Adapting a 12V Dell power supply to a Lenovo PC - very naive questions

Question

I bought on Amazon a chinese power supply for my Dell Latitude. It plugs into a 12V socket on my car and feeds 19.5V to the PC. So far, so good.

Now I want to feed another computer, a Lenovo thinkpad with a completely different socket:

I am thinking to cut the relevant cables and put at their places 2-wires plugs+sockets, so that I can swap the plugs depending on the PC that I am using:

I have cut the cable on the power supply and here comes the surprise: there are three wires in it.

I assume that there is a ground wire, but I wonder what its purpose is, as the 12V plug gets into the cigarette lighter of my car, which has no ground.

MY QUESTIONS:

• which of the three wires is the ground? (I reckon it's the red, because it is thinner than the white and the black)
• can I safely wire just the white and the black in the plugs+sockets that I have bought, or must I buy 3-wires plugs+sockets

Answer 1

We do not know which wire is which so you have to measure it yourself.

And connecting only the supply and return wire would most likely make the laptop to be unaware of how powerful power supply is connected and it will not charge at all.

There are three wires because there is supply wire, return wire, which supply the power, and the third wire is some kind of manufacturer specific ID or communication wire to let the laptop know how much power is available from the power supply.

The ID/communication wire can be specific and proprietary to laptop manufacturer so most likely they are not compatible and connecting ID wire may even damage something.

Basically what you are trying to do is not possible without finding a web page how to adapt a certain watt charger to certain laptop brand or reverse engineering how the original power supplies work.

So in short, power supplies are usually not be compatible with different brands.