ICE turning on for apparently no reason...?

i put it in neutral and use the brakes

 

I shift to B before the decent, or turn off the heater, or if it's really cold, I'll complete the warmup cycle in HV and pepper the cabin with a tiny bit of heat, than switch into EV as I'm headed down and at the bottom back into HV for some more heat. Obviously, not the exact same scenario as the one the OP keeps experiencing.

I think the simple way to avoid the ICE firing would be to back off the Go Pedal just before he reaches the top of the hill and shift to B for the decent. Not sure it would work, but I think it at least give the OP an idea or two how to make a change of two to avoid the problem.
And i sure hope he doesn't still want an explanation why, cause I doubt anyone could / would.

 

FWIW, I have a few thoughts on this.

To cut to the chase, make sure that your battery heater function is turned on. Apparently, for whatever reason, Toyota gives you the option to turn that off.

No battery warmer on my Prime | PriusChat

Secondarily, make sure that you charge the car just before you drive it. Because, near as I can tell, the only time the battery heater comes on is at end-of-charge?

So, in logical order.

Sounds like you're hitting a limit on regen charging the battery.

I can tell you, from using a Scangauge, that even moderate braking at 50 MPH can generate quite a bit of current. I easily exceed 50 amps (a "2 C" rate of charge). Off-ramps, I find it just about impossible to keep the current below 75 amps.

Cold lithium ion batteries cannot accept a high rate of charge without damage. Fast-charging when cold leads to "plating" of the anode. (I'm just parroting that, I don't actually know what that means. I just know it's bad.)

BU-410: Charging at High and Low Temperatures - Battery University

So, what would matter here is the temperature of the batteries, not the ambient temperature when you drive.

My casual observation (via Scanguage 3) is that those batteries have a lot of thermal mass, and normal EV driving doesn't heat them up much at all.

Sounds like you do not keep your car in a garage? And Colorado has cold nights.

Toyota specifically says that part of the advantage of the battery heater is that it reduces ICU use. "This enhances EV mileage and drivability in cold temperatures and reduces engine startup during EV Mode."

2022 Toyota Prius Prime FAQ

Finally, FWIW, my wife's 2021 Prime is garage-kept, and the only unexpected ICU start we've ever had was due to use of the windshield defroster, which we now avoid. But we have mostly moderate temperatures, and gently rolling hills -- no mile-long downhills around here.

When I run all that through the blender, I'd explore the "battery heater" chain of events. Do you have it turned on, do you charge the vehicle just prior to using it, and so on.

I would also not discount the possibility that your situation falls through the cracks -- cold, but not cold enough to trigger the battery heater. So you're just stuck with the car choosing to protect the battery in this fashion.

Why this would only have started after a considerable length of time, I cannot even guess. I assume your battery is healthy and/or you haven't lost a noticeable amount of range or anything?

 

My 2018 does this exact thing and my opinion is that it is classic piss-poor software design. Although the intention is likely good the outcome is bad. My drive ends on the downhill so I pull into work with a cold engine running for about 30 seconds. If I put the car in “EV” mode the software should consider that the overriding requirement and JUST USE THE STUPID BRAKES. That’s literally their purpose. If it can’t regenerate to the battery just use the damn friction brakes.

 

Yeah, it's the cold battery. You're asking it to accept energy faster than allowed on a cold battery. That's why it only does it at the start of your drive. As @chogan2 observed, charging it right before you leave will probably warm it enough to prevent that.

 

Not in my experience. It more seems to relate to total charge on my Prime. But regardless I put it back to stupid software engineering. And really this is my primary complaint with my Prime. LOTS of software is poorly thought out. Lots of hardware is poorly thought out. I'm teaching my eldest to drive and he constantly asks me questions about why the Prime does things so stupidly compared to the Corolla or Rav4.

 

That's fine. But the OP's SOC when it happened was 70%, so it's not a SOC issue in his case.

It may be stupid programming, no doubt, but I learned a long time ago that some things that appear stupid only look that way because I don't know the whole story.

 

If you aren't taking the time to explain to your car the exact details of your trip (especially the topography) then the best it can do is guess.

From where I sit, they're guessing pretty well, and we're saving a ton of time by not having to do all that data entry like pilots have to do with their flight manager computers.

 

I love it when people act like we should just accept crappy programming. From cars to home entertainment to PCs there is just this constant willingness to accept total mediocrity.

From the number of complaints I've seen about this behavior I'd say they are doing a pretty terrible job of guessing. This is the primary reason I believe that we are still decades out from SAFE self driving cars. As you point out, they are guessing.