[Septillion] and [Hugatry] have come up with a hack to manipulate a USB charger into becoming a variable voltage source. Their project QC2Control works with chargers that employ Quick Charge 2.0 ...

His solution is to add a P channel MOSFET which only allows power to flow when the polarity of the source voltage is correct. The schematic above shows the P-FET on the high side of the circuit.

If there are two resistors in series across a voltage source, then the circuit is a voltage divider. A voltage divider consisting of two \(500 Ω\) resistors is connected across a \(9V\) battery.