Does background reading drain the battery?
Not enough to notice on a modern phone. The field is produced in short bursts rather than held continuously, and an actual read costs less than the looking does. If your battery is disappearing, NFC is not where to look.
What it actually costs
The phone energises its field briefly, listens, and stops. That cycle repeats while the phone considers a tag plausible, which excludes a sleeping device.
An actual read is a short exchange of a few bytes, so it costs less than the periodic looking that preceded it.
Neither is a continuous drain. There is no radio holding a connection and nothing negotiating with a network.
Compared with what else runs
The screen dominates a phone's consumption, by a margin that makes most other comparisons uninteresting.
After that come the cellular radio searching for signal, location services, and applications refreshing in the background.
NFC polling sits well below all of those. Switching it off to save battery is optimising the wrong thing while the screen brightness sits untouched.
This is why most people leave it enabled permanently without ever thinking about it.
Why people worry
Because it is a radio, and radios have a reputation earned by the ones that hold connections and search for networks.
Because it is one of the few things in the settings that can be switched off, which makes it a candidate whenever a battery misbehaves.
And because battery problems have no obvious cause, so any toggle becomes a plausible suspect. Switching NFC off during a bad battery day and seeing improvement is a coincidence people remember.
If a battery is genuinely disappearing, look at screen time, an application misbehaving, or a failing cell, in that order.
Better reasons to switch it off
Handling tags you do not want triggered, such as a box of written tags on a desk you keep resting the phone on.
Working somewhere with tags you keep brushing past, where the interruptions are the problem rather than the power.
A deliberate choice not to have the phone act on objects in the world, which is a reasonable position and has nothing to do with battery.
All three are about control rather than consumption, and they are the honest arguments.
Frequently asked questions
Should I switch NFC off overnight?
There is no battery reason to. A sleeping phone is generally not looking for tags anyway.
Does background reading cost more than reading in an app?
The periodic looking is the cost, and it is small. A read in an application is a deliberate single exchange, so it costs less overall.
My battery drains fast. Is NFC to blame?
Almost certainly not. Look at screen time, a misbehaving application, or cellular signal searching, in that order.
Does an active tag nearby drain it?
No. A tag has no battery and does not initiate anything, so a tag near a phone changes nothing until the phone looks.