Express Transit cards with power reserve
You can use power reserve with your Express Transit cards to quickly pay for rides. On your compatible iPhone with the latest iOS version, power reserve is available for
up to five hours when your iPhone needs to be charged.
To see if Express Transit cards are available when your iPhone needs to be charged, press the side button. Doing this often may significantly reduce the power reserve for Express Transit cards. If you choose to power off your iPhone, this feature will not be available.*
*You can use Express Transit cards with power reserve with iPhone XS, iPhone XS Max, iPhone XR, or later.
https://support.apple.com/en-hk/HT209495