Very interesting!
With a 5 volt input, the peak output voltage is about 1.3 volts lower, or 3.7volts.
This is due to the voltage drop from the schottky diode (~0.3v) and the 1.0 voltage drop from the 3.3v voltage regulator (HM333B).
(the schematics for these cheap boards are easily found, just google the name on the board: XY-LPWM)
So... with your 12 volt input you are running about a 10.7v PWM waveform, which I don't think will hurt anything.
For anyone interested what the typical power control operation is,
here is the battery voltage on my G37 Sport before starting, during starting and driving for just over 8 minutes (500 seconds):
• Battery voltage drops to 10 volts while starter is engaged.
• After starting the engine, the charging level is higher, resulting in a battery voltage of 14.6 volts.
• After about 6 minutes (350 seconds) the charging level is reduced and the batter voltage is ~13.2 volts
