[brandent]

2018 PHEV here. I have had to jump start mine....I'm one of the 'miserable roadside assistance experience' people. Few comments on the process...

1. The HEV and PHEV are different. The PHEV version has a completely separate standard 12v car battery found behind the plastic panel in the right rear of the hatch. The HEV does not.
2. The HEV has a 'reset' for the 12v...they are physically the same unit. The PHEV has some kind of recharge scheme that the main battery will charge the 12v up if the 12v drains when the car is off. However it only runs a set number of cycles and will stop so as not to run the high voltage battery dead.
3. Yes you can (and in my case must) jump start the vehicle if the 12v is dead. In my case the high voltage battery was still nearly full. For whatever reason while the car was sitting plugged in overnight the 12v drained, was refilled by the big battery, then drained again. I think it'll only do this a couple times before giving up. In any case car was stone dead to the point of no interior light. A full main battery does not help in this situation. It WILL NOT start regardless of the status of the main battery.
4. There are two spots to jump start it. Under the hood there's a big electrical box with plastic cover. Pop it off and there's a positive battery post there among other things. Well not exactly a post but a designated metal piece to clamp on to. The negative is to be latched onto whatever metal piece you like for ground. There is no negative post. This is the approved procedure in the manual. Alternatively I suppose you could connect the jumpers straight up to the 12v battery in back. I'd just use the one they tell you to use though.
5. Similarly you can jump start someone else using the same connections.