I wonder if the phone requires a special cable by design. It could also be resistance in the cable or connectors, or a weak voltage regulator. A linear regulator only needs maybe 2V above the output voltage, but they waste power and could even go into thermal shutdown, I think. Not sure how much input voltage a switching regulator needs, which is probably what the USB charger uses, and it also has a current limit. Anyway, even if the bike's charging system isn't working right, the batter should have enough power to charge a phone!