I just opened up my 2018 Mini and measured the voltage (not plural) coming from the power supply connector as well as a continuity measurement of the logic board connector.MacMini2018_DC_Connector.pdf
As with the 2012 & 2014 models, the logic board needs ~12VDC. That’s it. The people claiming the 2018 power supply is outputting multiple voltages have clearly never opened one up and measured it. There is no basis to support this information as I have just confirmed for myself. As for the claims that these machines run on a “strict voltage tolerance”, I’m not buying that either. I ran my last Mini (i5 2014) on 13.6VDC because the point was to power it with a LiFEPO4 backup battery.
However (as noted in the drawing) the logic board connector shorts the pin marked with an asterisk (*) to 0V or GND. This is a little suspicious considering this connection is not made on the 20pin cable that connects the PSU to it. All other connections are mirrored. This could be Apple’s way of trying to foul up the not-so-savvy DIY enthusiast trying to convert his/her 2018 Mini to run on straight DC power. Meaning… it could be designed to where the power supply has to see that connection or else it’ll shut the computer down. Doubtful since I was able to measure voltage potential with the 20pin cable not connected. Probably more like ground loop prevention in some other part of the machine.
As for why they chose to use a 20pin connector to carry a single voltage… in a word - amperage. The previous models had a 85W PSU. 85W / 12V = 7A This one is 150W. So, 150W / 12V = 12.5A. They chose to use multiple runs of thinner wire instead of two big fat wires which is perfectly acceptable. Also because if one of the smaller wires break, it will probably still work. If too many break, it will have issues starting up. The only reason it needs such a big supply is for when it is starting up. It has large voltage spikes during that process. I measured them as high as 11A. It seems to idle around 2A. Any decent size Lead Acid or Lithium battery you use will simply laugh at that.
Keep in mind while looking at the diagram that it is drawn from the POV of looking at the CONNECTION SIDE of both the male and female counterparts. In other words, they are a mirror image of one another. I made sure to include the key on the end of the connector for reference. Hopefully this all makes sense.
Be sure to let me know how it goes if you are still planning on carrying out your initial plan. These machines are still pretty new and I imagine there are plenty of people like myself who are hesitant to start hacking up their machines which are probably still under warranty.