in the transmitter, and that's how they are powered and how they send information. It is not that their is an actively powered component that is always searching for cards (at least in the passive version). It is simply the fact that when the Card gets powered, it says "Ooh, I'm awake. OK, I'll just read/transmit my information, and then sleep again." But of course, this is why the device needs to be very close. Passive cards can only read and or transmit with their inductive fields, but when actives are be powered by the device, their range is amplified, and the active card stores information.
So as a long story short. No.
Any process or service running actively or hidden in the background will use some battery resources. It may be helpful to know, if you use a custom rom, it allows you to analyze battery usage in much deeper detail, and control aspects of your hardware many stock Operating systems have no control over. But in all fairness, you will waste battery power by constantly analyzing each new feature, that they constantly install into our phones. The short answer is just about anything you don't use should be turned off to get the most life from a battery. I found it seemed like my phone died much quicker, when I enabled NFC. And I never really used it, it seems like an obsolete feature. It gave me concerns about the security of my device.
