I sometimes use an HP Envy laptop to record SDR data overnight. The AC power adapter is way too RF-noisy, so I generally run it from an external 20vdc battery bank -- the one I have is called a MaxOak K2, bought from Amazon. It's worked fine. The laptop's internal battery by itself doesn't last through the night.
Unfortunately I misplaced the cable to connect the battery bank to the laptop, and the U.S. MaxOak people said they didn't have one available. It appeared that the HP laptop takes a 4.5mm x 3.0 mm barrel connector, sometimes called a "blue tip". I ordered what seemed like a generic replacement from Amazon.
Checking the HP-provided AC power adapter with a meter, there's 20vdc between the center pin and the outer barrel surface, and also 20vdc between the inner barrel surface and the outer barrel surface.
Checking the generic cable I just bought, the center pin goes to a blue wire, the outer barrel goes to a black wire, and the inner barrel goes to a white wire.
I'm inclined to connect +20vdc from the battery bank to the blue wire (center pin) and -vdc to the black wire (outer barrel), leaving the white wire (inner barrel) disconnected.
Does this sound best? Does a laptop want or need to sense voltage from the inner barrel for any reason?