NFC Battery Drain Question

[demarcmj]

So I assume that the battery drain from leaving on NFC 24/7 is probably pretty small since it's not transmitting constantly... similar to leaving on bluetooth all day vs. actually being connected to something all day.

My question is this...

Suppose I have an NFC tag on my desk, and rather than just tapping my phone on it, I place the phone on it and leave it there. How will the phone/tag react in this situation and what are the battery implications of doing this? I assume that the phone will be smart enough to not repeatedly execute whatever that tag's action is, but I'm really curious how the two communicate when they are left in constant contact. Will the phone constantly receive (and dismiss) this NFC tag until they're moved apart? I feel like this constant "seeing" would drain the hell out of my battery, since it's actually engaging the NFC (like the bluetooth example above)? Maybe it acts exactly like that, but doesn't drain as much battery as I think? Maybe it doesn't act like that at all? That's what I'm trying to find out.

This is important because I'd like to stick a tag on the back of my car dock, and this would create a similar situation where the tag and phone would be left in constant contact. If anybody knows how this technology works, please let me know.

 

[ericlmccormick]

I want to do the same thing. I have modified my phone to use a TouchStone and I have a base at work, home and car and I want(when they come in) a NFC tag on each so that my phone knows which location I am at instead of using my GPS location. Mainly for changing things through Locale like bluetooth and volume. While my phone will be on the charger, I wonder how much power draw there will be.

 

[demarcmj]

I want to do the same thing. I have modified my phone to use a TouchStone and I have a base at work, home and car and I want(when they come in) a NFC tag on each so that my phone knows which location I am at instead of using my GPS location. Mainly for changing things through Locale like bluetooth and volume. While my phone will be on the charger, I wonder how much power draw there will be.

Hopefully somebody that is more knowledgeable about the technology itself will be able to answer.

 

[zmore]

Hopefully somebody that is more knowledgeable about the technology itself will be able to answer.

AFAICT, a passive NFC tag is only powered by your phone's NFC RF field for the few hundred milliseconds that it's either being read or written, and that's 15mA.

I'd bet $100 that even if you took a tag and swiped it all day that it'd barely put a dent in the battery life, but, that's an experiment I'll leave to someone else.

(If NFC was a hog, it'd be bigger news.)

 

[demarcmj]

UPDATE: According to the folks in this thread, NFC is off when the screen is off.

So for my desk example, it probably wouldn't be a problem. In that case, the screen will go off after a while, and you'll probably pick it up again when you want to turn the screen back on.

But for my car dock scenario, it's a little different. First, I might have the screen on longer, which means that I still want to know about the battery drain when the two are kept in constant contact with the screen on. Second, if the screen goes off, I probably won't be moving the phone to turn the screen back on (it's secured in the dock)... so every time I turn the screen on, will it read the tag and execute the task?

EDIT: Ok so this post came in as I was typing...

AFAICT, a passive NFC tag is only powered by your phone's NFC RF field for the few hundred milliseconds that it's either being read or written, and that's 15mA.

I'd bet $100 that even if you took a tag and swiped it all day that it'd barely put a dent in the battery life, but, that's an experiment I'll leave to someone else.

(If NFC was a hog, it'd be bigger news.)

So if it only reads for the first couple hundred milliseconds, then the constant contact issue wouldn't be a factor.

But what about the turning the screen off and then on again while still in contact with the tag? How will that one go down? (note, this isn't a battery question... this is something someone can actually verify)

 

[ascsa]

But what about the turning the screen off and then on again while still in contact with the tag? How will that one go down? (note, this isn't a battery question... this is something someone can actually verify)

After turning the screen on (and unlocking it) ... the tag will be scanned again ....

and for the battery drain ... i had a look at the logs on my international gs3 while taped to a tag ...a lot of log messages and probably cpu activity while taped to a tag, repeated approx. every 150 milliseconds ...

Have a look --> this is just 1 second in the log .... and it´s repeated as long as the tag is taped to the device ...

07-12 02:47:25.057: D/NFC_LIST(10575): Allocated node: 0x1702e60 (0x5d5efbf4)
07-12 02:47:25.057: D/NFC JNI(10575): phLibNfc_RemoteDev_CheckPresence()
07-12 02:47:25.057: D/NFC JNI(10575): phLibNfc_RemoteDev_CheckPresence() returned 0x000d[NFCSTATUS_PENDING]
07-12 02:47:25.067: D/NFC JNI(10575): Callback: nfc_jni_presencecheck_callback() - status=0x0000[NFCSTATUS_SUCCESS]
07-12 02:47:25.067: D/NFC_LIST(10575): Deallocating node: 0x1702e60 (0x5d5efbf4)
07-12 02:47:25.192: D/NFC_LIST(10575): Allocated node: 0x17c6f28 (0x5d5efbf4)
07-12 02:47:25.192: D/NFC JNI(10575): phLibNfc_RemoteDev_CheckPresence()
07-12 02:47:25.192: D/NFC JNI(10575): phLibNfc_RemoteDev_CheckPresence() returned 0x000d[NFCSTATUS_PENDING]
07-12 02:47:25.202: D/NFC JNI(10575): Callback: nfc_jni_presencecheck_callback() - status=0x0000[NFCSTATUS_SUCCESS]
07-12 02:47:25.202: D/NFC_LIST(10575): Deallocating node: 0x17c6f28 (0x5d5efbf4)
07-12 02:47:25.332: D/NFC_LIST(10575): Allocated node: 0x1702e60 (0x5d5efbf4)
07-12 02:47:25.332: D/NFC JNI(10575): phLibNfc_RemoteDev_CheckPresence()
07-12 02:47:25.332: D/NFC JNI(10575): phLibNfc_RemoteDev_CheckPresence() returned 0x000d[NFCSTATUS_PENDING]
07-12 02:47:25.342: D/NFC JNI(10575): Callback: nfc_jni_presencecheck_callback() - status=0x0000[NFCSTATUS_SUCCESS]
07-12 02:47:25.342: D/NFC_LIST(10575): Deallocating node: 0x1702e60 (0x5d5efbf4)
07-12 02:47:25.467: D/NFC_LIST(10575): Allocated node: 0x17c6f28 (0x5d5efbf4)
07-12 02:47:25.467: D/NFC JNI(10575): phLibNfc_RemoteDev_CheckPresence()
07-12 02:47:25.467: D/NFC JNI(10575): phLibNfc_RemoteDev_CheckPresence() returned 0x000d[NFCSTATUS_PENDING]
07-12 02:47:25.477: D/NFC JNI(10575): Callback: nfc_jni_presencecheck_callback() - status=0x0000[NFCSTATUS_SUCCESS]
07-12 02:47:25.477: D/NFC_LIST(10575): Deallocating node: 0x17c6f28 (0x5d5efbf4)
07-12 02:47:25.607: D/NFC_LIST(10575): Allocated node: 0x1702e60 (0x5d5efbf4)
07-12 02:47:25.607: D/NFC JNI(10575): phLibNfc_RemoteDev_CheckPresence()
07-12 02:47:25.607: D/NFC JNI(10575): phLibNfc_RemoteDev_CheckPresence() returned 0x000d[NFCSTATUS_PENDING]
07-12 02:47:25.612: D/NFC JNI(10575): Callback: nfc_jni_presencecheck_callback() - status=0x0000[NFCSTATUS_SUCCESS]
07-12 02:47:25.612: D/NFC_LIST(10575): Deallocating node: 0x1702e60 (0x5d5efbf4)
07-12 02:47:25.672: D/KeyguardViewMediator(2098): handleTimeout
07-12 02:47:25.742: D/NFC_LIST(10575): Allocated node: 0x17c6f28 (0x5d5efbf4)
07-12 02:47:25.742: D/NFC JNI(10575): phLibNfc_RemoteDev_CheckPresence()
07-12 02:47:25.742: D/NFC JNI(10575): phLibNfc_RemoteDev_CheckPresence() returned 0x000d[NFCSTATUS_PENDING]
07-12 02:47:25.747: D/NFC JNI(10575): Callback: nfc_jni_presencecheck_callback() - status=0x0000[NFCSTATUS_SUCCESS]
07-12 02:47:25.752: D/NFC_LIST(10575): Deallocating node: 0x17c6f28 (0x5d5efbf4)
07-12 02:47:25.877: D/NFC_LIST(10575): Allocated node: 0x1702e60 (0x5d5efbf4)
07-12 02:47:25.877: D/NFC JNI(10575): phLibNfc_RemoteDev_CheckPresence()
07-12 02:47:25.877: D/NFC JNI(10575): phLibNfc_RemoteDev_CheckPresence() returned 0x000d[NFCSTATUS_PENDING]
07-12 02:47:25.887: D/NFC JNI(10575): Callback: nfc_jni_presencecheck_callback() - status=0x0000[NFCSTATUS_SUCCESS]
07-12 02:47:25.887: D/NFC_LIST(10575): Deallocating node: 0x1702e60 (0x5d5efbf4)

EDIT: Don´t get me wrong, having NFC enabled, usually doesn´t make a relevant difference for your battery life ... but keeping a tag permanently attached, seems to have a much higher impact ...