The reason for 5 Volts is obvious, baseline compatibility with USB protocol, lowest common denominator. Higher voltage options were required to deliver the wattage required by modern devices.
Allowing a PD charger to deliver a continuously variable voltage range, for example passing through vehicle supply voltage, or LiPo pack voltage, seems like a simple efficient method of achieving that goal. Yet, my reading of the USB PD spec seems to prohibit that option.
First, is my understanding correct, that such pass through via USB Type C PD 3.0 is a violation of the spec?
Please suggest any way(within PD 3.0 spec, perhaps an alt mode) to pass vehicle supply voltage(always < 20v), current limited to 5A, from a charger/OBDII device I am designing, via USB Type C full featured cable, to an Android device I am also designing.
Second, why are 4 fixed voltage levels required to be provided by charger instead of a continuous range(as in Qualcomm Quick Charge)?
Third, what are the advantages for a charger to supply each of those specific voltages, 9, 15, 20v.
As I understand it, maximum wattage 100W was chosen due to international safety requirements.
Any answer that states the reason for the voltages is to maintain safe current within existing cables must also explain how this is relevant given that PD is only allowed over USB Type C cables and that 5A is allowed at 20V, but only 3A at the lower voltages over the same cable.