Indeed, a bit of controversy has arisen lately regarding this topic - with this thread:
The PD checked my FAA numbers on my Drone !
Some understand that the number must be written on the aircraft. Others believe the number can be written on a battery since the battery becomes part of the aircraft when in flight.
However, see msinger's post here:
The PD checked my FAA numbers on my Drone !
In the post linked above, msinger has provided a link and effectively pointed out that the language of this rule is a bit different than the language used in other areas of the FAA site, and it's also different than the language posted in Post #3 (sorry GG). FAA often uses the phrase "duration of the flight" elsewhere on their site, but in this situation, the language used substitutes the word "flight" for the word "operation." You might ask yourself why that may be?
It seems logical that FAA used "operation" instead of "flight" because FAA understands that there are more than a few model flights which end in disaster (e.g. batteries pop out of aircrafts mid-flight, or crashes occur where the aircraft is found without a battery, etc.).
The actual statement reads:
"The unique identifier must be affixed to the small unmanned aircraft by any means necessary to ensure that it will remain affixed for the duration of each operation."
It's
obvious this is a
carefully crafted statement. The key phrases are
"unique identifier",
"aircraft",
"any means necessary to ensure..." and
"remain affixed for the duration of each operation."
Since we know from harsh experience reading posts on this site and others that a battery can become dislodged from an aircraft during flight,
it's clear that placing an FA number on a battery would
not ensure that the FA number would remain affixed to the aircraft
for the duration of the operation.
Hence,
one might conclude that the reason the FAA wants the pilot to adhere the FA number to the aircraft itself, rather than just an aircraft's battery, is so as to ensure the FA number will be found in any wreckage.