Is my traction battery stealing energy from the 12V battery?!!

I don't know why your car wouldn't start all the way.
If you checked the 12 volt batteries voltage before you jumped it, that might offer some clue.

The EV range thing is really hard to explain. There are at least two things going on when shutting down the Prius Plugin with --- EV range and letting the car sit for a day or two and not plugging into a charger..

One is voltage rebound, which happens to a battery (any battery ) after the LOAD that battery is powering gets removed / shut off. And the voltage rebound amount can depend on how big of a load ( how fast you were driving or how steep an incline you were climbing when the car ran out of EV range) was removed.
Two is that the Plugin behaves like a regular Prius when it doesn't have any EV range. So it's constantly charging and discharging the hybrid battery. I never got to measure the amount of voltage the Plugin would charge and discharge, but with the Prius Prime it's only about 6 volts most of the time. And in the 6 volt range there is never any EV range showing, even though the car will still power the electric motors and shut off the engine for short periods.

So, unless you get to where you shut the car off at the exact same time as the Plugin ran out of EV range, the HV mode will have charged the hybrid battery without showing you how much it was charged.

Than the next time you start the car it's a surprise that there is now a bit of EV range, that wasn't there when the car was shut off.

 

possibly a bad 12v is doing funny things with the computers. have it load tested at an auto parts store for free.
positively, there isn't enough juice in a 12v to move your car across the street

 

I have noticed what seems to be a bit of voltage rebound in my RAV4 Prime several times, after consuming the full EV range then letting it rest a while. Though I haven't yet explored it enough to distinguish from ordinary vagaries of HV-mode.

 

There's a lot more to the description of voltage rebound than what's explained above, but both voltage depression and rebound are happening all the time while in EV mode, you just have to understand what it is and how to look for it. DrPrius makes it real easy to see, once the concept is understood. Many who have done hybrid battery conditioning or rebuilds learn about it over the course of time.

To make things more confusing and complex, a few years ago I posted about what I'd experienced at very low EV range ( less than a mile of EV range left ) and stopping at a store, shutting off the Prime. When I shut off the Prime I had enough EV range to make it back to the charger a few 1/10s of a mile away. A few minutes later ( 20 minutes at most ) I started the Prime again and the EV range was now less than the couple of 1/10s of a mile I needed to make it to the charger and the engine started on my way to the charger from the store.

Special case, not for me but for most others I'd presume. In the previous 40 mile or so trip, I'd been using every hyper-miler trick I'd learned up until than to see how many EV miles I could squeeze out of a single charge, so as one might assume I"d been super gentle of the Go pedal and keeping speeds, Amp draw really low.

So, if you want to see how much of the EV range difference is voltage rebound and how much is regular charging while the engine is running, the best way is to slow way done a few miles from destination and when getting near 0.1 mile of EV range start using creep home mode and drain the battery as much as possible and park it before the engine starts up.

There's way more to talk about how voltage works both at the top of 100% EV (soc) charge and how far the voltage depression can drop below where the car will normally start the engine ( under certain specific conditions ), but most owners don't have enough time or interest to care.

 

My RAV4 Prime's EV range displays only whole miles, not tenths, so that puts limits how much detail is available.

 

edit this is not right sorry - hard to keep all the details about gauges straight -The Prius Prime is the same until it gets below 1 mile EV range, than it starts showing 1/10th of a mile. --
The EV range of the MID screen shows only miles of EV range. The similar screen on the MFD is the one that shows 1/.10s of a mile increments.
I have that screen on the Home screen dashboard at 1/3 the size sharing space with Audio and weather. It also has a full screen view with a few more option screen to pick from the full screen on the MFD.

 

How old is the 12 volt battery?

 

So far, I've been watching only one guage, the same as the former on yours. I'll have to explore more to see if it has anything else behaving as the latter. It is still new to me, with lots more features not yet found.

 

just to throw some sauce into this dish... when is the last time you charged the hv battery? I'm leaning on bad 12v or.... a roident chewed up some wires and they arent completely separated nor touching and if any are.. you might have one strand of wire (no nevermind, the harness wires are the thicker strand than the thinner like speaker wire) or just a short somewhere...

I know we don't know 100% of the secret sauce from Toyota but I do know, I like to use my air fryer on the 2000W inverter with the care not in ready mode while charging a lot!

Charging a PHEV Prius and Cooking with the Air Fryer...the car, not in Ready Mode.
https://youtube.com/shorts/iI9Zvs1xlYU