Controller to use 18 volt power tool battery to power 20 volt laptop
something as a one off design, I would just use an evaluation design for a car charger, like Texas Instruments PMP20774 eval design. But just to let you know, there is USB-c automotive chargers that will accept an input anywhere from 12 to 24 volts for the two voltage standards used for the automotive dc port (cigarette lighter @ 12V or 24V). So you don't necessarily have to build one from the ground up if you don't want to. The advantage of this over just using a 20V boost regulator, is the usb-c auto voltage modes (5v/12v/20v), and be able to charge any usb-c device