It's your responsibility to have valid fare with you at all times. Fare inspectors patrol the transit system randomly, so be sure to keep your phone charged, bring a battery backup or carry a separate plastic Hop card. On newer iPhones, the Power Reserve feature lets you continue to pay for rides even when your phone needs to be charged — but Power Reserve will not help if a fare inspector asks for proof of fare (your phone must be charged). Power Reserve works for Hop cards that have the “Express Transit” feature enabled.