2017 Prime Engine didn’t start when EV depleated

Sounds like a random glitch... Disconnecting the 12v for a few minutes will reset the whole system, which might be worth a shot. Also keeping an inexpensive OBD2 reader in your glove box so you can monitor battery health in real time on your phone with Dr. Prius app would be wise if it happens again. It also can be the difference between making it to wherever you're going on time or waiting for a tow truck.

 

I also drive a 2017 Prime.

I am fortunate, we have a local mechanic that we trust. I would take it to him, and describe the behavior [failure of ICE to start].

 

It's unlikely that the 12V battery had anything to do with this issue, since the car was in ready mode when this issue occurred. Once the car's in Ready mode, the DC-DC convert is suppose to take over - supplying power for all 12V electronics in the car. You may have an issue with the DC-DC converter but I'm pretty sure the ECU would throw a code for that issue.
The dealership scan tool can do a deep dive into what happened, it all depends on the skill level of the Toyota technician.

Hope this helps.

 

Automobile engineers try to think of every possible malfunction and have onboard diagnostics that trigger the codes. But there is always the possibility that some malfunction could happen for which no code is to be set.

 

Are you sure the engine did not start up. Because what you describe is normal behaviour if you were going 80 at the end of the EV range. It's normal for the engine to kick in and run in a start up mode that feels like limp mode, definitely not enough power to keep going at 80mph. The engine takes those seconds to start and get up to normal conditions and limit power to protect itself. In a situation like that(80mph) it is better to switch into Hybrid mode before the EV battery is depleted to give the engine time to start up and get to temperature. Easiest way to do that is to floor the accelerator for a second when you see the EV battery is almost depleted (5% or so) which will start the engine while the car continues using the EV battery and keeps maintaining speed.

 

That would be a serious design flaw, and a danger to everyone on the road

 

Not much different than running out of gas on the freeway. I have had that experience, you just slow down and get off the road.

 

Not the design engineers fault

 

Watching data from ODB-II, that's easy to confirm. A cold start on the highway will deplete the HV reserve really fast, bringing you down to what equates to a limp mode temporarily.

 

While I'm thankful I haven't had to do it more than a handful of times, I am equally thankful that I learned how to gracefully depart traffic in the event of breakdown.

I wish more people knew how, or got some kind of coverage in drivers' ed courses.

 

That's another plus of running Hybrid Assistant. You see the engine's RPM on screen. I don't know why Toyota didn't think it would be useful to have.

 

It might have been fun to read the battery temps using DrPrius, when the engine didn't start right away after running the battery pack dry at 80mph with ambient temps around 80 F.
On the other hand it might not have been much fun, waiting for the engine to warmup enough to pickup where the battery left off.

 

Toyota's approach with Prius has always been KISS. Like you said, there are ways of seeing that info. Here's a sample with Hybrid Assistant, a drive that has frequent engine on & off... perfect for this discussion: