12V Battery near-dead after charging HV battery :-(

Alright guys, I know you have all been anxious for an update on this. I have good news and bad news.
The good news is that the dealership says they've fixed it.
The bad news is they don't exactly know how or why.

I brought in my PiP along with two pages of notes on the problem and links to this forum thread and my video, with the hope they would actually take me seriously this time. They really appreciated the documentation and assigned my car to the master technician immediately. The master tech spent all day on my car yesterday and was on the phone with Toyota Tech Line four times. Apparently it was even hard to convince the tech line folks that my 12V system was at 11.5V and draining the battery!

The first thing they did was notice my 12V battery was low (of course! I had just charged the car at home and it's only 4 miles to the dealership, hardly any useful time for the DC-DC converter to recharge it...) and tried to use their shop charger to replete it. It failed to take a full charge (which we already knew), so they considered the battery bad and replaced it (huzzah!). No idea on why the battery check they did a month ago said it was fine; perhaps it was coincidence, or the tech made a mistake. On the other hand, had it just been replaced then, the problem would still exist... so we'll call that a lucky coincidence.

So, brand new 12V battery installed, and it's still discharging it when plugging in the car, just like I had found at home. They pulled a brand new PiP off their lot to test it, and of course it is at 13.5V just like others had reported here. So now they know for sure the system is malfunctioning, and they have a normal working car to reference and compare results.

They hooked up the Techstream and found some latched codes related to 12V power issues buried deep in the system (hmm, my OBD-II reader didn't show any codes!), which they cleared. Then they performed some kind of deep reset on the charging ECU(s). Now, after all of that, they say it is working normally, measuring 13.5V as expected.

Near as anyone can figure, the ECU that controls the 12V charging circuitry while the car is plugged in was locked in some fault state where it just refused to run every time. Obviously it was capable of running, since it would run for a few seconds after plugging in, but it just kept going into a fail-safe mode of some sort and shutting down.

This one really stumped everyone. They said that charging issues are basically unheard of, which I think we can all agree on, so they are learning as they go. I'll definitely be checking the system when I bring it home tonight, and probably check it periodically for a while to make sure it doesn't come back. I'm glad at any rate that I have a new 12V battery after all that abuse, but had I just replaced it myself last week, it wouldn't have actually fixed the problem at all. Plus they covered it under warranty, so I'm happy about that too.

Thanks again to all that helped on this, particularly Rozi and Potorap for making the measurements on their PiPs that made all the difference in my diagnosis. This one definitely falls in the "Wth" category. I'll provide more updates later if I have anything interesting to share.

 

Tested the PiP back at home last night and it is happily at 13.5-14.5V now when charging. Woohoo! I'll definitely be checking up on it occasionally. I'm just really happy I can stop worrying about this thing.

They REALLY reset that system though, both my tripometers were reset, as were both the EV ratio screens (goodbye lifetime stats ).

The codes they pulled out of the car were U0142/U0155/B1579/C2300, in case anyone cares. Seems mostly to be related to lost communications when the 12V system crashed.