The Apple USB-C Digital AV Multiport Adapter can be used with the Apple 15" MacBook Pro (2018) not only to connect a HDMI monitor or external drive but also charge the Mac over USB Type-C. The USB-C Digital AV Multiport Adapter can be connected to the Mac's AC adapter for charging. Here we observe and compare how the USB-C Digital AV Multiport Adapter charges the Apple 15" MacBook Pro (2018) versus how the USB-C Digital AV Multiport Adapter gets power supplied by the Apple 87W USB-C Power Adapter that ships with the Mac.
We first connected the Apple 87W USB-C Power Adapter using the supplied Apple USB-C Charge Cable (2m) accessory to the USB Type-C receptacle connector on the USB-C Digital AV Multiport Adapter. We then plugged in the USB-C Digital AV Multiport Adapter to the Mac's Thunderbolt USB-C port to see how the Mac draws power to charge with the Apple adapter using USB Power Delivery. Initially no current was pulled while the voltage started off at 5.2 volts during the first few seconds of plugin. Once the voltage has risen to 19.6 volts, the Mac started pulling current to about 2.7 amps to get charged at around 53 watts by the USB-C Digital AV Multiport Adapter. Click here to download the trace captured by the GRL-A1 power analyzer to see more details.
To see how the Apple 87W USB-C Power Adapter delivered power to the USB-C Digital AV Multiport Adapter to support charging the Mac, again we observed no charging happened upon plugin as no current was pulled by the adapter when the voltage initiated at 5.2 volts. While this was similar to when the Apple adapter was charging the Mac, it took up to about 17 seconds for the adapter to only start pulling current from the 87W charger. (Comparatively it only took around 2 seconds for the Mac to start off charging with the adapter.) The adapter also displayed similar behavior as the Mac by pulling current to about 2.7 amps once the voltage went up to reach close to 20 volts to also get around 54 watts charging. View more details here by downloading the GRL-A1 trace.
As we further looked at the USB Power Delivery negotiations between the USB-C Digital AV Multiport Adapter and the Apple 15" MacBook Pro (2018), we could see different Power Data Objects (PDO's) being advertised by the Apple adapter to the Mac. Initially the adapter advertised PDO of 7.5 watts (5 volts @ 1.5 amps) which was later followed by PDO's of 0.5 watts (5 volts @ 0.1 amps), 2.5 watts (5 volts @ 0.5 amps), and variable power supply of 49.2-55.2 watts (18.7-21 volts @ 2.63 amps) as it charged the Mac. By comparison, the Apple 87W USB-C Power Adapter was seen to advertise the following PDO's- 12 watts (5 volts @ 2.4 amps), 27 watts (9 volts @ 3 amps) and 86 watts (20 volts @ 4.3 amps) to the USB-C Digital AV Multiport Adapter. The difference between the PDO's advertised by the adapter and 87W charger clearly shows the adapter advertising lower current than what the charger advertised which takes into account the power consumption of the adapter and any downstream devices connected to the adapter.
When observed how the Apple 15" MacBook Pro (2018) charged directly with the Apple 87W USB-C Power Adapter in this review, the Mac was able to pull current of about 4.4 amps to achieve around 88 watts (20 volts @ 4.4 amps) charging with the 87W charger. Comparatively using the USB-C Digital AV Multiport Adapter with the Mac actually reduces the power the 87W charger can deliver to the Mac given the much lower current provided by the adapter, which is a problem when the Mac needs to draw full 88W power.