The Nintendo Switch always needs a battery - even when docked - to boot as it uses the battery power to do so. That means the Switch will became e-waste when no more batteries are produced for it. Because of that and because I want to do a "consolized" Switch - for overclocking it and having better thermals - I started a little project for having a Switch without a battery,I'm doing a "consolized" Nintendo Switch where I want to mod it and overclock the hell of it xD. Anyways... I didn't want to use the battery so I used a buck converter wired to 15v from the dock and then step it down to 4.2v.
It works! During the startup there is 0.4A consumed for booting the system and from there on the energy will be supplied by the dock itself. The only thing needed is to ground the sensor wire (grey one in the middle of the connector).
As said before a battery is needed for booting the device which requires ~0.4A in the process. Other than that, a fully charged battery gives ~4.3V. For achieving the results I wanted I have used the battery's PCB in conjunction with a buck converter so the PCB will tell the Switch that no charge is required while the buck converter will give power to the PCB making it believe that battery is fully charged.
The buck converter I'm using is this one https://www.ebay.com/itm/255283221975 but any converter capable of outputting 4.3V from 15.3V is good enough. The power is supplied to the converter from a 15.3V line from the dock board as shown in the image bellow.