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If you have a laptop PC which supports USB Power Delivery over USB Type-C, you can use it as a power bank to supply charging power to USB Power Delivery enabled phones like the Samsung Galaxy S8 (assuming you don't mind sacrificing your laptop's battery charge).
Here we connect an Apple 15" MacBook Pro (Late 2016) with the Samsung Galaxy S8 directly using a USB Type-C to Type-C cable and see how well the MacBook charges the phone over USB Power Delivery. Note that in this case the laptop is fully charged but the phone's battery is fully depleted.
To initiate charging for the Samsung Galaxy S8, we select 'Charge this Phone' under the phone's USB charging options.
Looking at the initial USB Power Delivery negotiations, the Apple 15" MacBook Pro (Late 2016) constantly declares its source capabilities of a single 15 watts (5 volts @ 3 amps) power profile.
We then observe that the Samsung Galaxy S8 finally requests to pull 15 watts (5 volts @ 3 amps) from the Apple 15" MacBook Pro (Late 2016) which is accepted.
Several hard resets later, both the Apple 15" MacBook Pro (Late 2016) and the Samsung Galaxy S8 again repeat the same USB Power Delivery protocol negotiation with the same 15 watts (5 volts @ 3 amps) power contract established.
We also observe that the Samsung Galaxy S8 also advertises its power sink capabilities of 2.5 watts (5 volts @ 0.5 amps).
Interestingly we find in between the USB Power Delivery protocol negotiations that both the Apple 15" MacBook Pro (Late 2016) and the Samsung Galaxy S8 also established a power contract of 7.5 watts (5 volts @ 1.5 amps).
In fact, we have observed that the Samsung Galaxy S8 was being charged by pulling around 7.5 watts (5 volts @ 1.5 amps) for about 13 minutes, but after that charging became unstable with frequent drops in the current level.
