PowerPC wrote:Even if I take both outputs from a single unit, assuming they work at the same time?
If not, why is this being recommended as an alternative?
You are referring to the 3rd option posted above by
RealBlackStuff, I assume? It's not really presented as an alternative for charging ThinkPad laptops, because the product in OP does not charge laptops at all. It can only charge USB devices. Also, the Anker has a single DC output port that outputs either 9V or 12V, so they do not work at the same time.
But back to the question: assuming there is a power pack which outputs DC 9V and DC 12V at the same time, why can't one combine them to get a DC 21V output?
Because "voltage" stands for "electrical potential difference". Voltage does not exist at one point: it's always the
difference between the electrical potential of
two points.
"A power pack outputs 9V DC" actually means the power pack is capable of keeping the electrical potential difference between the positive output pin and the negative output pin at 9 volts while current is flowing out of the + pin and into the - pin. Similarly, a 12V power pack maintains 12 volts of potential difference between its + and - pins.
And, DC consumer electronics always connect the - pin of everything together and call it "ground" (even when it's not connected to the ground). Therefore in a power pack with separate 9V DC and 12V DC outputs, you'll have 3 points of different potential: 0V (ground), +9V and +12V. There's no potential difference larger than 12V anywhere.