First of all let my say that i'm NOT an electrician or an electrical engineer. i'm just a guy who wanted to be able to chagre my laptop via my powerbank and this is how i did it.
The power bank i have is the Aukey 30000mAh. Specs for the USB ports:
Type-C Input/Output: 5V 3A
Output (Quick Charge 3.0): 3.6V-6.5V 3A | 6.5V-9V 2A | 9V-12V 1.5A
Output (AiPower): 5V 2.4A
The power bank is able to put out 42W simultaneously, if you are charging via all the 3 ports at the same time.
The laptop needs 12V 2A to charge.
First i bought the LTC1871 DC-DC STEP-UP converter (google and you will find it) This was the only step up converter i could find which was able to handle a 2 amp output and at the same time step up the voltage from 5v to 12v there are a few tests of this converter on youtube but i think this is the best one: https://www.youtube.com/watch?v=Y1YtPG4fFeM
Remember to set the correct output voltage on the converter before you test this setup... The converter has a lcd display that shows you in and output voltage, but it is not 100% accurate. use a voltmeter/multimeter to make sure.
Then i took two standard USB cables and cut away the end that wasn't A-Male. Cut away the wires for data (usually green and white). Twisted together the red wires (+) from each usb cable and did same thing with the black (-) these wires are then attached to the input side of the converter. The converter is clearly marked with whats the input/output side and which is plus and minus.
I got lucky and found a power supply with the right DC-jack for my laptop. Cut the cable from the power supply and then attached it to the output side of the converter and voila! the laptop is charging :)
I haven't been able to measure the amps. But according to the battery indicator in windows it takes the same amount of time to fully charge the battery whether or not i use the standard power supply or my setup.
I do have some issues... I can't use my laptop while it's charging, the lid has to be closed or the converter shuts of. And even if the lid is closed, i still takes a few tries before converter starts and begin to charge. I have to push the on button on the powerbank a few times... Might be fixed with som better cables, not really sure.