It is important to use proper PD E-Marker enabled charging cables to ensure full power is delivered from the charger to the device, which otherwise will limit the VBUS voltage thus resulting in a lower power output. Here we compare between the original Apple USB-C to Lightning Cable and its alternative SensusTech USB-C to Lightning Charging Cable option to see how they affect charging performance.
First we used the Apple USB-C to Lightning Cable to connect the Apple iPhone XS Max with the Apple 18W USB-C Power Adapter that shipped with the Apple 11-inch iPad Pro (2018). Over USB Power Delivery negotiation, we could see in the first few seconds after plugin no current was pulled at a 5 volts rail startup voltage. The iPhone only began pulling current when reaching 10 seconds to initially charge to around 7.6 watts (5.1 volts @ 1.5 amps). After a couple hard resets later the voltage went up to a 9 volts rail while current was pulled in between 1.8-2.1 amps to achieve max charging at around 18.9 watts (9 volts @ 2.1 amps). Click here to download the trace captured by the GRL-A1 power analyzer to view more details.
By replacing the Apple cable with the SensusTech USB-C to Lightning Charging Cable, we also observed a similar trend over USB Power Delivery where current was only pulled a bit later after plugin for about 10.2 watts (5.1 volts @ 2 amps) initial charging. After a hard reset however the voltage only stayed within 5V while the iPhone was charging at mostly around 11.7 watts (5.1 volts @ 2.3 amps). To see more details download the trace captured by the GRL-A1 power analyzer here.
Comparatively we can see that the SensusTech USB-C to Lightning Charging Cable supports only 5V thus limiting down the power that flows through to just about 11W. While the Apple USB-C to Lightning Cable supports 9V to deliver 18W which is the max power capability of the Apple 18W charger. Thus if using a PD E-Marker enabled cable that lowers voltage down to 5V, the Apple iPhone XS Max won't be able to draw the full 18W needed for fastest charging.
To further confirm this we also tested using the AUKEY PA-Y12 Amp 60W PD 3.0 USB-C Wall Charger to charge the Apple iPhone XS Max while connected over the Apple USB-C to Lightning Cable. When plugged in on a 24% full battery, the iPhone also took nearly 10 seconds to only start pulling current along a 5 volts rail startup voltage. After initially charging to about 10 watts (5.3 volts @ 1.9 amps), a hard reset caused the voltage to increase to a 9 volts rail where charging eventually happened at around 18.6 watts (9.3 volts @ 2 amps). Click here to download the trace captured by the GRL-A1 power analyzer to view more details. We can see that even when using a non Apple charger, the iPhone is still able to achieve 9V charging as long as the Apple cable is connected in between.