What is passive vs. active NFC?
Passive NFC devices (tags and cards) have no battery — they're powered by the radio field emitted by an active NFC device. Active NFC devices (smartphones, payment terminals) generate their own RF field and can both read and write tags.
How passive tags work
An NFC business card is a passive tag. It has no battery, no processor beyond a simple chip, and no power source. When a phone's NFC antenna comes close, the phone emits a radio field that inductively powers the chip. The chip uses that borrowed power to send its stored data back to the phone. Remove the phone and the chip goes dormant instantly. This is why NFC chips last indefinitely — there's nothing to drain.
Active NFC devices
Your smartphone is an active NFC device. It generates the RF field, reads passive tags, and can also communicate with other active NFC devices for peer-to-peer data exchange (Android Beam used this, though it's been discontinued). Payment terminals, transit readers, and NFC-enabled laptops are also active devices.
Can two NFC business cards read each other?
No — they're both passive. You need at least one active device (a phone) to facilitate the read.
Does an NFC card drain my phone battery?
Marginally. The NFC read consumes a small amount of power for about one second. In practice, it's undetectable in daily battery life.
Can passive NFC tags be written to?
Most passive tags support both read and write operations, but NFC business card platforms typically lock the chip after encoding to prevent overwriting.