The Nest API reference specifies a battery_health property for a Nest Protect (part of the smoke_co_alarms structure) which records whether the battery is ok or needs replacing soon.
But, a Nest Protect can also be wired to a 120V supply.
In this scenario, what will be the value of the battery_health property, or will this property simply be omitted?
The wired Nest Protect includes batteries for backup so it can function in the event of a power outage (see the "Tech Specs" section of the information page). The battery_health property will indicate the health of the backup battery.