The iPhone XS and XS Max, which ship this week, as well as the forthcoming iPhone XR, support certain NFC card transaction even when iOS is not running and the host device is powered because it needs to be charged.
Buried in Apple’s released technical specs for the new iphone XS and XR lines, due to arrive sept 21 and October 26. It is a first for the devices: Express Card with power reserve. The feature allows new iPhones to conduct supported NFC transactions without the assistance of iOS.
Apple recently published iOS security guide update provides more detail.
“If iOS isn’t running because iPhone needs to be charged, there may still be enough power in the battery to support Express Card transaction. Supported iphone devices automatically support this feature with a transit card designated as the Express transit card and student ID cards with Express Mode turned on”.
The security guide notes that in iOS 12, pressing the side button will show the text indicating both the low battery icon and showing that Express Cards may be Used.
Notably, users are only able to access power reserve functionality under low power conditions, meaning the feature is unavailable following a standard hardware shutdown.