ICE turns on inexplicably

Hard to say. I also have a 2021 and regularly leave my house with a full charge and drive 2 miles to the top of a long descent. When I start down I have 96%, and by the bottom I have 100% again without triggering ICE. Even in 90°F temps the last couple of days. I suspect it's because the car was in the garage and the battery temp is nowhere near 90°F.

Do you have the ability to see the traction battery temperature? If you were just charging prior to departure it might be pretty warm already, which could trigger the observed behavior.

 

sounds typical for toyota plug ins...

 

My 2017 did the same thing today. First time,
under the same conditions you described.
I don't normally do this route but the engine started toward the bottom of a mile long decline.

It was a bit odd since i had about 12 miles of electric range remaining.

The weather here was pretty cold 38° F. I'm guessing the engine started to heat the cabin.

I didn't have the rear window defroster on.

 

Yeah, ICE starting no reason (at least to the mind of the driver) is a very usual thing on this car. The car has a mind of its own. It is covered by the manual. LOL

 

Have you tried starting the trip with different levels of SOC? Warm battery vs cold battery?

 

the pip manual states 14 reasons why the engine may come on, and adds, 'there may be other reasons'.

prime was supposed to improve on that, and i haven't read the manual, but toyota's mantra is, 'it is a phev, not a bev. the engine is there for more than one reason.
and they probably know what is best for the car, but it's all so top secret

 

That means it's a warm-up cycle. The battery temp could trigger ICE to start, but once it starts it will run until the engine gets up to full operating temperature, which can take a little bit. Turning the car off and back on stops the warming cycle.

 

State Of Charge.

Try leaving with, say, a 1/2 full batt and see what happens.

 

No. That's not how the Prius is programmed. The warm up cycle doesn't initiate an engine start, it only controls engine shutoff. Once the engine starts it will run until the programmed temperature is reached, but it will not start on its own for the sake of getting the temperature up.

 

When the car determines that the current being supplied from braking (engine or pedal) will damage the battery, it will use the energy to turn the gas motor instead of charge the battery. That power has to go somewhere.

On steep, long hills even that strategy doesn't dissipate all of the extra power and it does appear to go to the battery anyway. After a 15-mile descent my Prime was at 100% charge and continued displaying 100% for the next 5 miles of driving or so.

 

Finally a right answer. @fotomoto for the win. The ICE starts in order to burn off excess battery charge that occurred after going down a hill with an already full battery. In point of fact, the ICE is not running, it's being spun by the electric motor in order to use up battery charge. But I think I've read that it does use a tiny bit of fuel thus requiring the warmup cycle to complete.

I'd guess that those five miles were either flat or very slightly downhill. What kind of miles/kWh were you getting in those five miles after the decent? Very small changes in grade make big differences in m/kWh.

 

A little uphill, actually, with a lot of stop signs. I hadn't been expecting what I saw so I did not reset any trip meters to measure the mi/kWh for that bit of the drive. Based on past experience I would guess about 4.5 mi/kWh or so.