The modern USB charging circuitry of mobile phones is pretty sophisticated. For example they can negotiate higher charging voltages etc.
This question, inspired by a discussion on worldbuilding, is simpler: I suppose I can charge any phone by simply providing 5V DC on the VCC/GND pins of the receptacle without even connecting the other pins, let alone communicating with the device. Think of a replica of Volta's first battery or a potato battery.
If this is correct, the actual question is which voltage range the charging electronics typically support on the two pins without being damaged. We will roughly know the voltage of one battery cell and can stack them in order to get in the vicinity of 5V but we cannot actually measure it and may end up half a Volt or so high.