Unfortunately, charging your iPhone or iPad faster via USB-C is not as straightforward as it may appear due to an incredibly fragmented market of USB Type-C cables that limit power delivery and charging speeds. To help customers avoid any charging issues, Apple will allegedly require that all third-party chargers work at full speed with 2018 iPhones.
The Japanese blog Mac Otakara (Google Translate) has reportedly learned from suppliers that third-party power adapters may need to pass USB-C Authentication certification in order to be permitted to charge iPhones faster.
iOS will allegedly limit non-sanctioned adapters to up to a 2.5-watt charging speed (5V and 0.5A) and display a warning that the device has not passed USB-C Authentication certification. By comparison, current iPhones can be fast-charged with 15 watts of power (5V and 3).
The next iPhone might support even faster charging at 18 watts of power (9V and 2A).
The USB-C Authentication certification ensures that a charger meets the USB-C Power Delivery specification and is free of maliciously embedded code. As such, it is intended to protect customers from possible uncertified shady chargers.
The current iPhone X, iPhone 8 Plus and iPhone 8 models support fast-charge capability, as defined by the USB-C Power Delivery specification. Customers who want to take advantage of that feature are looking at an upfront cost of at least $74 because current iPhones ship with the old 5-watt power brick and non-USB-C cable.
Several rumors have claimed that 2018 iPhones will come with a faster 18-watt power adapter along with a Lightning-to-USB-C cable. 2019 iPhones may even ditch Apple’s proprietary Lightning port for industry-standard USB-C connectivity, according to another rumor.