Yes, it is counter intuitive, but the DC to DC converter doesn't work when the car is charging. Kevin, the Toyota factory guy explained it to me that the 12v battery is only charged when the car is running, just like a gas model.
I disagree with the statement above. The 12V battery is supported during both ready-on and during charging of the main traction battery. If one measures voltage at the 12V aux battery they'll find about 14.4V during both ready-on and charging, but something around 12V when the car is off.
Well that wouldn't be the first time Kevin misinformed me. LOL. That is good news for those that put a trickle charger on their 12v battery when on vacation. Just to clarify, is it your understanding that if the car has finished charging but the J1772 plug is still in the car, the 12v battery will not be charged? Only when the onboard charger is running is the 12v battery charged.